path: root/docs/html/guide/practices/ui_guidelines/activity_task_design.jd

page.title=Activity and Task Design Guidelines
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>Activity and task design quickview</h2>

<ul>
<li>Activities are the main building blocks of Android applications. </li>
<li>In addition to writing your own activities, you are free to re-use activities from many other applications through intents.</li>
<li>You can enable activities in your application to be started from intents in other applications.</li>
<li>In nearly all cases, the activity stack just works as expected.</li>
    In a couple of cases you might need to ensure the right thing happens by setting a string or flag.</li>
</ul>

<h2>In this document</h2>

<ol>
  <li><a href=#applications_activities>Applications, Activities, Activity Stack and Tasks</a>
  </li>
  <li><a href=#tour>A Tour of Activities and Tasks
    <ol>
      <li><a href=#starting_an_activity_from_home>Starting an Activity from Home</a></li>
      <li><a href=#navigating_away_from_an_activity>Navigating Away from an Activity</a></li>
      <li><a href=#reusing_an_activity>Re-using an Activity</a></li>
      <li><a href=#replacing_an_activity>Replacing an Activity</a></li>
      <li><a href=#multitasking>Multitasking</a></li>
      <li><a href=#launching_from_two_entry_points>Launching from Two Entry Points</a></li>
      <li><a href=#intents>Intents</a></li>
      <li><a href=#switching_between_tasks>Switching Between Tasks</a></li>
    </ol>
  </li>
  <li><a href=#tips>Design Tips
    <ol>
      <li><a href=#activity_not_reused_tip>Don't specify intent filters in an activity that won't be re-used</a></li>
      <!-- <li><a href=#others_to_reuse_tip>Don't define your own URI schemes</a></li> -->
      <li><a href=#reusing_tip>Handle case where no activity matches</a></li>
      <li><a href=#activity_launching_tip>Consider how to launch your activities</a></li>
      <li><a href=#activities_added_to_task_tip>Allow activities to add to current task</a></li>
      <li><a href=#notifications_get_back_tip>Notifications should let user easily get back</li>
      <li><a href=#use_notification_tip>Use the notification system</a></li>
      <li><a href=#taking_over_back_key>Don't take over BACK key unless you absolutely need to</a></li>
    </ol>
  </li>
</ol>

<h2>See also</h2>

<ol>
  <li><a href="{@docRoot}guide/topics/fundamentals.html">Application Fundamentals</a></li>
</ol>

</div>
</div>

<p>
  This document describes core principles of the Android application
  framework, from a high-level, user-centric perspective useful to
  interaction and application designers as well as application
  developers.
</p>

<p>
  It illustrates activities and tasks with examples, and describes some
  of their underlying principles and mechanisms, such as navigation,
  multitasking, activity re-use, intents, and the activity stack. 
  The document also highlights design decisions that are available to you
  and what control they give you over the UI of your application.
</p>

<p>
  This document draws examples from several Android applications,
  including default applications (such as Dialer) and Google
  applications (such as Maps).  You can try out the examples yourself in
  the Android emulator or on an Android-powered device. If you are using
  a device, note that your device may not offer all of the example
  applications used in this document.
</p>

<p>
  Be sure to look at the <a href="#design_tips">Design Tips</a> section
  for guidelines, tips, and things to avoid. This document is a
  complement to <a href={@docRoot}guide/topics/fundamentals.html
  title="Application Fundamentals">Application Fundamentals</a>, 
  which covers the underlying mechanics for programmers.
</p>

<h2 id="applications_activities">Applications, Activities, Activity Stack and Tasks</h2>

<p>
  Four fundamental concepts in the Android system that are helpful for you to understand are:
</p>

<ul>
  <li>Applications
  <li>Activities
  <li>Activity Stack
  <li>Tasks
</ul>

<h3 id=applications>Applications</h3>

<p>
  An Android <em>application</em> typically consists of one or more
  related, loosely bound activities <!--(and possibly 
  <a href=#services_broadcast_receivers title="other components">other
  components</a>)--> for the user to interact with, typically bundled up
  in a single file (with an .apk suffix). Android ships with a rich set
  of applications that may include email, calendar, browser, maps, text
  messaging, contacts, camera, dialer, music player, settings and
  others.
</p>

<p>
  Android has an application launcher available at the Home screen,
  typically in a sliding drawer which displays applications as icons,
  which the user can pick to start an application.
</p>


<h3 id=activities>Activities</h3>

<p>
  <em>Activities</em> are the main building blocks of Android
  applications. When you create an application, you can assemble it from
  activities that you create and from activities you re-use from other
  applications. These activities are bound at runtime, so that newly
  installed applications can take advantage of already installed
  activities.  Once assembled, activities work together to form a
  cohesive user interface. An activity has a distinct visual user
  interface designed around a single, well-bounded purpose, such as
  viewing, editing, dialing the phone, taking a photo, searching,
  sending data, starting a voice command, or performing some other type
  of user action. Any application that presents anything on the display
  must have at least one activity responsible for that display.
</p>

<p>
  When using an Android device, as the user moves through the user
  interface they start activities one after the other, totally oblivious
  to the underlying behavior &mdash; to them the experience should be
  seamless, activity after activity, <a href="#tasks">task</a> after
  task.
</p>
  
<p>
  An activity handles a particular type of content (data) and accepts a
  set of related user actions. In general, each activity has a 
  <a href={@docRoot}guide/topics/fundamentals.html#actlife
  title=lifecycle>lifecycle</a> that is independent of the other
  activities in its application or task &mdash; each activity is
  launched (started) independently, and the user or system can start,
  run, pause, resume, stop and restart it as needed. Because of this
  independence, activities can be re-used and replaced by other
  activities in a variety of ways.
</p>

<p>
  The Dialer application is an example of an application that consists
  basically of four activities: dialer, contacts list, view contact, and
  new contact, as shown in the following screenshots:
</p>

      <table style="border: none;">
        <tbody>
        <tr>
          <td style="border: none !important;">
            <img src={@docRoot}images/activity_task_design/ContactsDialer.png>
            <div style=TEXT-ALIGN:center>
              Dialer
            </div>
          </td>
          <td style="border: none !important;">
            <img src={@docRoot}images/activity_task_design/ContactsList.png>
            <div style=TEXT-ALIGN:center>
              Contacts
            </div>
          </td>
          <td style="border: none !important;">
            <img src={@docRoot}images/activity_task_design/ContactView.png>
            <div style=TEXT-ALIGN:center>
              View Contact
            </div>
          </td>
          <td style="border: none !important;">
            <img src={@docRoot}images/activity_task_design/ContactNew.png>
            <div style=TEXT-ALIGN:center>
               New Contact
            </div>
          </td>
        </tr>
        </tbody>
      </table>

<p>
  Here are other examples of applications and the activities they might contain:
</p>

    <ul>
      <li>
        Email - activities to view folders, view list of messages,
        view a message, compose a message, and set up an account
      </li>
      <li>
        Calendar - activities to view day, view week, view month, view
        agenda, edit an event, edit preferences, and view an alert
      </li>
      <li>
        Camera - activities for running the camera, viewing the list
        of pictures, viewing a picture, cropping a picture, running
        the camcorder, viewing the list of movies, and viewing a movie
      </li>
      <li>
        Game - one activity to play the game, typically another for setup
      </li>
      <li>
        Maps - one activity to view a location on a map, a second for lists
        (such as turn list or friend list), and a third for details
        (friend location, status, photo)
      </li>
    </ul>


<p>
  An activity is the most prominent of four <em>components</em> of an
  application. The other components are service, content provider and
  broadcast receiver. For more details on activities, see Activity in
  <a href={@docRoot}guide/topics/fundamentals.html#appcomp
  title="Application Components">Application Components</a>.
</p>


<h3 id="activity_stack">Activity Stack</h3>

<p>
  As the user moves from activity to activity, across applications, the
  Android system keeps a linear navigation history of activities the
  user has visited. This is the <em>activity stack</em>, also known as the
  back stack. In general, when a user starts a new activity, it is added
  to the activity stack, so that pressing BACK displays the previous
  activity on the stack. However, the user cannot use the BACK key to go
  back further than the last visit to Home. The adding of an activity to
  the current stack happens whether or not that activity begins a new 
  <a href=#tasks title=task>task</a> (as long as that task was started
  without going Home), so going back can let the user go back to
  activities in previous tasks. The user can get to tasks earlier than
  the most recent Home by selecting its root activity from the
  application launcher, a shortcut, or the "Recent tasks" screen.
</p>

<p>
  Activities are the only things that can be added to the activity stack
  &mdash; views, windows, menus, and dialogs cannot. That is, when
  designing the navigation, if you have screen A and you want the user
  to be able go to a subsequent screen B and then use the BACK key to go
  back to screen A, then the screen A needs to be implemented as an
  activity. The one exception to this rule is if your application 
  <a href=#taking_over_back_key title="takes control of the BACK key"
  takes control of the BACK key</a> and manages the navigation itself.
</p>



<h3 id=tasks>Tasks</h3>

<p>
  A <em>task</em> is the sequence of activities the user follows to
  accomplish an objective, regardless of which applications the
  activities belong to. Until a new task is explicitly specified (see
  "Interrupting the Task"), all activities the user starts are
  considered to be part of the current task. It's notable that these
  activities can be in any application &mdash; that is, all in the same
  application or in different ones. That is, a task that starts out in
  contacts can continue, by choosing an email address, to an email
  activity and then, by attaching a file, to a picture gallery to pick
  from. Contacts, email and picture gallery are all separate
  applications.
</p>

<p>
  The activity that starts a task is called the <em>root activity</em>.
  It is often, but not necessarily, started from the application
  launcher, Home screen shortcut or "Recent tasks" switcher (a long
  press on Home on some devices). The user can return to a task by
  choosing the icon for its root activity the same way they started the
  task. Once inside a task, the BACK key goes to previous activities in
  that task. The activity stack is made up of one or more tasks.
</p>

<p>
  Here are some examples of tasks:
</p>

    <ul>
      <li>
        Send a text message with an attachment
      </li>
      <li>
        View a YouTube video and share it by email with someone else 
      </li>
    </ul>

<p>
  <b>Interrupting the Task</b> - An important property of a task is that
  the user can interrupt what they're doing (their task) to perform a
  different task, then are able to return to where they left off to
  complete the original task. The idea is that users can run multiple
  tasks simultaneously and switch between them. There are two primary
  ways to jump off to that other task &mdash; in both cases the user
  should be able to return to where they were before the interruption:
</p>


  <ul>
    <li>
      User is interrupted by a notification – a notification appears and the user wants to act on it
    </li>
    <li>
      User deciding to perform another task – user just presses Home and starts an application
    </li>
  </ul>

<p>
  Of course, there are exceptions to the rules. Beyond the two ways just
  mentioned, there is a third way to start a task, and that is by
  starting an activity that defines itself as a new task. Maps and
  Browser are two applications that do this. For example, choosing an
  address in an email starts the Maps activity as a new task, and
  choosing a link in an email starts the Browser activity as a new
  task. In these cases, the BACK key will return to the previous
  activity in a different task (Email), because it was not started from
  Home.
</p>


<h2 id="tour">A Tour of Activities and Tasks</h2>

<p>
  The following examples illustrate basic principles for applications,
  activities, the activity stack, the BACK key, tasks and intents.  It
  shows how the system responds to user actions such as starting
  activities and switching between tasks.  With most of these examples
  you can follow along, launching activities on your device as
  indicated.
</p>


<h3 id=starting_an_activity_from_home>Starting an Activity from Home</h3>

<p>
  Home is the starting place for most applications. (Some applications
  can be launched only from other applications.) When the user touches
  an icon in the application launcher (or a shortcut on the Home
  screen), the main activity for that application is launched into the
  foreground where it has user focus. As shown in the following figure,
  the user action of going Home and touching the Email icon launches the
  List Messages activity of the Email application. The Home activity
  remains stopped in the background, ready to restart when called on by
  the user.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/HomeTaskBasics1a.png>
</p>

<h3 id=navigating_away_from_an_activity>Navigating Away from an Activity with BACK and HOME keys</h3>

<p>
  An activity can keep or lose its state depending on how the user
  leaves the activity &mdash; by the HOME or BACK key.
</p>

<p>
  By default, pressing the BACK key finishes (destroys) the current
  activity and displays the previous activity to the user. In the
  following figure, the user starts email by touching the Email icon in
  the Home screen, which displays a list of email messages. The user
  scrolls down the list (changing its initial state). Pressing BACK
  destroys the List Messages activity and returns to the previous
  activity, which is Home. If the user re-launches Email, it would
  re-load the messages and display its initial, non-scrolled state.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/HomeTaskBasics1b.png>
</p>

<p>
  In the above example, pressing BACK goes to Home because it was the
  last activity the user was viewing. But if the user had gotten to List
  Message from some other activity, then pressing BACK would have
  returned there.
</p>

<p>
  By contrast, the next figure shows the user leaving List Messages by
  pressing HOME instead of BACK &mdash; the List Messages activity is
  stopped and moved to the background rather than being
  destroyed. Starting Email again from its icon would simply bring the
  List Messages activity to the foreground (changing it from stopped to
  running) in the same scrolled state the user last left it.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/HomeTaskBasics1c.png>
</p>

<p>
  <b>Exceptions.</b> Some background activities return to their initial
  screen (they lose any state, such as scrolling) when they are brought
  to the foreground. This is true for Contacts and Gallery. If the user
  chooses Home &gt; Contacts then chooses a contact, they are viewing
  the details of a contact. If they start again by choosing Home &gt;
  Contacts, they are presented with the initial list of contacts rather
  than the contact they were last viewing. Contacts is designed this way
  because this initial screen is the main entry point for the
  application with four tabs for accessing the full range of features.
</p>

<p>
  In addition, not all activities have the behavior that they are
  destroyed when BACK is pressed. When the user starts playing music in
  the Music application and then presses BACK, the application overrides
  the normal back behavior, preventing the player activity from being
  destroyed, and continues playing music, even though its activity is no
  longer visible &mdash; as a visual substitute, the Music application
  places a notification in the status bar so the user still has an easy
  way to get to the application to stop or control the music. Note that
  you can write an activity to stop when its screen is no longer
  visible, or to continue running in the background &mdash; the latter
  was chosen for the music player.
</p>


<h3 id=reusing_an_activity>Re-using an Activity</h3>

<p>
  When activity A starts activity B in a different application, activity
  B is said to be <em>re-used</em>. This use case normally takes place
  because activity A is lacking a capability and can find it in activity B.
</p>

<p>
  <b>Contacts Re-Uses Gallery to Get a Picture</b> - The Contacts
  activity has a field for a picture of a contact, but the Gallery is
  normally where pictures are kept. So Contacts can re-use the Gallery
  activity to get a picture. This is a good example of re-use of the
  Gallery activity. The following figure illustrates the sequence of
  activities to do this (up to crop). This is how it's done: The user
  chooses Contacts, selects the contact for viewing, chooses MENU &gt;
  Edit contact and touches the picture field, which launches the Gallery
  activity. The user then chooses the picture they want, crops and saves
  it. Saving it causes the picture to be inserted into the picture field
  in the contact.
</p>

<p>
  Notice the Gallery returns a picture to the Contacts application that
  started it. The next example illustrates re-use of an activity that
  does not return a result. Also notice that the following figure is
  illustrates the navigation history through the activities, or the
  activity stack &mdash; the user can back up through each activity all
  the way to Home.
</p>

<p>
  When designing an application, it's good to think about how it can
  re-use activities in other applications, and how your activities might
  be re-used by other applications. If you add an activity with the same
  <a href=#intents title="intent filter">intent filter</a> as an
  existing activity, then the system presents the user with a choice
  between the activities.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/ReusingAnActivity1.png>
</p>

<p>
  <b>Gallery Re-Uses Messaging for Sharing a Picture</b> - Sharing is
  another good example of one application re-using an activity from a
  different application. As shown in the following figure, the user
  starts Gallery, picks a picture to view, chooses MENU &gt; Share, and
  picks "Messaging". This starts the Messaging activity, creates a new
  message and attaches the original picture to it. The user then fills
  in the "To" field, writes a short message and sends it. User focus
  remains in the Messaging program. If the user wants to go back to the
  Gallery, they must press the BACK key. (The user can back up through
  each activity all the way to Home.)
</p>

<p>
  In contrast to the previous example, this re-use of the Messaging
  activity does not return anything to the Gallery activity that started it.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/ReusingAnActivity2.png>
</p>

<p>
  Both of these examples illustrate tasks &mdash; a sequence of
  activities that accomplish an objective. Each case uses activities
  from two different applications to get the job done.
</p>


<h3 id=replacing_an_activity>Replacing an Activity</h3>

<p>
  This is the use case where activity A replaces activity B in a
  different application. This situation normally happens because
  activity A is better at doing the job than activity B. In other words,
  A and B are equivalent enough that A can replace B. This case stands
  in contrast with re-using an activity, where A and B are quite
  different activities and supplement each other.
</p>

<p>
  In this example, the user has downloaded a replacement for the Phone
  Ringtone activity, called Rings Extended. Now when they go to
  Settings, Sound &amp; Display, Phone Ringtone, the system presents
  them with a choice between the Android System's ringtone activity and
  the new one. This dialog box has an option to remember their choice
  "Use by default for this action". When they choose "Rings Extended",
  that activity loads, replacing the original Android ringtone activity.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/ReplacingAnActivity.png>
</p>

<h3 id=multitasking>Multitasking</h3>

<p>
  As previously noted, when an activity has been launched, the user can
  go to Home and launch a second activity without destroying the first
  activity. This scenario demonstrates launching the Maps application.
</p>

  <ul>
    <li>
      State 1 - The user launches the View Map activity and searches
      for a map location. Let's say the network is slow, so the map is
      taking an unusually long taking time to draw.
    </li>
  </ul>
  <ul>
    <li>
      State 2 - The user wants to do something else while they're
      waiting, so they press HOME, which does not interrupt the map's
      network connection and allows the map to continue loading in the
      background.

      <p>
      Note that when you write an activity, you can make it stop or
      continue running when it is moved to the background (see
      onStop() in <a href={@docRoot}guide/topics/fundamentals.html#actlife
      title="Activity Lifecycle">Activity Lifecycle</a>).
      For activities that download data from the network, it's recommended
      to let them continue downloading so the user can multi-task.
      </p>

    </li>

    <li>
      State 3 - The map activity is now running in the background,
      with Home in the foreground. The user then launches the Calendar
      activity, which launches into the foreground, taking user focus,
      where they view today's calendar (as indicated by the heavy
      outline).
    </li>
  </ul>

<p>
<img src={@docRoot}images/activity_task_design/HomeTaskBasics1d.png>
</p>
      <ul>
        <li>
          State 4 - The user presses Home, then Maps to return to the map, which by now has fully loaded.
        </li>
      </ul>

<p>
  <img src={@docRoot}images/activity_task_design/HomeTaskBasics1e.png>
</p>

<p>
  The application launcher at Home has launched "View Map" and "Day
  View" activities into separate <em>tasks</em>, hence the system is
  multitasking &mdash; running multiple <a href=#tasks
  title=tasks>tasks</a>.
</p>


<h3 id=launching_from_two_entry_points>Launching from Two Entry Points</h3>

<p>
  Every application must have at least one entry point &mdash; a way
  for the user or system to access activities inside the
  application. Each icon in the application launcher at home
  represents an entry point. Applications can also from another
  application. Each activity is a potential entry point into the
  application.&nbsp;
</p>

<p>
  The phone application has two entry points: Contacts and Dialer. A
  user entering from Contacts can choose a phone number to launch the
  Dialer. As shown in the following figure, a user could choose the
  Contacts icon to launch the Contacts activity, then pick a phone
  number to launch the Dialer activity and dial the phone.
</p>

<p>
  Once the user is inside the application, they can access other
  activities, such as New Contact and Edit Contact, through tabs, menu
  items, list items, onscreen buttons, or other user interface
  controls.
</p>

<p>
<img src={@docRoot}images/activity_task_design/PhoneActivitiesDiagram.png>
</p>

<h3 id=intents>Intents</h3>

<p>
  When the user takes an action on some data, such as touching a
  mailto:info@example.com link, they are actually initiating an Intent
  object, or just an <em>intent</em>,  which then gets resolved to a
  particular component (we consider only activity components here).
  So, the result of a user touching a mailto: link is an Intent object 
  that the system tries to match to an activity. If that Intent object was
  written explicitly naming an activity (an <em>explicit intent</em>),
  then the system immediately launches that activity in response to the user
  action. However, if that Intent object was written without naming an
  activity (an <em>implicit intent</em>), the system compares the Intent
  object to the <em>intent filters</em> of available activities. If more
  than one activity can handle the action and data, the system
  displays an activity chooser for the user to choose from.
</p>

<p>
  This example of touching the mailto: link is shown in the following
  figure. If the device has two email applications set up, when a user
  touches a mailto: email address on a web page, the result is an
  Intent object which displays a dialog box with a choice between the
  two activities to compose an email (Gmail and Email).
</p>

<p>
  <img src={@docRoot}images/activity_task_design/IntentsDiagram.png>
</p>

<p>
  Here are some examples of Intent objects and the activities they resolve to:
</p>

  <ul>
    <li>
      View the list of contacts - resolves to a contact list viewer activity
    </li>

    <li>
      View a particular contact - resolves to a contact viewer activity
    </li>

    <li>
      Edit a particular contact - resolves to a contact editor activity
    </li>

    <li>
      Send to a particular email - resolves to an email activity
    </li>

    <li>
      Dial a phone number - resolves to a phone dialer activity
    </li>

    <li>
       View the list of images - resolves to an image list viewer activity
    </li>

    <li>
        View a particular image - resolves to an image viewer activity
    </li>

    <li>
        Crop a particular image - resolves to an image cropper activity
    </li>
  </ul>

<p>
  Notice that an Intent object specifies two things, an action and data:
</p>

    <ul>
      <li>
        A generic action to be performed. In these examples: view, edit, dial or crop
      </li>

      <li>
        The specific data to be acted on. In these examples: the list of contacts, a particular contact, a phone number, the list of images, or a particular image
      </li>
    </ul>

    <p>
      Note that any user action to start an activity from the
      application launcher at Home is an explicit intent to a specific
      activity. Likewise, some activities launch private activities
      within their application as explicit intents so no other activity
      can access them.
    </p>

    <p>
      For more on intents, see {@link android.content.Intent Intent class} and
      <a href={@docRoot}guide/topics/fundamentals.html#ifilters
      title="intent filters">intent filters</a>.
    </p>


<h3 id=switching_between_tasks>Switching Between Tasks</h3>

<p>
  This scenario shows how the user can switch between two tasks. In
  this example, the user writes a text message, attaches a picture,
  but before they are done they glance at their calendar. They then
  return to where they left off, attaching the picture and sending the
  message.
</p>

    <ol>
      <li>
        <b>Start first task.</b> You want to send a text message and attach a photo. You would choose:

        <p>
          Home &gt; Messaging &gt; New message &gt; MENU &gt; Attach
          &gt; Pictures. This last step launches the picture gallery
          for picking a photo. Notice that picture gallery is an
          activity in a separate application.
        </p>


        <table>
          <tbody>
            <tr>
            <td valign=top style="border: none !important;">
              <img src={@docRoot}images/activity_task_design/HomeTaskSwitching1a.png>
            </td>
            <td valign=top style="border: none !important;">
              <img src={@docRoot}images/activity_task_design/HomeTaskSwitching1b.png>
            </td>
            <td valign=top style="border: none !important;">
              <img src={@docRoot}images/activity_task_design/HomeTaskSwitching1c.png>
            </td>
            </tr>
          </tbody>
        </table>

        <p>
          At this point, before you have picked a picture, you decide
          to stop and glance at your calendar, which is a separate
          task. Because the current activity has no button to go
          directly to the Calendar, you need to start from Home.
        </p>

      </li>
      <li>
        <b>Start second task.</b> You choose Home &gt; Calendar to
        look at a calendar event. Calendar launches from Home as a new
        task because the application launcher creates a new task for
        each application it launches.

        <p>
      <img src={@docRoot}images/activity_task_design/HomeTaskSwitching2.png>
        </p>
      </li>

      <li>
        <b>Switch to first task and complete it.</b> When done looking
        at the Calendar, you can return to attaching the picture by
        starting the root activity again for that task: choose Home
        &gt; Messaging, which takes you not to Messaging, but directly
        to the Picture gallery, where you left off. You can then pick
        a photo, which is added to the message, you send the message
        and you're done with the first task.

      <table>
        <tbody>
        <tr>

          <td valign=top style="border: none !important;">
            <img src={@docRoot}images/activity_task_design/HomeTaskSwitching3a.png>
          </td>

          <td valign=top style="border: none !important;">
            <img src={@docRoot}images/activity_task_design/HomeTaskSwitching3b.png>
          </td>

          <td valign=top style="border: none !important;">
            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<img src={@docRoot}images/activity_task_design/HomeTaskSwitching3c.png>
          </td>

        </tr>
        </tbody>
      </table>
      </li>
    </ol>


<h2 id="tips">Design Tips</h2>

<p>
  The following are tips and guidelines for application designers and developers.
</p>

<h3 id=activity_not_reused_tip>When writing an activity that won't be re-used, don't specify intent filters &mdash; use explicit intents</h3>

<p>
  If you're writing an activity that you don't want other activities
  to use, be sure not to add any intent filters to that activity. This
  applies to an activity that will be launched only from the
  application launcher or from other activities inside your
  application. Instead, just create intents specifying the explicit
  component to launch &mdash; that is, explicit intents. In this case,
  there's just no need for intent filters. Intent filters are
  published to all other applications, so if you make an intent
  filter, what you're doing is publishing access to your activity,
  which means you can cause unintentional security holes.
</p>

<!--
<h3 id="others_to_reuse_tip">When writing an activity for others to re-use, don't define your own URI schemes</h3>

<p>
  If publishing to others, don't define your own URI schemes in an
  Intent type. Schemes (such as http: and mailto:) are an Internet
  standard with a universal namespace outside of just Android, so you
  aren't allowed to just make up your own. Instead, you should just
  define your own actions. The action namespace is designed to not
  have conflicts. Typically, your activity has one scheme with many
  different actions.
</p>

<p>
  Example: You want to show the user a bar code for some text. The
  wrong way to do this is for the intent filter protocol to be
  &lt;action android:name="android.intent.action.VIEW" /&gt; and
  &lt;data android:scheme="barcode" /&gt;. Do not do this.
</p>

<p>
  Instead you should define &lt;action
  android:name="com.example.action.SHOW_BARCODE" /&gt; and have the
  invoker supply the data as an extra field in the Intent object.
</p>

<p>
  Be aware this intent filter protocol
  ("com.example.action.SHOW_BARCODE", in this example) is a public API
  that you can't change once it's defined. You must support it in the
  future because others are going to be relying on it. If you want to
  add new features that are incompatible with the current protocol,
  just define a new protocol and continue to support the old one.
</p>
-->

<h3 id="reusing_tip"> When reusing an activity owned by others, handle the case where no activity matches</h3>

<p>
  Your applications can re-use activities made available from other
  applications. In doing so, you cannot presume your intent will always
  be resolved to a matching external activity &mdash; you must handle the case
  where no application installed on the device can handle the intent.
</p>

<p>
  You can either test that an activity matches the intent, which you can do
  before starting the activity, or catch an exception if starting the
  activity fails. Both approaches are described in the blog posting
  <a href="http://android-developers.blogspot.com/2009/01/can-i-use-this-intent.html">Can
  I use this Intent?</a>.
</p>

<p>
  To test whether an intent can be resolved, your code can query the package manager.
  The blog post provides an example in the isIntentAvailable() helper method.
  You can perform this test when initializing the user interface.
  For instance, you could disable the user control that initiates
  the Intent object, or display a message to the user that lets them go
  to a location, such as the Market, to download its application.
  In this way, your code can start the activity (using either startActivity() 
  or startActivityForResult()) only if the intent has tested to resolve
  to an activity that is actually present.
</p>

<h3 id=activity_launching_tip>Consider how you want your activities to be launched or used by other applications</h3>

<p>
  As a designer or developer, it's up to you to determine how users
  start your application and the activities in it. As an application
  is a set of activities, the user can start these activities from
  Home or from another application.
</p>

    <ul>
      <li>
        <b>Launch your main activity from an icon at Home </b>- If
        your application can run standalone, it should probably be
        started by the user touching an icon in <em>application
        launcher</em> (typically implemented as a sliding drawer on the
        Home screen), or from a shortcut icon on the Home screen, or
        from the task switcher.  (The mechanism for this is for the
        activity to have an 
        <a href={@docRoot}guide/topics/fundamentals.html#ifilters
        title="Intent filter">intent filter</a> with action MAIN and
        category LAUNCHER.)
      </li>
    </ul>

    <ul>
      <li>
        <b>Launch your activity from within another application</b> -
        Perhaps your activities are meant for re-use. For example,
        many applications have data they want to share with other
        users. Activities that can share data with other users include
        email, text messaging and uploading to a public website.  <p>
        If one or more of your activities can be an alternative to an
        existing activity in another application, you can make it
        available to users at the point they request that
        activity.&nbsp;For example, if your activity can send data to
        others (such as by email, text messaging, or uploading),
        consider setting up that activity to appear as a choice to the
        user. To give a specific example, Gallery enables a user to
        view and share pictures. When the user chooses "Share" from
        the menus, the system compares the "Share" request (an Intent
        object) to available activities (by looking at their intent
        filters) and displays choices to share. In this case, it
        matches Email, Gmail, Messaging and Picasa. If your activity
        can send a picture or upload it to a website, all it needs to
        do is make itself available for sharing (by setting its intent
        filter).
        </p>
<p>
  Another activity can start your activity either with or without expecting a result back.&nbsp;
</p>
      </li>

      <ul>
        <li>
          <b>Start an activity expecting a result</b> - This approach
          is closed loop, where the activity being started must either
          return a valid result or be canceled. In the previous
          examples of sharing a photo from a Gallery, the user ends up
          back in the Gallery after completing the send or upload
          procedure. These are examples of starting an activity
          external to the Gallery. (Such an activity is started with
          {@link
          android.app.Activity#startActivityForResult(android.content.Intent,
          int) startActivityForResult()}.)
        </li>

        <li>
          <b>Start an activity not expecting a result</b> - This
          approach is open-ended. An example is choosing an house
          address in an email message (or web page), where the Maps
          activity is started to map the location. No result from maps
          is expected to be returned to the email message; the user
          can return by pressing the BACK key. (Such an activity is
          started with {@link
          android.content.Context#startActivity(android.content.Intent)
          startActivity()}.)
        </li>
      </ul>

      <li>
        <b>Launch your activity <em>only</em> from within another
        application</b> - The previous cases of sharing by way of
        Email, Gmail, Messaging and Picasa (from within Gallery) are
        all activities that can also be started from icons in the
        application launcher at Home. In contrast, the activities for
        cropping a picture and attaching a file cannot be started from
        Home, because they do not stand alone and require a
        context.&nbsp;
      </li>

<p>
  In fact, not all applications have icons and can be started from
  Home. Take for example a small app that is infrequently used and
  replaces existing functionality, that already has a natural entry
  point inside an existing application. For example, an Android phone
  typically has a built-in ringtone picker that can be selected from
  the sound settings of the Settings application. A custom ringtone
  picker application that you write could be launched by an intent
  identical to the built-in ringtone picker. At the point where the
  user chooses "Phone ringtone", they are presented with a dialog
  letting them choose between "Android System" and your ringtone
  picker (and letting them save their choice) as shown in the
  following figure. A ringtone is something you set infrequently, and
  already has a well-defined starting point, so probably does not need
  an application icon at Home.
</p>

<p>
  <img src={@docRoot}images/activity_task_design/ActivityChooser.png>
</p>

      <li>
          <b>Launch two or more main activities within a single
          application from separate icon at Home</b> - As we have
          defined it, all the code in a single .apk file is considered
          to be one <em>application.</em> You can write an application
          that contains two main activities launchable from Home.
      </li>

<p>
  The Camera.apk application is a good example of an application that
  contains two independent main activities &mdash; Camera and
  Camcorder &mdash; that each have their own icons in application
  launcher, that can be launched separately, and so appear to the user
  as separate applications. They both share use of the same lens, and
  both store their images (still and moving) in the Gallery.&nbsp;
</p>

<p>
  In order for your application to contain two different, independent
  activities launchable from Home, you must define them to be
  associated with different tasks. (This means setting the main
  activity for each task to a different <!--a href=#affinities
  title=affinity-->task affinity<!--/a--> &mdash; in this case,
  "com.android.camera" and "com.android.videocamera".)
</p>

<p>
  Contacts and Dialer are another example of two main activities
  launchable from Home that reside in the same application.
</p>

      <li>
        <b>Making your application available as a widget</b> - An
        application can also display a portion of itself as an <a
        href={@docRoot}guide/topics/appwidgets/index.html title="app
        widget">app widget</a>, embedded in Home or another
        application, and receive periodic updates.
      </li>

    </ul>


<h3 id=activities_added_to_task_tip>Allow your activities to be added to the current task</h3>

<p>
  If your activities can be started from another application, allow
  them to be added to the current <a href=#tasks title=Tasks>task</a>
  (or an existing task it has an affinity with). Having activities
  added to a task enables the user to switch between a task that
  contains your activities and other tasks. <!--See <a href=#tasks
  title=Tasks>Tasks</a> for a fuller explanation.--> Exceptions are
  your activities that have only one instance.&nbsp;
</p>

<p>
  For this behavior, your activity should have a <!--a
  href=launch_modes title="standard or singleTop"-->launch
  mode<!--/a--> of standard or singleTop rather than singleTask or
  singleInstance. These modes also enable multiple instances of your
  activity to be run.
</p>

    
<h3 id="notifications_get_back_tip">Notifications should let the user easily get back to the previous activity</h3>
<p>
  Applications that are in the background or not running can have
  services that send out notifications to the user letting them know about
  events of interest. Two examples are Calendar, which can send out notifications of
  upcoming events, and Email, which can send out notifications when new
  messages arrive. One of the user interface guidelines is that when the
  user is in activity A, gets a notification for activity B and
  picks that notification, when they press the BACK key, they should
  go back to activity A.&nbsp;
</p>

<p>
  The following scenario shows how the activity stack should work
  when the user responds to a notification.
</p>

<ol>
  <li>
    User is creating a new event in Calendar. They realize they
    need to copy part of an email message into this event
  </li>
  <li>
    The user chooses Home &gt; Gmail
  </li>
  <li>
    While in Gmail, they receive a notification from Calendar for an upcoming meeting
  </li>
  <li>
    So they choose that notification, which takes them to a
    dedicated Calendar activity that displays brief details of the
    upcoming meeting
  </li>
  <li>
    The user chooses this short notice to view further details
  </li>
  <li>
    When done viewing the event, the user presses the BACK
    key. They should be taken to Gmail, which is where they were
    when they took the notification
  </li>
</ol>

<p>
This behavior doesn't necessarily happen by default.
</p>

<p>
Notifications generally happen primarily in one of two ways:
</p>

  <ul>
    <li>
      <b>The chosen activity is dedicated for notification only</b> -
      For example, when the user receives a
      Calendar notification, choosing that
      notification starts a special activity that displays a list
      of upcoming calendar events &mdash; this view is available only
      from the notification, not through the Calendar's own user
      interface. After viewing this upcoming event, to ensure that
      the user pressing the BACK key will return to the activity
      the user was in when they picked the notification, you would
      make sure this dedicated activity does not have the same
      task affinity as the Calendar or any other activity. (You do
      this by setting task affinity to the empty string, which
      means it has no affinity to anything.) The explanation for
      this follows.

      <p>
      Because of the way tasks work, if the taskAffinity of the
      dedicated activity is kept as its default, then pressing the
      BACK key (in step 6, above) would go to Calendar, rather
      than Gmail. The reason is that, by default, all activities
      in a given application have the same task
      affinity. Therefore, the task affinity of the dedicated
      activity matches the Calendar task, which is already running
      in step 1. This means in step 4, choosing the notification
      brings the existing Calendar event (in step 1) forward and
      starts the dedicated activity on top of it.  This is not
      what you want to have happen. Setting the dedicated
      activity's taskAffinity to empty string fixes this.
      </p>
    </li>

    <li>
      <b>The chosen activity is not dedicated, but always comes to
      the foreground in its initial state</b> - For example, in
      response to a notification, when the Gmail application comes
      to the foreground, it always presents the list of conversations.
      You can ensure this happens by setting a "clear top" flag in the
      intent that the notification triggers.  This ensures that when the
      activity is launched, it displays its initial activity, preventing
      Gmail from coming to the foreground in whatever state the user last
      happened to be viewing it. (To do this, you put {@link
      android.content.Intent#FLAG_ACTIVITY_CLEAR_TOP
      FLAG_ACTIVITY_CLEAR_TOP} in the intent you pass to startActivity()). 
    </li>
  </ul>

<p>
  There are other ways to handle notifications, such as bringing the
  activity to the foreground, set to display specific data, such as
  displaying the text message thread for the person who just sent a
  new text message.
</p>

<p>
  A notification always starts an activity as a new task (that is, it
  puts <font size=1>FLAG_ACTIVITY_NEW_TASK</font> in the intent it
  passes to startActivity()). This is done because interruptions to a
  task should not become part of that task.
</p>

<h3 id=use_notification_tip>Use the notification system &mdash; don't use dialog boxes in place of notifications</h3>

<p>
  If your background service needs to notify a user, use the standard
  notification system &mdash; don't use a dialog or toast to notify
  them. A dialog or toast would immediately take focus and interrupt
  the user, taking focus away from what they were doing: the user
  could be in the middle of typing text the moment the dialog appears
  and could accidentally act on the dialog. Users are used to dealing
  with notifications and can pull down the notification shade at their
  convenience to respond to your message.
</p>

<h3 id=taking_over_back_key>Don't take over the BACK key unless you absolutely need to</h3>

<p>
  As a user navigates from one activity to the next, the system adds
  them to the activity stack. This forms a navigation history that is
  accessible with the BACK key. Most activities are relatively limited
  in scope, with just one set of data, such as viewing a list of
  contacts, composing an email, or taking a photo. But what if your
  application is one big activity with several pages of content and
  needs finer-grained control of the BACK key? Examples of such Google
  applications are the Browser, which can have several web pages open
  at once, and Maps, which can have several layers of geographic data
  to switch between. Both of these applications take control of the
  BACK key and maintain their own internal back stacks that operate
  only when these applications have focus.
</p>

<p>
  For example, Maps uses <em>layers</em> to present different
  information on a map to the user: displaying the location of a
  search result, displaying locations of friends, and displaying a
  line for a street path providing direction between points. Maps
  stores these layers in its own history so the BACK key can return to
  a previous layer.
</p>

<p>
  Similarly, Browser uses <em>browser windows</em> to present different
  web pages to the user. Each window has its own navigation history,
  equivalent to tabs in a browser in a desktop operating system (such
  as Windows, Macintosh or Linux). For example, if you did a Google
  web search in one window of the Android Browser, clicking on a link
  in the search results displays a web page in that same window, and
  then pressing BACK would to the search results page. Pressing
  BACK goes to a previous window only if the current window was
  launched from that previous window.  If the user keeps pressing
  back, they will eventually leave the browser activity and return
  Home.
</p>