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diff --git a/docs/html/resources/articles/layout-tricks-merge.jd b/docs/html/resources/articles/layout-tricks-merge.jd new file mode 100644 index 0000000..3e165d2 --- /dev/null +++ b/docs/html/resources/articles/layout-tricks-merge.jd @@ -0,0 +1,198 @@ +page.title=Layout Tricks: Merging Layouts +@jd:body + +<p>The <a href=""></a> articles showed you how to use the <code><include /></code> tag in XML layouts, to reuse and share your layout code. This article explains the <code><merge /></code> tag and how it complements the <code><include /></code> tag.</p> + +<p>The <code><merge /></code> tag was created for the purpose of +optimizing Android layouts by reducing the number of levels in view trees. It's +easier to understand the problem this tag solves by looking at an example. The +following XML layout declares a layout that shows an image with its title on top +of it. The structure is fairly simple; a {@link android.widget.FrameLayout} is +used to stack a {@link android.widget.TextView} on top of an +{@link android.widget.ImageView}:</p> + +<pre class="prettyprint"><FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" + android:layout_width="fill_parent" + android:layout_height="fill_parent"> + + <ImageView + android:layout_width="fill_parent" + android:layout_height="fill_parent" + + android:scaleType="center" + android:src="@drawable/golden_gate" /> + + <TextView + android:layout_width="wrap_content" + android:layout_height="wrap_content" + android:layout_marginBottom="20dip" + android:layout_gravity="center_horizontal|bottom" + + android:padding="12dip" + + android:background="#AA000000" + android:textColor="#ffffffff" + + android:text="Golden Gate" /> + +</FrameLayout></pre> + +<p>This layout renders nicely and nothing seems wrong with it:</p> + +<div style="text-align: center;"><img src="images/merge1.jpg" alt="A FrameLayout is used to overlay a title on top of an image"></div> + +<p>Things get more interesting when you inspect the result with <a +href="{@docRoot}guide/developing/tools/hierarchy-viewer.html">HierarchyViewer</a>. +If you look closely at the resulting tree, you will notice that the +<code>FrameLayout</code> defined in our XML file (highlighted in blue below) is +the sole child of another <code>FrameLayout</code>:</p> + +<div style="text-align: center;"><img src="images/merge2.png" alt="A layout with only one child of same dimensions can be removed"></div> + +<p>Since our <code>FrameLayout</code> has the same dimension as its parent, by +the virtue of using the <code>fill_parent</code> constraints, and does not +define any background, extra padding or a gravity, it is <em>totally +useless</em>. We only made the UI more complex for no good reason. But how could +we get rid of this <code>FrameLayout</code>? After all, XML documents require a +root tag and tags in XML layouts always represent view instances.</p> + +<p>That's where the <code><merge /></code> tag comes in handy. When the +{@link android.view.LayoutInflater} encounters this tag, it skips it and adds +the <code><merge /></code> children to the <code><merge /></code> +parent. Confused? Let's rewrite our previous XML layout by replacing the +<code>FrameLayout</code> with <code><merge /></code>:</p> + +<pre class="prettyprint"><merge xmlns:android="http://schemas.android.com/apk/res/android"> + + <ImageView + android:layout_width="fill_parent" + android:layout_height="fill_parent" + + android:scaleType="center" + android:src="@drawable/golden_gate" /> + + <TextView + android:layout_width="wrap_content" + android:layout_height="wrap_content" + android:layout_marginBottom="20dip" + android:layout_gravity="center_horizontal|bottom" + + android:padding="12dip" + + android:background="#AA000000" + android:textColor="#ffffffff" + + android:text="Golden Gate" /> + +</merge></pre> + +<p>With this new version, both the <code>TextView</code> and the +<code>ImageView</code> will be added directly to the top-level +<code>FrameLayout</code>. The result will be visually the same but the view +hierarchy is simpler:</p> + +<div style="text-align: center;"><img src="images/merge3.png" alt="Optimized view hierarchy using the merge tag"></div> + +<p>Obviously, using <code><merge /></code> works in this case because the +parent of an activity's content view is always a <code>FrameLayout</code>. You +could not apply this trick if your layout was using a <code>LinearLayout</code> +as its root tag for instance. The <code><merge /></code> can be useful in +other situations though. For instance, it works perfectly when combined with the +<code><include /></code> tag. You can also use <code><merge +/></code> when you create a custom composite view. Let's see how we can use +this tag to create a new view called <code>OkCancelBar</code> which simply shows +two buttons with customizable labels. You can also <a +href="http://progx.org/users/Gfx/android/MergeLayout.zip">download the complete +source code of this example</a>. Here is the XML used to display this custom +view on top of an image:</p> + +<pre class="prettyprint"><merge + xmlns:android="http://schemas.android.com/apk/res/android" + xmlns:okCancelBar="http://schemas.android.com/apk/res/com.example.android.merge"> + + <ImageView + android:layout_width="fill_parent" + android:layout_height="fill_parent" + + android:scaleType="center" + android:src="@drawable/golden_gate" /> + + <com.example.android.merge.OkCancelBar + android:layout_width="fill_parent" + android:layout_height="wrap_content" + android:layout_gravity="bottom" + + android:paddingTop="8dip" + android:gravity="center_horizontal" + + android:background="#AA000000" + + okCancelBar:okLabel="Save" + okCancelBar:cancelLabel="Don't save" /> + +</merge></pre> + +<p>This new layout produces the following result on a device:</p> + +<div style="text-align: center;"><img src="images/merge4.jpg" alt="Creating a custom view with the merge tag"></div> + +<p>The source code of <code>OkCancelBar</code> is very simple because the two +buttons are defined in an external XML file, loaded using a +<code>LayoutInflate</code>. As you can see in the following snippet, the XML +layout <code>R.layout.okcancelbar</code> is inflated with the +<code>OkCancelBar</code> as the parent:</p> + +<pre class="prettyprint">public class OkCancelBar extends LinearLayout { + public OkCancelBar(Context context, AttributeSet attrs) { + super(context, attrs); + setOrientation(HORIZONTAL); + setGravity(Gravity.CENTER); + setWeightSum(1.0f); + + LayoutInflater.from(context).inflate(R.layout.okcancelbar, this, true); + + TypedArray array = context.obtainStyledAttributes(attrs, R.styleable.OkCancelBar, 0, 0); + + String text = array.getString(R.styleable.OkCancelBar_okLabel); + if (text == null) text = "Ok"; + ((Button) findViewById(R.id.okcancelbar_ok)).setText(text); + + text = array.getString(R.styleable.OkCancelBar_cancelLabel); + if (text == null) text = "Cancel"; + ((Button) findViewById(R.id.okcancelbar_cancel)).setText(text); + + array.recycle(); + } +}</pre> + +<p>The two buttons are defined in the following XML layout. As you can see, we +use the <code><merge /></code> tag to add the two buttons directly to the +<code>OkCancelBar</code>. Each button is included from the same external XML +layout file to make them easier to maintain; we simply override their id:</p> + +<pre class="prettyprint"><merge xmlns:android="http://schemas.android.com/apk/res/android"> + <include + layout="@layout/okcancelbar_button" + android:id="@+id/okcancelbar_ok" /> + + <include + layout="@layout/okcancelbar_button" + android:id="@+id/okcancelbar_cancel" /> +</merge></pre> + +<p>We have created a flexible and easy to maintain custom view that generates +an efficient view hierarchy:</p> + +<div style="text-align: center;"><img src="images/merge5.png" alt="The resulting hierarchy is simple and efficient"></div> + +<p>The <code><merge /></code> tag is extremely useful and can do wonders +in your code. However, it suffers from a couple of limitations:</p> + +<ul> +<li><code><merge /></code> can only be used as the root tag of an XML layout</li> +<li>When inflating a layout starting with a <code><merge /></code>, you <strong>must</strong> +specify a parent <code>ViewGroup</code> and you must set <code>attachToRoot</code> to +<code>true</code> (see the documentation for +{@link android.view.LayoutInflater#inflate(int, android.view.ViewGroup, boolean)} method)</li> +</ul> + |