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Diffstat (limited to 'docs/html')
| -rw-r--r-- | docs/html/guide/topics/data/data-storage.jd | 7 | ||||
| -rw-r--r-- | docs/html/training/basics/data-storage/databases.jd | 322 | ||||
| -rw-r--r-- | docs/html/training/basics/data-storage/files.jd | 382 | ||||
| -rw-r--r-- | docs/html/training/basics/data-storage/index.jd | 55 | ||||
| -rw-r--r-- | docs/html/training/basics/data-storage/shared-preferences.jd | 121 | ||||
| -rw-r--r-- | docs/html/training/training_toc.cs | 20 |
6 files changed, 906 insertions, 1 deletions
diff --git a/docs/html/guide/topics/data/data-storage.jd b/docs/html/guide/topics/data/data-storage.jd index e9d2d25..2603a06 100644 --- a/docs/html/guide/topics/data/data-storage.jd +++ b/docs/html/guide/topics/data/data-storage.jd @@ -232,7 +232,12 @@ save files. This can be a removable storage media (such as an SD card) or an int (non-removable) storage. Files saved to the external storage are world-readable and can be modified by the user when they enable USB mass storage to transfer files on a computer.</p> -<p class="caution"><strong>Caution:</strong> External files can disappear if the user mounts the +<p>It's possible that a device using a partition of the +internal storage for the external storage may also offer an SD card slot. In this case, +the SD card is <em>not</em> part of the external storage and your app cannot access it (the extra +storage is intended only for user-provided media that the system scans).</p> + +<p class="caution"><strong>Caution:</strong> External storage can become unavailable if the user mounts the external storage on a computer or removes the media, and there's no security enforced upon files you save to the external storage. All applications can read and write files placed on the external storage and the user can remove them.</p> diff --git a/docs/html/training/basics/data-storage/databases.jd b/docs/html/training/basics/data-storage/databases.jd new file mode 100644 index 0000000..3a717dd --- /dev/null +++ b/docs/html/training/basics/data-storage/databases.jd @@ -0,0 +1,322 @@ +page.title=Saving Data in SQL Databases +parent.title=Data Storage +parent.link=index.html + +trainingnavtop=true +previous.title=Saving Data in Files +previous.link=files.html + +@jd:body + + +<div id="tb-wrapper"> +<div id="tb"> + +<h2>This lesson teaches you to</h2> +<ol> + <li><a href="#DefineContract">Define a Schema and Contract</a></li> + <li><a href="#DbHelper">Create a Database Using a SQL Helper</a></li> + <li><a href="#WriteDbRow">Put Information into a Database</a></li> + <li><a href="#ReadDbRow">Read Information from a Database</a></li> + <li><a href="#DeleteDbRow">Delete Information from a Database</a></li> + <li><a href="#UpdateDbRow">Update a Database</a></li> +</ol> + +<h2>You should also read</h2> +<ul> + <li><a href="{@docRoot}guide/topics/data/data-storage.html#db">Using Databases</a></li> +</ul> + +<!-- +<h2>Try it out</h2> + +<div class="download-box"> + <a href="{@docRoot}shareables/training/Sample.zip" class="button">Download the sample</a> + <p class="filename">Sample.zip</p> +</div> +--> + +</div> +</div> + + +<p>Saving data to a database is ideal for repeating or structured data, +such as contact information. This class assumes that you are +familiar with SQL databases in general and helps you get started with +SQLite databases on Android. The APIs you'll need to use a database +on Android are available in the {@link android.database.sqlite} package.</p> + + +<h2 id="DefineContract">Define a Schema and Contract</h2> + +<p>One of the main principles of SQL databases is the schema: a formal +declaration of how the database is organized. The schema is reflected in the SQL +statements that you use to create your database. You may find it helpful to +create a companion class, known as a <em>contract</em> class, which explicitly specifies +the layout of your schema in a systematic and self-documenting way.</p> + +<p>A contract class is a container for constants that define names for URIs, +tables, and columns. The contract class allows you to use the same constants +across all the other classes in the same package. This lets you change a column +name in one place and have it propagate throughout your code.</p> + +<p>A good way to organize a contract class is to put definitions that are +global to your whole database in the root level of the class. Then create an inner +class for each table that enumerates its columns.</p> + +<p class="note"><strong>Note:</strong> By implementing the {@link +android.provider.BaseColumns} interface, your inner class can inherit a primary +key field called {@code _ID} that some Android classes such as cursor adaptors +will expect it to have. It's not required, but this can help your database +work harmoniously with the Android framework.</p> + +<p>For example, this snippet defines the table name and column names for a +single table:</p> + + +<pre> +public static abstract class FeedEntry implements BaseColumns { + public static final String TABLE_NAME = "entry"; + public static final String COLUMN_NAME_ENTRY_ID = "entryid"; + public static final String COLUMN_NAME_TITLE = "title"; + public static final String COLUMN_NAME_SUBTITLE = "subtitle"; + ... +} +</pre> + + +<p>To prevent someone from accidentally instantiating the contract class, give +it an empty constructor. </p> + +<pre> +// Prevents the FeedReaderContract class from being instantiated. +private FeedReaderContract() {} +</pre> + + + +<h2 id="DbHelper">Create a Database Using a SQL Helper</h2> + +<p>Once you have defined how your database looks, you should implement methods +that create and maintain the database and tables. Here are some typical +statements that create and delete a table:</P> + +<pre> +private static final String TEXT_TYPE = " TEXT"; +private static final String COMMA_SEP = ","; +private static final String SQL_CREATE_ENTRIES = + "CREATE TABLE " + FeedReaderContract.FeedEntry.TABLE_NAME + " (" + + FeedReaderContract.FeedEntry._ID + " INTEGER PRIMARY KEY," + + FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID + TEXT_TYPE + COMMA_SEP + + FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP + + ... // Any other options for the CREATE command + " )"; + +private static final String SQL_DELETE_ENTRIES = + "DROP TABLE IF EXISTS " + TABLE_NAME_ENTRIES; +</pre> + +<p>Just like files that you save on the device's <a +href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">internal +storage</a>, Android stores your database in private disk space that's associated +application. Your data is secure, because by default this area is not +accessible to other applications.</p> + +<p>A useful set of APIs is available in the {@link +android.database.sqlite.SQLiteOpenHelper} class. +When you use this class to obtain references to your database, the system +performs the potentially +long-running operations of creating and updating the database only when +needed and <em>not during app startup</em>. All you need to do is call +{@link android.database.sqlite.SQLiteOpenHelper#getWritableDatabase} or +{@link android.database.sqlite.SQLiteOpenHelper#getReadableDatabase}.</p> + +<p class="note"><strong>Note:</strong> Because they can be long-running, +be sure that you call {@link +android.database.sqlite.SQLiteOpenHelper#getWritableDatabase} or {@link +android.database.sqlite.SQLiteOpenHelper#getReadableDatabase} in a background thread, +such as with {@link android.os.AsyncTask} or {@link android.app.IntentService}.</p> + +<p>To use {@link android.database.sqlite.SQLiteOpenHelper}, create a subclass that +overrides the {@link +android.database.sqlite.SQLiteOpenHelper#onCreate onCreate()}, {@link +android.database.sqlite.SQLiteOpenHelper#onUpgrade onUpgrade()} and {@link +android.database.sqlite.SQLiteOpenHelper#onOpen onOpen()} callback methods. You may also +want to implement {@link android.database.sqlite.SQLiteOpenHelper#onDowngrade onDowngrade()}, +but it's not required.</p> + +<p>For example, here's an implementation of {@link +android.database.sqlite.SQLiteOpenHelper} that uses some of the commands shown above:</p> + +<pre> +public class FeedReaderDbHelper extends SQLiteOpenHelper { + // If you change the database schema, you must increment the database version. + public static final int DATABASE_VERSION = 1; + public static final String DATABASE_NAME = "FeedReader.db"; + + public FeedReaderDbHelper(Context context) { + super(context, DATABASE_NAME, null, DATABASE_VERSION); + } + public void onCreate(SQLiteDatabase db) { + db.execSQL(SQL_CREATE_ENTRIES); + } + public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { + // This database is only a cache for online data, so its upgrade policy is + // to simply to discard the data and start over + db.execSQL(SQL_DELETE_ENTRIES); + onCreate(db); + } + public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) { + onUpgrade(db, oldVersion, newVersion); + } +} +</pre> + +<p>To access your database, instantiate your subclass of {@link +android.database.sqlite.SQLiteOpenHelper}:</p> + +<pre> +FeedReaderDbHelper mDbHelper = new FeedReaderDbHelper(getContext()); +</pre> + + + + +<h2 id="WriteDbRow">Put Information into a Database</h2> + +<p>Insert data into the database by passing a {@link android.content.ContentValues} +object to the {@link android.database.sqlite.SQLiteDatabase#insert insert()} method:</p> + +<pre> +// Gets the data repository in write mode +SQLiteDatabase db = mDbHelper.getWritableDatabase(); + +// Create a new map of values, where column names are the keys +ContentValues values = new ContentValues(); +values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID, id); +values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, title); +values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_CONTENT, content); + +// Insert the new row, returning the primary key value of the new row +long newRowId; +newRowId = db.insert( + FeedReaderContract.FeedEntry.TABLE_NAME, + FeedReaderContract.FeedEntry.COLUMN_NAME_NULLABLE, + values); +</pre> + +<p>The first argument for {@link android.database.sqlite.SQLiteDatabase#insert insert()} +is simply the table name. The second argument provides +the name of a column in which the framework can insert NULL in the event that the +{@link android.content.ContentValues} is empty (if you instead set this to {@code "null"}, +then the framework will not insert a row when there are no values).</p> + + + + +<h2 id="ReadDbRow">Read Information from a Database</h2> + +<p>To read from a database, use the {@link android.database.sqlite.SQLiteDatabase#query query()} +method, passing it your selection criteria and desired columns. +The method combines elements of {@link android.database.sqlite.SQLiteDatabase#insert insert()} +and {@link android.database.sqlite.SQLiteDatabase#update update()}, except the column list +defines the data you want to fetch, rather than the data to insert. The results of the query +are returned to you in a {@link android.database.Cursor} object.</p> + +<pre> +SQLiteDatabase db = mDbHelper.getReadableDatabase(); + +// Define a <em>projection</em> that specifies which columns from the database +// you will actually use after this query. +String[] projection = { + FeedReaderContract.FeedEntry._ID, + FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, + FeedReaderContract.FeedEntry.COLUMN_NAME_UPDATED, + ... + }; + +// How you want the results sorted in the resulting Cursor +String sortOrder = + FeedReaderContract.FeedEntry.COLUMN_NAME_UPDATED + " DESC"; + +Cursor c = db.query( + FeedReaderContract.FeedEntry.TABLE_NAME, // The table to query + projection, // The columns to return + selection, // The columns for the WHERE clause + selectionArgs, // The values for the WHERE clause + null, // don't group the rows + null, // don't filter by row groups + sortOrder // The sort order + ); +</pre> + +<p>To look at a row in the cursor, use one of the {@link android.database.Cursor} move +methods, which you must always call before you begin reading values. Generally, you should start +by calling {@link android.database.Cursor#moveToFirst}, which places the "read position" on the +first entry in the results. For each row, you can read a column's value by calling one of the +{@link android.database.Cursor} get methods, such as {@link android.database.Cursor#getString +getString()} or {@link android.database.Cursor#getLong getLong()}. For each of the get methods, +you must pass the index position of the column you desire, which you can get by calling +{@link android.database.Cursor#getColumnIndex getColumnIndex()} or +{@link android.database.Cursor#getColumnIndexOrThrow getColumnIndexOrThrow()}. +For example:</p> + +<pre> +cursor.moveToFirst(); +long itemId = cursor.getLong( + cursor.getColumnIndexOrThrow(FeedReaderContract.FeedEntry._ID) +); +</pre> + + + + +<h2 id="DeleteDbRow">Delete Information from a Database</h2> + +<p>To delete rows from a table, you need to provide selection criteria that +identify the rows. The database API provides a mechanism for creating selection +criteria that protects against SQL injection. The mechanism divides the +selection specification into a selection clause and selection arguments. The +clause defines the columns to look at, and also allows you to combine column +tests. The arguments are values to test against that are bound into the clause. +Because the result isn't handled the same as a regular SQL statement, it is +immune to SQL injection.</p> + +<pre> +// Define 'where' part of query. +String selection = FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?"; +// Specify arguments in placeholder order. +String[] selelectionArgs = { String.valueOf(rowId) }; +// Issue SQL statement. +db.delete(table_name, mySelection, selectionArgs); +</pre> + + + +<h2 id="UpdateDbRow">Update a Database</h2> + +<p>When you need to modify a subset of your database values, use the {@link +android.database.sqlite.SQLiteDatabase#update update()} method.</p> + +<p>Updating the table combines the content values syntax of {@link +android.database.sqlite.SQLiteDatabase#insert insert()} with the {@code where} syntax +of {@link android.database.sqlite.SQLiteDatabase#delete delete()}.</p> + +<pre> +SQLiteDatabase db = mDbHelper.getReadableDatabase(); + +// New value for one column +ContentValues values = new ContentValues(); +values.put(FeedReaderContract.FeedEntry.COLUMN_NAME_TITLE, title); + +// Which row to update, based on the ID +String selection = FeedReaderContract.FeedEntry.COLUMN_NAME_ENTRY_ID + " LIKE ?"; +String[] selelectionArgs = { String.valueOf(rowId) }; + +int count = db.update( + FeedReaderDbHelper.FeedEntry.TABLE_NAME, + values, + selection, + selectionArgs); +</pre> + diff --git a/docs/html/training/basics/data-storage/files.jd b/docs/html/training/basics/data-storage/files.jd new file mode 100644 index 0000000..dd081a6 --- /dev/null +++ b/docs/html/training/basics/data-storage/files.jd @@ -0,0 +1,382 @@ +page.title=Saving Files +parent.title=Data Storage +parent.link=index.html + +trainingnavtop=true + +@jd:body + + +<div id="tb-wrapper"> +<div id="tb"> + +<h2>This lesson teaches you to</h2> +<ol> + <li><a href="#InternalVsExternalStorage">Choose Internal or External Storage</a></li> + <li><a href="#GetWritePermission">Obtain Permissions for External Storage</a></li> + <li><a href="#WriteInternalStorage">Save a File on Internal Storage</a></li> + <li><a href="#WriteExternalStorage">Save a File on External Storage</a></li> + <li><a href="#GetFreeSpace">Query Free Space</a></li> + <li><a href="#DeleteFile">Delete a File</a></li> +</ol> + +<h2>You should also read</h2> +<ul> + <li><a href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">Using the Internal +Storage</a></li> + <li><a href="{@docRoot}guide/topics/data/data-storage.html#filesExternal">Using the External +Storage</a></li> +</ul> + +</div> +</div> + +<p>Android uses a file system that's +similar to disk-based file systems on other platforms. This lesson describes +how to work with the Android file system to read and write files with the {@link java.io.File} +APIs.</p> + +<p>A {@link java.io.File} object is suited to reading or writing large amounts of data in +start-to-finish order without skipping around. For example, it's good for image files or +anything exchanged over a network.</p> + +<p>This lesson shows how to perform basic file-related tasks in your app. +The lesson assumes that you are familiar with the basics of the Linux file system and the +standard file input/output APIs in {@link java.io}.</p> + + +<h2 id="InternalVsExternalStorage">Choose Internal or External Storage</h2> + +<p>All Android devices have two file storage areas: "internal" and "external" storage. These names +come from the early days of Android, when most devices offered built-in non-volatile memory +(internal storage), plus a removable storage medium such as a micro SD card (external storage). +Some devices divide the permanent storage space into "internal" and "external" partitions, so even +without a removable storage medium, there are always two storage spaces and +the API behavior is the same whether the external storage is removable or not. +The following lists summarize the facts about each storage space.</p> + +<div class="col-5" style="margin-left:0"> +<p><b>Internal storage:</b></p> +<ul> +<li>It's always available.</li> +<li>Files saved here are accessible by only your app by default.</li> +<li>When the user uninstalls your app, the system removes all your app's files from +internal storage.</li> +</ul> +<p>Internal storage is best when you want to be sure that neither the user nor other apps can +access your files.</p> +</div> + +<div class="col-7" style="margin-right:0"> +<p><b>External storage:</b></p> +<ul> +<li>It's not always available, because the user can mount the external storage as USB storage +and in some cases remove it from the device.</li> +<li>It's world-readable, so +files saved here may be read outside of your control.</li> +<li>When the user uninstalls your app, the system removes your app's files from here +only if you save them in the directory from {@link android.content.Context#getExternalFilesDir +getExternalFilesDir()}.</li> +</ul> +<p>External storage is the best +place for files that don't require access restrictions and for files that you want to share +with other apps or allow the user to access with a computer.</p> +</div> + + +<p class="note" style="clear:both"> +<strong>Tip:</strong> Although apps are installed onto the internal storage by +default, you can specify the <a +href="{@docRoot}guide/topics/manifest/manifest-element.html#install">{@code +android:installLocation}</a> attribute in your manifest so your app may +be installed on external storage. Users appreciate this option when the APK size is very large and +they have an external storage space that's larger than the internal storage. For more +information, see <a +href="{@docRoot}guide/topics/data/install-location.html">App Install Location</a>.</p> + + +<h2 id="GetWritePermission">Obtain Permissions for External Storage</h2> + +<p>To write to the external storage, you must request the + {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE} permission in your <a +href="{@docRoot}guide/topics/manifest/manifest-intro.html">manifest file</a>:</p> + +<pre> +<manifest ...> + <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> + ... +</manifest> +</pre> + +<div class="caution"><p><strong>Caution:</strong> +Currently, all apps have the ability to read the external storage +without a special permission. However, this will change in a future release. If your app needs +to read the external storage (but not write to it), then you will need to declare the {@link +android.Manifest.permission#READ_EXTERNAL_STORAGE} permission. To ensure that your app continues +to work as expected, you should declare this permission now, before the change takes effect.</p> +<pre> +<manifest ...> + <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /> + ... +</manifest> +</pre> +<p>However, if your app uses the {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE} +permission, then it implicitly has permission to read the external storage as well.</p> +</div> + +<p>You don’t need any permissions to save files on the internal +storage. Your application always has permission to read and +write files in its internal storage directory.</p> + + + + + +<h2 id="WriteInternalStorage">Save a File on Internal Storage</h2> + +<p>When saving a file to internal storage, you can acquire the appropriate directory as a +{@link java.io.File} by calling one of two methods:</p> + +<dl> + <dt>{@link android.content.Context#getFilesDir}</dt> + <dd>Returns a {@link java.io.File} representing an internal directory for your app.</dd> + <dt>{@link android.content.Context#getCacheDir}</dt> + <dd>Returns a {@link java.io.File} representing an internal directory for your app's temporary +cache files. Be sure to delete each file once it is no +longer needed and implement a reasonable size limit for the amount of memory you use at any given +time, such as 1MB. If the system begins running low on storage, it may delete your cache files +without warning.</dd> +</dl> + +<p>To create a new file in one of these directories, you can use the {@link +java.io.File#File(File,String) File()} constructor, passing the {@link java.io.File} provided by one +of the above methods that specifies your internal storage directory. For example:</p> + +<pre> +File file = new File(context.getFilesDir(), filename); +</pre> + +<p>Alternatively, you can call {@link +android.content.Context#openFileOutput openFileOutput()} to get a {@link java.io.FileOutputStream} +that writes to a file in your internal directory. For example, here's +how to write some text to a file:</p> + +<pre> +String filename = "myfile"; +String string = "Hello world!"; +FileOutputStream outputStream; + +try { + outputStream = openFileOutput(filename, Context.MODE_PRIVATE); + outputStream.write(string.getBytes()); + outputStream.close(); +} catch (Exception e) { + e.printStackTrace(); +} +</pre> + +<p>Or, if you need to cache some files, you should instead use {@link +java.io.File#createTempFile createTempFile()}. For example, the following method extracts the +file name from a {@link java.net.URL} and creates a file with that name +in your app's internal cache directory:</p> + +<pre> +public File getTempFile(Context context, String url) { + File file; + try { + String fileName = Uri.parse(url).getLastPathSegment(); + file = File.createTempFile(fileName, null, context.getCacheDir()); + catch (IOException e) { + // Error while creating file + } + return file; +} +</pre> + +<p class="note"><strong>Note:</strong> +Your app's internal storage directory is specified +by your app's package name in a special location of the Android file system. +Technically, another app can read your internal files if you set +the file mode to be readable. However, the other app would also need to know your app package +name and file names. Other apps cannot browse your internal directories and do not have +read or write access unless you explicitly set the files to be readable or writable. So as long +as you use {@link android.content.Context#MODE_PRIVATE} for your files on the internal storage, +they are never accessible to other apps.</p> + + + + + +<h2 id="WriteExternalStorage">Save a File on External Storage</h2> + +<p>Because the external storage may be unavailable—such as when the user has mounted the +storage to a PC or has removed the SD card that provides the external storage—you +should always verify that the volume is available before accessing it. You can query the external +storage state by calling {@link android.os.Environment#getExternalStorageState}. If the returned +state is equal to {@link android.os.Environment#MEDIA_MOUNTED}, then you can read and +write your files. For example, the following methods are useful to determine the storage +availability:</p> + +<pre> +/* Checks if external storage is available for read and write */ +public boolean isExternalStorageWritable() { + String state = Environment.getExternalStorageState(); + if (Environment.MEDIA_MOUNTED.equals(state)) { + return true; + } + return false; +} + +/* Checks if external storage is available to at least read */ +public boolean isExternalStorageReadable() { + String state = Environment.getExternalStorageState(); + if (Environment.MEDIA_MOUNTED.equals(state) || + Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) { + return true; + } + return false; +} +</pre> + +<p>Although the external storage is modifiable by the user and other apps, there are two +categories of files you might save here:</p> + +<dl> + <dt>Public files</dt> + <dd>Files that +should be freely available to other apps and to the user. When the user uninstalls your app, +these files should remain available to the user. + <p>For example, photos captured by your app or other downloaded files.</p> + </dd> + <dt>Private files</dt> + <dd>Files that rightfully belong to your app and should be deleted when the user uninstalls + your app. Although these files are technically accessible by the user and other apps because they + are on the external storage, they are files that realistically don't provide value to the user + outside your app. When the user uninstalls your app, the system deletes + all files in your app's external private directory. + <p>For example, additional resources downloaded by your app or temporary media files.</p> + </dd> +</dl> + +<p>If you want to save public files on the external storage, use the +{@link android.os.Environment#getExternalStoragePublicDirectory +getExternalStoragePublicDirectory()} method to get a {@link java.io.File} representing +the appropriate directory on the external storage. The method takes an argument specifying +the type of file you want to save so that they can be logically organized with other public +files, such as {@link android.os.Environment#DIRECTORY_MUSIC} or {@link +android.os.Environment#DIRECTORY_PICTURES}. For example:</p> + +<pre> +public File getAlbumStorageDir(String albumName) { + // Get the directory for the user's public pictures directory. + File file = new File(Environment.getExternalStoragePublicDirectory( + Environment.DIRECTORY_PICTURES), albumName); + if (!file.mkdirs()) { + Log.e(LOG_TAG, "Directory not created"); + } + return file; +} +</pre> + + +<p>If you want to save files that are private to your app, you can acquire the +appropriate directory by calling {@link +android.content.Context#getExternalFilesDir getExternalFilesDir()} and passing it a name indicating +the type of directory you'd like. Each directory created this way is added to a parent +directory that encapsulates all your app's external storage files, which the system deletes when the +user uninstalls your app.</p> + +<p>For example, here's a method you can use to create a directory for an individual photo album:</p> + +<pre> +public File getAlbumStorageDir(Context context, String albumName) { + // Get the directory for the app's private pictures directory. + File file = new File(context.getExternalFilesDir( + Environment.DIRECTORY_PICTURES), albumName); + if (!file.mkdirs()) { + Log.e(LOG_TAG, "Directory not created"); + } + return file; +} +</pre> + +<p>If none of the pre-defined sub-directory names suit your files, you can instead call {@link +android.content.Context#getExternalFilesDir getExternalFilesDir()} and pass {@code null}. This +returns the root directory for your app's private directory on the external storage.</p> + +<p>Remember that {@link android.content.Context#getExternalFilesDir getExternalFilesDir()} +creates a directory inside a directory that is deleted when the user uninstalls your app. +If the files you're saving should remain available after the user uninstalls your +app—such as when your app is a camera and the user will want to keep the photos—you +should instead use {@link android.os.Environment#getExternalStoragePublicDirectory +getExternalStoragePublicDirectory()}.</p> + + +<p>Regardless of whether you use {@link +android.os.Environment#getExternalStoragePublicDirectory +getExternalStoragePublicDirectory()} for files that are shared or +{@link android.content.Context#getExternalFilesDir +getExternalFilesDir()} for files that are private to your app, it's important that you use +directory names provided by API constants like +{@link android.os.Environment#DIRECTORY_PICTURES}. These directory names ensure +that the files are treated properly by the system. For instance, files saved in {@link +android.os.Environment#DIRECTORY_RINGTONES} are categorized by the system media scanner as ringtones +instead of music.</p> + + + + +<h2 id="GetFreeSpace">Query Free Space</h2> + +<p>If you know ahead of time how much data you're saving, you can find out +whether sufficient space is available without causing an {@link +java.io.IOException} by calling {@link java.io.File#getFreeSpace} or {@link +java.io.File#getTotalSpace}. These methods provide the current available space and the +total space in the storage volume, respectively. This information is also useful to avoid filling +the storage volume above a certain threshold.</p> + +<p>However, the system does not guarantee that you can write as many bytes as are +indicated by {@link java.io.File#getFreeSpace}. If the number returned is a +few MB more than the size of the data you want to save, or if the file system +is less than 90% full, then it's probably safe to proceed. +Otherwise, you probably shouldn't write to storage.</p> + +<p class="note"><strong>Note:</strong> You aren't required to check the amount of available space +before you save your file. You can instead try writing the file right away, then +catch an {@link java.io.IOException} if one occurs. You may need to do +this if you don't know exactly how much space you need. For example, if you +change the file's encoding before you save it by converting a PNG image to +JPEG, you won't know the file's size beforehand.</p> + + + + +<h2 id="DeleteFile">Delete a File</h2> + +<p>You should always delete files that you no longer need. The most straightforward way to delete a +file is to have the opened file reference call {@link java.io.File#delete} on itself.</p> + +<pre> +myFile.delete(); +</pre> + +<p>If the file is saved on internal storage, you can also ask the {@link android.content.Context} to locate and +delete a file by calling {@link android.content.Context#deleteFile deleteFile()}:</p> + +<pre> +myContext.deleteFile(fileName); +</pre> + +<div class="note"> +<p><strong>Note:</strong> When the user uninstalls your app, the Android system deletes +the following:</p> +<ul> +<li>All files you saved on internal storage</li> +<li>All files you saved on external storage using {@link +android.content.Context#getExternalFilesDir getExternalFilesDir()}.</li> +</ul> +<p>However, you should manually delete all cached files created with +{@link android.content.Context#getCacheDir()} on a regular basis and also regularly delete +other files you no longer need.</p> +</div> + diff --git a/docs/html/training/basics/data-storage/index.jd b/docs/html/training/basics/data-storage/index.jd new file mode 100644 index 0000000..99bb261 --- /dev/null +++ b/docs/html/training/basics/data-storage/index.jd @@ -0,0 +1,55 @@ +page.title=Saving Data + +trainingnavtop=true +startpage=true + +@jd:body + +<div id="tb-wrapper"> +<div id="tb"> + +<h2>Dependencies and prerequisites</h2> +<ul> + <li>Android 1.6 (API Level 4) or higher</li> + <li>Familiarity with Map key-value collections</li> + <li>Familiarity with the Java file I/O API</li> + <li>Familiarity with SQL databases</li> +</ul> + +<h2>You should also read</h2> +<ul> + <li><a href="{@docRoot}guide/topics/data/data-storage.html">Storage Options</a></li> +</ul> + +</div> +</div> + +<p>Most Android apps need to save data, even if only to save information about the app state +during {@link android.app.Activity#onPause onPause()} so the user's progress is not lost. Most +non-trivial apps also need to save user settings, and some apps must manage large +amounts of information in files and databases. This class introduces you to the +principal data storage options in Android, including:</p> + +<ul> + <li>Saving key-value pairs of simple data types in a shared preferences +file</li> + <li>Saving arbitrary files in Android's file system</li> + <li>Using databases managed by SQLite</li> +</ul> + + +<h2>Lessons</h2> + +<dl> + <dt><b><a href="shared-preferences.html">Saving Data in Shared Preferences</a></b></dt> + <dd>Learn to use a shared preferences file for storing small amounts of information in +key-value pairs.</dd> + + <dt><b><a href="files.html">Saving Data in Files</a></b></dt> + <dd>Learn to save a basic file, such as to store long sequences of data that + are generally read in order.</dd> + + <dt><b><a href="databases.html">Saving Data in SQL Databases</a></b></dt> + <dd>Learn to use a SQLite database to read and write structured data.</dd> + +</dl> diff --git a/docs/html/training/basics/data-storage/shared-preferences.jd b/docs/html/training/basics/data-storage/shared-preferences.jd new file mode 100644 index 0000000..67f45cb --- /dev/null +++ b/docs/html/training/basics/data-storage/shared-preferences.jd @@ -0,0 +1,121 @@ +page.title=Saving Key-Value Sets +parent.title=Data Storage +parent.link=index.html + +trainingnavtop=true + +@jd:body + + +<div id="tb-wrapper"> +<div id="tb"> + +<h2>This lesson teaches you to</h2> +<ol> + <li><a href="#GetSharedPreferences">Get a Handle to a SharedPreferences</a></li> + <li><a href="#WriteSharedPreference">Write to Shared Preferences</a></li> + <li><a href="#ReadSharedPreference">Read from Shared Preferences</a></li> +</ol> + +<h2>You should also read</h2> +<ul> + <li><a href="{@docRoot}guide/topics/data/data-storage.html#pref">Using Shared Preferences</a></li> +</ul> + +</div> +</div> + + +<p>If you have a relatively small collection of key-values that you'd like to save, +you should use the {@link android.content.SharedPreferences} APIs. +A {@link android.content.SharedPreferences} object points to a file containing +key-value pairs and provides simple methods to read and write them. Each +{@link android.content.SharedPreferences} file is +managed by the framework and can be private or shared.</p> + +<p>This class shows you how to use the {@link android.content.SharedPreferences} APIs to store and +retrieve simple values.</p> + +<p class="note"><strong>Note:</strong> The {@link android.content.SharedPreferences} APIs are +only for reading and writing key-value pairs and you should not confuse them with the +{@link android.preference.Preference} APIs, which help you build a user interface +for your app settings (although they use {@link android.content.SharedPreferences} as their +implementation to save the app settings). For information about using the {@link +android.preference.Preference} APIs, see the <a href="{@docRoot}guide/topics/ui/settings.html" +>Settings</a> guide.</p> + +<h2 id="GetSharedPreferences">Get a Handle to a SharedPreferences</h2> + +<p>You can create a new shared preference file or access an existing +one by calling one of two methods:</p> +<ul> + <li>{@link android.content.Context#getSharedPreferences(String,int) +getSharedPreferences()} — Use this if you need multiple shared preference files identified +by name, which you specify with the first parameter. You can call this from any +{@link android.content.Context} in your app.</li> + <li>{@link android.app.Activity#getPreferences(int) getPreferences()} — Use this from an +{@link android.app.Activity} if you need +to use only one shared preference file for the activity. Because this retrieves a default shared +preference file that belongs to the activity, you don't need to supply a name.</li> +</ul> + +<p>For example, the following code is executed inside a {@link android.app.Fragment}. +It accesses the shared preferences file that's +identified by the resource string {@code R.string.preference_file_key} and opens it using +the private mode so the file is accessible by only your app.</p> + +<pre> +Context context = getActivity(); +SharedPreferences sharedPref = context.getSharedPreferences( + getString(R.string.preference_file_key), Context.MODE_PRIVATE); +</pre> + +<p>When naming your shared preference files, you should use a name that's uniquely identifiable +to your app, such as {@code "com.example.myapp.PREFERENCE_FILE_KEY"}</p> + +<p>Alternatively, if you need just one shared preference file for your activity, you can use the +{@link android.app.Activity#getPreferences(int) getPreferences()} method:</p> + +<pre> +SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE); +</pre> + +<p class="caution"><strong>Caution:</strong> If you create a shared preferences file +with {@link android.content.Context#MODE_WORLD_READABLE} or {@link +android.content.Context#MODE_WORLD_WRITEABLE}, then any other apps that know the file identifier +can access your data.</p> + + +<h2 id="WriteSharedPreference">Write to Shared Preferences</h2> + +<p>To write to a shared preferences file, create a {@link +android.content.SharedPreferences.Editor} by calling {@link +android.content.SharedPreferences#edit} on your {@link android.content.SharedPreferences}.</p> + +<p>Pass the keys and values you want to write with methods such as {@link +android.content.SharedPreferences.Editor#putInt putInt()} and {@link +android.content.SharedPreferences.Editor#putString putString()}. Then call {@link +android.content.SharedPreferences.Editor#commit} to save the changes. For example:</p> + +<pre> +SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE); +SharedPreferences.Editor editor = sharedPref.edit(); +editor.putInt(getString(R.string.saved_high_score), newHighScore); +editor.commit(); +</pre> + + +<h2 id="ReadSharedPreference">Read from Shared Preferences</h2> + +<p>To retrieve values from a shared preferences file, call methods such as {@link +android.content.SharedPreferences#getInt getInt()} and {@link +android.content.SharedPreferences#getString getString()}, providing the key for the value +you want, and optionally a default value to return if the key isn't +present. For example:</p> + +<pre> +SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE); +long default = getResources().getInteger(R.string.saved_high_score_default)); +long highScore = sharedPref.getInt(getString(R.string.saved_high_score), default); +</pre> + diff --git a/docs/html/training/training_toc.cs b/docs/html/training/training_toc.cs index b70ba3f..0952885 100644 --- a/docs/html/training/training_toc.cs +++ b/docs/html/training/training_toc.cs @@ -104,6 +104,26 @@ </li> <li class="nav-section"> + <div class="nav-section-header"><a href="<?cs var:toroot?>training/basics/data-storage/index.html"> + <span class="en">Saving Data</span> + </a></div> + <ul> + <li><a href="<?cs var:toroot ?>training/basics/data-storage/shared-preferences.html"> + <span class="en">Saving Key-Value Sets</span> + </a> + </li> + <li><a href="<?cs var:toroot ?>training/basics/data-storage/files.html"> + <span class="en">Saving Files</span> + </a> + </li> + <li><a href="<?cs var:toroot ?>training/basics/data-storage/databases.html"> + <span class="en">Saving Data in SQL Databases</span> + </a> + </li> + </ul> + </li> + + <li class="nav-section"> <div class="nav-section-header"><a href="<?cs var:toroot ?>training/basics/intents/index.html"> <span class="en">Interacting with Other Apps</span> </a></div> |