Android 4 學習(17):使用Content Resolver

Content Resolver簡介

每一個應用程序都有一個ContentResolver實例,經過getContentResolver()方法能夠獲取:異步

 

ContentResolver cr = getContentResolver();

 

Content Provider對應,Content Resolver用於使用Content Provider發佈的數據。使用ContentResolver查詢ConentProvider提供的數據:ide

 

// Get the Content Resolver.
ContentResolver cr = getContentResolver();
// Specify the result column projection. Return the minimum set of columns required to satisfy your requirements.
String[] result_columns = new String[] {
  MyHoardContentProvider.KEY_ID,
  MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN,
  MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN };
// Specify the where clause that will limit your results.
String where = MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN + 「=」 + 1;
// Replace these with valid SQL statements as necessary.
String whereArgs[] = null;
String order = null;
// Return the specified rows.
Cursor resultCursor = cr.query(MyHoardContentProvider.CONTENT_URI, result_columns, where, whereArgs, order);

 

Cursor中解析數據的方法和上一篇博文中介紹的同樣,首先要移動到指定行,而後從指定列中獲取對應類型的數據:ui

 

float largestHoard = 0f;
String hoardName = 「No Hoards」;
// Find the index to the column(s) being used.
int GOLD_HOARDED_COLUMN_INDEX = resultCursor.getColumnIndexOrThrow(
MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN);
int HOARD_NAME_COLUMN_INDEX = resultCursor.getColumnIndexOrThrow(
MyHoardContentProvider.KEY_GOLD_HOARD_NAME_COLUMN);
// Iterate over the cursors rows.
// The Cursor is initialized at before first, so we can
// check only if there is a 「next」 row available. If the
// result Cursor is empty, this will return false.
while (resultCursor.moveToNext()) {
float hoard = resultCursor.getFloat(GOLD_HOARDED_COLUMN_INDEX);
if (hoard > largestHoard) {
largestHoard = hoard;
hoardName = resultCursor.getString(HOARD_NAME_COLUMN_INDEX);
}
}
// Close the Cursor when you’ve finished with it.
resultCursor.close();

 

使用Cursor Loader進行異步查詢

Cursor Loader存在於在每一個ActivityFragment中,能夠實現異步查詢和監聽底層數據的變化。例如管理Cursor的生命週期,確保在Activity退出以前Cursor被關閉。this

 

實現LoaderCallbacks接口

 

若要使用Cursor Loader,首先要實現LoaderManager.LoaderCallbacks接口:spa

 

LoaderManager.LoaderCallbacks<Cursor> loaderCallback = new LoaderManager.LoaderCallbacks<Cursor>()

 

LoaderCallback主要有這幾個回調方法:code

 

  • onCreateLoader
  • onLoadFinished
  • onLoaderReset
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
  // Construct the new query in the form of a Cursor Loader. Use the id
  // parameter to construct and return different loaders.
  String[] projection = null;
  String where = null;
  String[] whereArgs = null;
  String sortOrder = null;
  // Query URI
  Uri queryUri = MyContentProvider.CONTENT_URI;
  // Create the new Cursor loader.
  return new CursorLoader(DatabaseSkeletonActivity.this, queryUri,projection, where, whereArgs, sortOrder);
}
public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
  // Replace the result Cursor displayed by the Cursor Adapter with the new result set.
  adapter.swapCursor(cursor);
  // This handler is not synchronized with the UI thread, so you will need to synchronize it before modifying any UI elements directly.
}
public void onLoaderReset(Loader<Cursor> loader) {
  // Remove the existing result Cursor from the List Adapter.
  adapter.swapCursor(null);
  // This handler is not synchronized with the UI thread, so you
  // will need to synchronize it before modifying any UI elements directly.
}

初始化和重啓Cursor Loader

ActivityFragment中,能夠調用getLoaderManager得到Cursor Loaderorm

 

LoaderManager loaderManager = getLoaderManager();

 

獲取Cursor Loaderblog

 

Bundle args = null;
loaderManager.initLoader(LOADER_ID, args, myLoaderCallbacks);

 

LOADER_ID用於標識得到的loaderargs是可選參數,myLoaderCallbacks則是前面LoaderManager.LoaderCallbacks接口的實現。若是對應LOADER_ID的Cursor Loader不存在,那麼系統會調用onCreateLoader方法來建立一個Loader接口

 

使用Content Resolver增、改、刪

插入數據:生命週期

// Create a new row of values to insert.
ContentValues newValues = new ContentValues();
// Assign values for each row.
newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_NAME_COLUMN, hoardName);
newValues.put(MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN, hoardValue);
newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN, hoardAccessible);
// [ ... Repeat for each column / value pair ... ]
// Get the Content Resolver
ContentResolver cr = getContentResolver();
// Insert the row into your table
Uri myRowUri = cr.insert(MyHoardContentProvider.CONTENT_URI, newValues);

刪除數據:

// Specify a where clause that determines which row(s) to delete.
// Specify where arguments as necessary.
String where = MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN + =」 + 0;
String whereArgs[] = null;
// Get the Content Resolver.
ContentResolver cr = getContentResolver();
// Delete the matching rows
int deletedRowCount = cr.delete(MyHoardContentProvider.CONTENT_URI, where, whereArgs);

修改數據:

// Create the updated row content, assigning values for each row.
ContentValues updatedValues = new ContentValues();
updatedValues.put(MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN, newHoardValue);
// [ ... Repeat for each column to update ... ]
// Create a URI addressing a specific row.
Uri rowURI = ContentUris.withAppendedId(MyHoardContentProvider.CONTENT_URI, hoardId);
// Specify a specific row so no selection clause is required.
String where = null;
String whereArgs[] = null;
// Get the Content Resolver.
ContentResolver cr = getContentResolver();
// Update the specified row.
int updatedRowCount = cr.update(rowURI, updatedValues, where, whereArgs);

使用Content Resolver讀寫Content Provider中的數據

 

public void addNewHoardWithImage(String hoardName, float hoardValue, boolean hoardAccessible, Bitmap bitmap) {
  // Create a new row of values to insert.
  ContentValues newValues = new ContentValues();
  // Assign values for each row.
  newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_NAME_COLUMN, hoardName);
  newValues.put(MyHoardContentProvider.KEY_GOLD_HOARDED_COLUMN, hoardValue);
  newValues.put(MyHoardContentProvider.KEY_GOLD_HOARD_ACCESSIBLE_COLUMN, hoardAccessible);
  // Get the Content Resolver
  ContentResolver cr = getContentResolver();
  // Insert the row into your table
  Uri myRowUri = cr.insert(MyHoardContentProvider.CONTENT_URI, newValues);
  try {
    // Open an output stream using the new row’s URI.
    OutputStream outStream = cr.openOutputStream(myRowUri);
    // Compress your bitmap and save it into your provider.
    bitmap.compress(Bitmap.CompressFormat.JPEG, 80, outStream);
  }catch (FileNotFoundException e) {
    Log.d(TAG, 「No file found for this record.」);
  }
}
public Bitmap getHoardImage(long rowId) {
  Uri myRowUri = ContentUris.withAppendedId(MyHoardContentProvider.CONTENT_URI, rowId);
  try {
    // Open an input stream using the new row’s URI.
    InputStream inStream = getContentResolver().openInputStream(myRowUri);
    // Make a copy of the Bitmap.
    Bitmap bitmap = BitmapFactory.decodeStream(inStream);
    return bitmap;
  }catch (FileNotFoundException e) {
    Log.d(TAG, 「No file found for this record.」);
  }
  return null;
}
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