Using the LoaderManager Callbacks
LoaderManager.LoaderCallbacks是一個回調介面讓使用者能與LoaderManager互動。
Loader，尤其是CursorLoader，預期在停止後保留它們的資料。這允許應用程式保留住它們的資料跨越activity或fragment的onStop()及onStrat()方法，以便當使用者回到應用程式，他們不需要等待資料重新載入。你可以使用LoaderManager.LoaderCallbacks方法知道何時建立新的loader，並告訴應用程式何時該停止使用loader的資料。
LoaderManager.LoaderCallbacks包含這些方法
- onCreateLoader() - 為特定ID建立及回傳新的loader
- onLoaderFinished() - 當先前建立的loader完成資料加載時呼叫
- onLoaderReset() - 當先前建立的loader被重置時呼叫，從而使得其資料不可用
onCreateLoader
當你試著存取loader時(例如，透過initLoader())，它會檢查特定ID的loader是否存在。如果不存在，它會觸發LoaderManager.Loadercallbacks方法的onCreateLoader()方法，這是建立新loader的地方，通常會是CursorLoader，但是你可以執行你自己的loader子類。
在此例中，這個onCreateLoader()方法會建立一個CursorLoader，你必須使用建構子建立CursorLoader，建構子需要完整的資訊去查詢Content Provider。具體來說，建構子需要以下資訊
- uri - 對於要取得content的URI
- projection - 哪些columns回傳的列表，傳入null將會回傳所有columns，這樣是無效率的
- selection - 過濾器宣告回傳哪些rows，格式如SQL WHERE條文(排除WHERE本身)。傳入null會回傳特定uri的所有rows
- selectionArgs - 在selection中可能會包含多個?s，這將從selectionArgs的值取代，依它們在selection出現的順序。這些值被用字串綁定
- sortOrder - 如何指揮rows，格式為SQL ORDER BY條文(排除ORDER BY本身)。傳入null使用預設的排序方式，它可能沒有排序
舉例
// If non-null, this is the current filter the user has provided.
String mCurFilter;
...
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
// This is called when a new Loader needs to be created. This
// sample only has one Loader, so we don't care about the ID.
// First, pick the base URI to use depending on whether we are
// currently filtering.
Uri baseUri;
if (mCurFilter != null) {
baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
Uri.encode(mCurFilter));
} else {
baseUri = Contacts.CONTENT_URI;
}
// Now create and return a CursorLoader that will take care of
// creating a Cursor for the data being displayed.
String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
+ Contacts.HAS_PHONE_NUMBER + "=1) AND ("
+ Contacts.DISPLAY_NAME + " != '' ))";
return new CursorLoader(getActivity(), baseUri,
CONTACTS_SUMMARY_PROJECTION, select, null,
Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}
onLoadFinished
此方法當先前loader已經完成加載資料時被呼叫，此方法保證在最後供應給loader的資料釋放之前被呼叫。在此刻，你應該移除所有使用的舊資料，但不應該自己完成釋放資料，因為擁有資料的loader會照料它們。
一旦loader知道應用程式不會在使用資料，loader就會釋放資料。例如，如果資料是來自CursorLoader的cursor，你不應該呼叫close()。如果此cursor被放入CursorAdapter，你應該使用SwapCursor()方法，如此舊的cursor不會被關閉。
舉例
// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
...
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
// Swap the new cursor in. (The framework will take care of closing the
// old cursor once we return.)
mAdapter.swapCursor(data);
}
onLoaderReset
當先前的loader被重置時會呼叫此方法，如此使資料為不可用的。此回調方法讓你找出資料和時被釋放，如此你可以移除你的參考。
以下執行為swapCursor()方法，傳入參數為null
// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
...
public void onLoaderReset(Loader<Cursor> loader) {
// This is called when the last Cursor provided to onLoadFinished()
// above is about to be closed. We need to make sure we are no
// longer using it.
mAdapter.swapCursor(null);
}
Example
以下例子為在Fragment上用ListView顯示查詢聯絡人content provider的結果，使用CursorLoader管理作用在proivder的查詢。
為了讓應用程式存取使用者的聯絡人資料，它的manifest必須包含權限READ_CONTACTS。
public static class CursorLoaderListFragment extends ListFragment
implements OnQueryTextListener, LoaderManager.LoaderCallbacks<Cursor> {
// This is the Adapter being used to display the list's data.
SimpleCursorAdapter mAdapter;
// If non-null, this is the current filter the user has provided.
String mCurFilter;
@Override public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// Give some text to display if there is no data. In a real
// application this would come from a resource.
setEmptyText("No phone numbers");
// We have a menu item to show in action bar.
setHasOptionsMenu(true);
// Create an empty adapter we will use to display the loaded data.
mAdapter = new SimpleCursorAdapter(getActivity(),
android.R.layout.simple_list_item_2, null,
new String[] { Contacts.DISPLAY_NAME, Contacts.CONTACT_STATUS },
new int[] { android.R.id.text1, android.R.id.text2 }, 0);
setListAdapter(mAdapter);
// Prepare the loader. Either re-connect with an existing one,
// or start a new one.
getLoaderManager().initLoader(0, null, this);
}
@Override public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
// Place an action bar item for searching.
MenuItem item = menu.add("Search");
item.setIcon(android.R.drawable.ic_menu_search);
item.setShowAsAction(MenuItem.SHOW_AS_ACTION_IF_ROOM);
SearchView sv = new SearchView(getActivity());
sv.setOnQueryTextListener(this);
item.setActionView(sv);
}
public boolean onQueryTextChange(String newText) {
// Called when the action bar search text has changed. Update
// the search filter, and restart the loader to do a new query
// with this filter.
mCurFilter = !TextUtils.isEmpty(newText) ? newText : null;
getLoaderManager().restartLoader(0, null, this);
return true;
}
@Override public boolean onQueryTextSubmit(String query) {
// Don't care about this.
return true;
}
@Override public void onListItemClick(ListView l, View v, int position, long id) {
// Insert desired behavior here.
Log.i("FragmentComplexList", "Item clicked: " + id);
}
// These are the Contacts rows that we will retrieve.
static final String[] CONTACTS_SUMMARY_PROJECTION = new String[] {
Contacts._ID,
Contacts.DISPLAY_NAME,
Contacts.CONTACT_STATUS,
Contacts.CONTACT_PRESENCE,
Contacts.PHOTO_ID,
Contacts.LOOKUP_KEY,
};
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
// This is called when a new Loader needs to be created. This
// sample only has one Loader, so we don't care about the ID.
// First, pick the base URI to use depending on whether we are
// currently filtering.
Uri baseUri;
if (mCurFilter != null) {
baseUri = Uri.withAppendedPath(Contacts.CONTENT_FILTER_URI,
Uri.encode(mCurFilter));
} else {
baseUri = Contacts.CONTENT_URI;
}
// Now create and return a CursorLoader that will take care of
// creating a Cursor for the data being displayed.
String select = "((" + Contacts.DISPLAY_NAME + " NOTNULL) AND ("
+ Contacts.HAS_PHONE_NUMBER + "=1) AND ("
+ Contacts.DISPLAY_NAME + " != '' ))";
return new CursorLoader(getActivity(), baseUri,
CONTACTS_SUMMARY_PROJECTION, select, null,
Contacts.DISPLAY_NAME + " COLLATE LOCALIZED ASC");
}
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
// Swap the new cursor in. (The framework will take care of closing the
// old cursor once we return.)
mAdapter.swapCursor(data);
}
public void onLoaderReset(Loader<Cursor> loader) {
// This is called when the last Cursor provided to onLoadFinished()
// above is about to be closed. We need to make sure we are no
// longer using it.
mAdapter.swapCursor(null);
}
}
