目錄

• 1.載入器特征
• 2. Loader API
• 3. 在應用中使用Loader
  • 3.1. 啟動載入器
  • 3.2. 重啟載入器
  • 3.3. 使用LoaderManager回調
• 4. 實例： 訪問用戶聯繫人

1.載入器特征

• 用於每個 Activity 和 Fragment
  
• 支持非同步載入數據。
• 監控其數據源併在內容變化時傳遞新結果。

2. Loader API

3. 在應用中使用Loader

主要步驟

1. Activity 或 Fragment
2. LoaderManager 的實例
3. 一個 CursorLoader，用於載入由ContentProvider 支持的數據。您也可以實現自己的 Loader或 AsyncTaskLoader 子類，從其他源中載入數據。
4. 一個 LoaderManager.LoaderCallbacks 實現。您可以使用它來創建新載入器，並管理對現有載入器的引用。
5. 一種顯示載入器數據的方法，如 SimpleCursorAdapter。
6. 使用 CursorLoader 時的數據源，如 ContentProvider。

3.1. 啟動載入器

在 Activity 的 onCreate() 方法或片段的onActivityCreated() 方法內初始化 Loader

// Prepare the loader.  Either re-connect with an existing one,
// or start a new one.
// 標識ID + 構建時給載入器的可選參數 + LoaderManager.LoaderCallbacks 實現
getLoaderManager().initLoader(0, null, this);
// 如果 ID 指定的載入器已存在，則將重覆使用上次創建的載入器。
// 如果 ID 指定的載入器不存在，則 initLoader() 將觸發 LoaderManager.LoaderCallbacks 方法 onCreateLoader()

3.2. 重啟載入器

public boolean onQueryTextChanged(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;
}

3.3. 使用LoaderManager回調

LoaderManager.LoaderCallbacks 是一個支持客戶端與LoaderManager 交互的回調介面。

• 載入器（特別是 CursorLoader）在停止運行後，仍需保留其數據。這樣，應用即可保留 Activity 或片段的 onStop()和 onStart() 方法中的數據。當用戶返回應用時，無需等待它重新載入這些數據。您可使用 LoaderManager.LoaderCallbacks 方法瞭解何時創建新載入器，並告知應用何時停止使用載入器的數據。
  
• LoaderManager.LoaderCallbacks 包括以下方法：
  • onCreateLoader()：針對指定的 ID 進行實例化並返回新的 Loader
  • onLoadFinished()：將在先前創建的載入器完成載入時調用
  • onLoaderReset()：將在先前創建的載入器重置且其數據因此不可用時調用

4. 實例： 訪問用戶聯繫人

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);
    }
}