> 編寫:[fastcome1985](https://github.com/fastcome1985) - 原文:[http://developer.android.com/training/basics/fragments/communicating.html](http://developer.android.com/training/basics/fragments/communicating.html) - 為了重用Fragment UI組件，我們應該把每一個fragment都構建成完全的自包含的、模塊化的組件，定義他們自己的布局與行為。定義好這些模塊化的Fragment后，就可以讓他們關聯[activity](# "An activity represents a single screen with a user interface.")，使他們與application的邏輯結合起來，實現全局的復合的UI。 - 通常fragment之間可能會需要交互，比如基于用戶事件改變fragment的內容。所有fragment之間的交互需要通過他們關聯的[activity](# "An activity represents a single screen with a user interface.")，兩個fragment之間不應該直接交互。 ### 定義一個接口 - 為了讓fragment與[activity](# "An activity represents a single screen with a user interface.")交互，可以在Fragment 類中定義一個接口，并在[activity](# "An activity represents a single screen with a user interface.")中實現。Fragment在他們生命周期的onAttach()方法中獲取接口的實現，然后調用接口的方法來與[Activity](# "An activity represents a single screen with a user interface.")交互。 下面是一個fragment與[activity](# "An activity represents a single screen with a user interface.")交互的例子： ~~~ public class HeadlinesFragment extends ListFragment { OnHeadlineSelectedListener mCallback; // Container Activity must implement this interface public interface OnHeadlineSelectedListener { public void onArticleSelected(int position); } @Override public void onAttach(Activity activity) { super.onAttach(activity); // This makes sure that the container activity has implemented // the callback interface. If not, it throws an exception try { mCallback = (OnHeadlineSelectedListener) activity; } catch (ClassCastException e) { throw new ClassCastException(activity.toString() + " must implement OnHeadlineSelectedListener"); } } ... } ~~~ - 現在Fragment就可以通過調用`OnHeadlineSelectedListener`接口實例的`mCallback`中的`onArticleSelected()`（也可以是其它方法）方法與[activity](# "An activity represents a single screen with a user interface.")傳遞消息。 - 舉個例子，在fragment中的下面的方法在用戶點擊列表條目時被調用，fragment 用回調接口來傳遞事件給父[Activity](# "An activity represents a single screen with a user interface."). ~~~ @Override public void onListItemClick(ListView l, View v, int position, long id) { // Send the event to the host activity mCallback.onArticleSelected(position); } ~~~ ### 實現接口 - 為了接收回調事件，宿主[activity](# "An activity represents a single screen with a user interface.")必須實現在Fragment中定義的接口。 - 舉個例子，下面的[activity](# "An activity represents a single screen with a user interface.")實現了上面例子中的接口。 ~~~ public static class MainActivity extends Activity implements HeadlinesFragment.OnHeadlineSelectedListener{ ... public void onArticleSelected(int position) { // The user selected the headline of an article from the HeadlinesFragment // Do something here to display that article } } ~~~ ### 傳消息給Fragment - 宿主[activity](# "An activity represents a single screen with a user interface.")通過[findFragmentById()](http://developer.android.com/reference/android/support/v4/app/FragmentManager.html#findFragmentById(int))方法獲取[fragment](http://developer.android.com/reference/android/support/v4/app/Fragment.html)的實例，然后直接調用Fragment的public方法來向fragment傳遞消息。 - 例如，假設上面所示的[activity](# "An activity represents a single screen with a user interface.")可能包含另外一個fragment,這個fragment用來展示從上面的回調方法中返回的指定的數據。在這種情況下，[activity](# "An activity represents a single screen with a user interface.")可以把從回調方法中接收到的信息傳遞給這個展示數據的Fragment. ~~~ public static class MainActivity extends Activity implements HeadlinesFragment.OnHeadlineSelectedListener{ ... public void onArticleSelected(int position) { // The user selected the headline of an article from the HeadlinesFragment // Do something here to display that article ArticleFragment articleFrag = (ArticleFragment) getSupportFragmentManager().findFragmentById(R.id.article_fragment); if (articleFrag != null) { // If article frag is available, we're in two-pane layout... // Call a method in the ArticleFragment to update its content articleFrag.updateArticleView(position); } else { // Otherwise, we're in the one-pane layout and must swap frags... // Create fragment and give it an argument for the selected article ArticleFragment newFragment = new ArticleFragment(); Bundle args = new Bundle(); args.putInt(ArticleFragment.ARG_POSITION, position); newFragment.setArguments(args); FragmentTransaction transaction = getSupportFragmentManager().beginTransaction(); // Replace whatever is in the fragment_container view with this fragment, // and add the transaction to the back stack so the user can navigate back transaction.replace(R.id.fragment_container, newFragment); transaction.addToBackStack(null); // Commit the transaction transaction.commit(); } } } ~~~