> 编写:[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) - 为了重用Fragment UI组件，我们应该把每一个fragment都构建成完全的自包含的、模块化的组件，定义他们自己的布局与行为。定义好这些模块化的Fragment后，就可以让他们关联[activity](# "An activity represents a single screen with a user interface.")，使他们与application的逻辑结合起来，实现全局的复合的UI。 - 通常fragment之间可能会需要交互，比如基于用户事件改变fragment的内容。所有fragment之间的交互需要通过他们关联的[activity](# "An activity represents a single screen with a user interface.")，两个fragment之间不应该直接交互。 ### 定义一个接口 - 为了让fragment与[activity](# "An activity represents a single screen with a user interface.")交互，可以在Fragment 类中定义一个接口，并在[activity](# "An activity represents a single screen with a user interface.")中实现。Fragment在他们生命周期的onAttach()方法中获取接口的实现，然后调用接口的方法来与[Activity](# "An activity represents a single screen with a user interface.")交互。 下面是一个fragment与[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"); } } ... } ~~~ - 现在Fragment就可以通过调用`OnHeadlineSelectedListener`接口实例的`mCallback`中的`onArticleSelected()`（也可以是其它方法）方法与[activity](# "An activity represents a single screen with a user interface.")传递消息。 - 举个例子，在fragment中的下面的方法在用户点击列表条目时被调用，fragment 用回调接口来传递事件给父[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); } ~~~ ### 实现接口 - 为了接收回调事件，宿主[activity](# "An activity represents a single screen with a user interface.")必须实现在Fragment中定义的接口。 - 举个例子，下面的[activity](# "An activity represents a single screen with a user interface.")实现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 } } ~~~ ### 传消息给Fragment - 宿主[activity](# "An activity represents a single screen with a user interface.")通过[findFragmentById()](http://developer.android.com/reference/android/support/v4/app/FragmentManager.html#findFragmentById(int))方法获取[fragment](http://developer.android.com/reference/android/support/v4/app/Fragment.html)的实例，然后直接调用Fragment的public方法来向fragment传递消息。 - 例如，假设上面所示的[activity](# "An activity represents a single screen with a user interface.")可能包含另外一个fragment,这个fragment用来展示从上面的回调方法中返回的指定的数据。在这种情况下，[activity](# "An activity represents a single screen with a user interface.")可以把从回调方法中接收到的信息传递给这个展示数据的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(); } } } ~~~