Fragment——Fragment之间的交互
Communicating with Other Fragments
In order to reuse the Fragment UI components, you should build each as a completely self-contained, modular component that defines its own layout and behavior. Once you have defined these reusable Fragments, you can associate them with an Activity and connect them with the application logic to realize the overall composite UI.
Often you will want one Fragment to communicate with another, for example to change the content based on a user event. All Fragment-to-Fragment communication is done through the associated Activity. Two Fragments should never communicate directly.
Define an Interface
To allow a Fragment to communicate up to its Activity, you can define an interface in the Fragment class and implement it within the Activity. The Fragment captures the interface implementation during its onAttach() lifecycle method and can then call the Interface methods in order to communicate with the Activity.
Here is an example of Fragment to Activity communication:
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"); } } ... }
Now the fragment can deliver messages to the activity by calling the onArticleSelected() method (or other methods in the interface) using the mCallback instance of the OnHeadlineSelectedListener interface.
For example, the following method in the fragment is called when the user clicks on a list item. The fragment uses the callback interface to deliver the event to the parent activity.
@Override public void onListItemClick(ListView l, View v, int position, long id) { // Send the event to the host activity mCallback.onArticleSelected(position); }
Implement the Interface
In order to receive event callbacks from the fragment, the activity that hosts it must implement the interface defined in the fragment class.
For example, the following activity implements the interface from the above example.
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 } }
Deliver a Message to a Fragment
The host activity can deliver messages to a fragment by capturing the Fragment instance withfindFragmentById(), then directly call the fragment's public methods.
For instance, imagine that the activity shown above may contain another fragment that's used to display the item specified by the data returned in the above callback method. In this case, the activity can pass the information received in the callback method to the other fragment that will display the item:
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(); } } }
为了能重用UI部件，你应该自定义每一个Fragment的布局和功能，使他们能成为完全独立的模块。定义好这些模块化的Fragment后，就可以让他们关联activity，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，两个fragment之间不应该直接交互。
定义一个接口
为了让fragment与activity交互，可以在Fragment 类中定义一个接口，并在activity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交互。
下面是一个fragment与activity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传递消息。
举个例子，下面这个在fragment中的方法将会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.
@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必须实现在Fragment中定义的接口。
举个例子，下面的activity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通过findFragmentById()方法获取fragment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(); } } }
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