为了重用Fragment UI组件,你应该作为一个定义了它自己的布局和行为的,完全独立的,模块化的组建来构建。一旦你定义了这些可重用的Fragment,你使用一个Activity关联它们,和结合应用程序的逻辑以实现整体复合界面。
经常你会想让一个fragment和另一个通信,例如基于用户事件改变内容。所有Fragment和Fragment的通信是通过相关的Activity完成。两个Fragment不能直接通信。
定义一个接口
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为了允许fragment和它的activity通信,你可以在这个Fragment类中定义一个接口,并在这个Activity中实现它。这个fragment在它的onAttach()生命周期方法中获取接口的实现,然后调用这个接口方法和activity通信。
- 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能使用onHeadlineSelectedListener接口的mCallback实例,调用onArticleSelected()方法(或者在接口中的其它方法)发送消息给activity。
例如,下面在fragment中的方法当用户点击列表项的时候被调用。fragment使用回调接口来想父activity发送这个事件。
- @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
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为了从fragment获取事件回调方法,寄存它的activity必须实现被定义在fragment类中的接口。
例如,下面的activity实现了上面例子的接口:
- 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
- }
- }
向Fragment发送消息
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寄主activity能向使用findFragmentById()方法捕获的Fragment实例发送消息,然后直接调用fragment的公共方法。
例如,想象一下,上面显示的activity可能包含另一个fragment,它被用于显示选项详情,通过上面的回调方法被返回的数据。在这种情况下,activity能想另一个显示选项的fragment,传递在回调方法中获取的信息。
- 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();
- }
- }
- }