Fragments之间的交互

转载

• 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，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
• 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

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，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.

@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。

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();        }    }}