這個是我一直想要找的，沒想到偉大的官網竟然有。感動到流鼻涕了。。

為了重用Fragment UI元件，我們應該建立一個完全獨立的自控的，定義自己佈局和行為的模型元件。一旦我們定義了這些可重用的Fragments，我們就可以把它們和Activity關聯起來並且用應用邏輯連線它們來實現一個整體的混合的UI。

所有的Fragment和Fragment的互動，都是通過它們共同關聯的Activity。就是說兩個Fragment永遠都不可能直接互動的。

定義一個介面

為了允許一個Fragment和它上級的Activity互動，我們可以在Fragment類中來定義一個Interface並且那個Activity要實現這個介面。Fragment在自己的onAttach()生命週期方法中獲得這個interface的實現類物件，並且為了和Activity互動可以接著呼叫介面的方法。

下面是官網的例子：

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");
        }
    }
    
    ...
}
 

  @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Send the event to the host activity
        mCallback.onArticleSelected(position);
    }

為了接收Fragment事件回撥，宿主的Activity必須實現那個介面。

例子：

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
    }
}
 

宿主的Activity可以傳遞訊息給Fragment通過使用findFragmentById()捕獲Fragment例項，接著呼叫Fragment的public方法。

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呼叫public方法更新佈局介面
            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();
        }
    }
}