為了重用Fragment UI組件，我們應(yīng)該把每一個(gè)fragment都構(gòu)建成完全的自包含的、模塊化的組件，定義他們自己的布局與行為。定義好這些模塊化的Fragment后，就可以讓他們關(guān)聯(lián)activity，使他們與application的邏輯結(jié)合起來(lái)，實(shí)現(xiàn)全局的復(fù)合的UI。
通常fragment之間可能會(huì)需要交互，比如基于用戶事件改變fragment的內(nèi)容。所有fragment之間的交互需要通過(guò)他們關(guān)聯(lián)的activity，兩個(gè)fragment之間不應(yīng)該直接交互。

定義一個(gè)接口

為了讓fragment與activity交互，可以在Fragment 類(lèi)中定義一個(gè)接口，并在activity中實(shí)現(xiàn)。Fragment在他們生命周期的onAttach()方法中獲取接口的實(shí)現(xiàn)，然后調(diào)用接口的方法來(lái)與Activity交互。

下面是一個(gè)fragment與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");
        }
    }
    ...
}

現(xiàn)在Fragment就可以通過(guò)調(diào)用OnHeadlineSelectedListener接口實(shí)例的mCallback中的onArticleSelected()（也可以是其它方法）方法與activity傳遞消息。
舉個(gè)例子，在fragment中的下面的方法在用戶點(diǎn)擊列表?xiàng)l目時(shí)被調(diào)用，fragment 用回調(diào)接口來(lái)傳遞事件給父Activity.

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

實(shí)現(xiàn)接口

為了接收回調(diào)事件，宿主activity必須實(shí)現(xiàn)在Fragment中定義的接口。
舉個(gè)例子，下面的activity實(shí)現(xiàn)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
    }
}

傳消息給Fragment

宿主activity通過(guò)findFragmentById()方法獲取fragment的實(shí)例，然后直接調(diào)用Fragment的public方法來(lái)向fragment傳遞消息。
例如，假設(shè)上面所示的activity可能包含另外一個(gè)fragment,這個(gè)fragment用來(lái)展示從上面的回調(diào)方法中返回的指定的數(shù)據(jù)。在這種情況下，activity可以把從回調(diào)方法中接收到的信息傳遞給這個(gè)展示數(shù)據(jù)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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