Definition

• A Fragment represents a behavior or a potion of user interface in an Activity.
• You can combile multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities.
• A Fragment has its own life cycle, receives its own input events, and you can add or remove it while the activity is running. (sub activity)
• A Fragment‘s lifecycle is directly affected by its host activity‘s lifecycle. It follows the activity‘s lifecycle, except for in resuming state. The fragment‘s transaction can be pushed into the backstack.

Two ways to add a fragment to acitvity

• Define <fragment>...</fragment> in layout file.
• In code, add a fragment to an existing ViewGroup.

Design philosophy

• 在3.0（API 11）出现，目的是满足动态、灵活的UI设计，避免view hierarchy的频繁变化。
• 对于目的是复用的Fragment A，尽量避免在另一个Fragment B里操作它的生命周期（因为在另一个场景下很可能就没有B了）

Creating a fragment

在实现一个Fragment时，通常需要override以下方法

• onCreate()，完成初始化功能
• onCreateView()，Fragment首次绘制UI时会调用该方法，返回根View，如果Fragment不需要UI，则返回null
• onPause()，commit changes

几种Fragment

• DialogFragment
• ListFragment
• PreferenceFragment

创建Fragment的几种方法

• 布局文件里声明

<fragment android:name="com.example.news.ArticleListFragment"
            android:id="@+id/list"
　　　　　　　android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="match_parent" />

• Programmatically (in Activity)

FragmentManager fm = getFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
SampleFragment f = new SampleFragment();
ft.add(R.id.fragment_container, f);
fm.commit();

无UI的Fragment

• 使用 add(Fragment fragment, String tag) 进行添加
• 无需重写 onCreateView 方法
• 使用 findFragmentByTag 获取该Fragment

Managing Fragments

FragmentManager 的作用

• 获取Fragment：findFragmentById（有UI），findFragmentByTag（无UI）
• 使Fragment出栈，popBackStack()，模拟用户点击Back按钮。（用处何在？）
• 在BackStack中增加Listener，addOnBackStackChangeListener()

Performing Fragment Transactions

使用BackStack来存储Fragment的状态

// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();

在commit()前使用setTransaction()来增加动画效果

Communicating with the Activity

彼此获取引用

• Fragment通过getActivity()来获取宿主Activity的引用
• Activity通过getFragmentManager().findFragmentById()来获取嵌入的Fragment引用

在Fragment中实现对Activity的Callback：声明一个接口，在onAttach()时将实现该接口的Activity引用保存，在需要通知Activity时调用接口中的方法

public static class FragmentA extends ListFragment {　　private OnArticleSelectedListener mListener;
    ...
    // Container Activity must implement this interface
    public interface OnArticleSelectedListener {
        public void onArticleSelected(Uri articleUri);
    }
    ...
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnArticleSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
        }
    }
    ...
    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Append the clicked item‘s row ID with the content provider Uri
        Uri noteUri = ContentUris.withAppendedId(ArticleColumns.CONTENT_URI, id);
        // Send the event and Uri to the host activity
        mListener.onArticleSelected(noteUri);
    }
    ...
}

Fragment可以注册Options Menu与Context Menu，从而接收到onOptionsItemSelected()等事件

Handling the Fragment Lifecycle

Fragment的生命周期与Activity类似

• create阶段，onAttach() -> onCreate() -> onCreateView() -> onActivityCreated()
• 最大的不同在于Activity自动入栈，而Fragment需要显式声明

在宿主Activity的Resume阶段，可以自由进行Fragment的增添／删除