Android 拾遗（二）Fragment

• Fragment 为什么被称为第五大组件

  • 有自己的生命周期
  • 能够动态的附加到 Activity 中去
  • 能够适应不同的屏幕尺寸，尤其是 pad
  • 组件化、可以复用

• Fragment 加载到 Activity 的两种方式

  • 静态加载

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">
    
        <fragment android:name="com.example.android.fragments.HeadlinesFragment"
                  android:id="@+id/headlines_fragment"
                  android:layout_weight="1"
                  android:layout_width="0dp"
                  android:layout_height="match_parent" />      
    
    </LinearLayout>
    

  • 动态加载

    public class MainActivity extends FragmentActivity {
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.news_articles);
    
            // Check that the activity is using the layout version with
            // the fragment_container FrameLayout
            if (findViewById(R.id.fragment_container) != null) {
    
                // However, if we're being restored from a previous state,
                // then we don't need to do anything and should return or else
                // we could end up with overlapping fragments.
                if (savedInstanceState != null) {
                    return;
                }
    
                // Create a new Fragment to be placed in the activity layout
                HeadlinesFragment firstFragment = new HeadlinesFragment();
    
                // In case this activity was started with special instructions from an
                // Intent, pass the Intent's extras to the fragment as arguments
                firstFragment.setArguments(getIntent().getExtras());
    
                // Add the fragment to the 'fragment_container' FrameLayout
                getSupportFragmentManager().beginTransaction()
                        .add(R.id.fragment_container, firstFragment).commit();
            }
        }
    
    

Fragment 的生命周期

• onCreate()
  系统会在创建片段时调用此方法。您应该在实现内初始化您想在片段暂停或停止后恢复时保留的必需片段组件
• onCreateView()
  系统会在片段首次绘制其用户界面时调用此方法。 要想为您的片段绘制 UI，您从此方法中返回的 View 必须是片段布局的根视图。如果片段未提供 UI，您可以返回 null。
• onPause()
  系统将此方法作为用户离开片段的第一个信号（但并不总是意味着此片段会被销毁）进行调用。 您通常应该在此方法内确认在当前用户会话结束后仍然有效的任何更改（因为用户可能不会返回）。

Fragment 之间的通信

• Fragment 和 Activity 之间的通信

  1.Fragment可以通过 getActivity() 访问 Activity 实例，并轻松地执行在 Activity 布局中查找视图等任务
  如：View listView = getActivity().findViewById(R.id.list);

  2.通过接口回调。
  To allow a Fragment to communicate up to its Activity, you can define an interface in the Fragment class and implement it within the Activity. The Fragment captures the interface implementation during its onAttach() lifecycle method and can then call the Interface methods in order to communicate with the Activity.

  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);
              }
  

  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
          }
      }
  

• Fragment 和 Fragment 之间的通信

  All Fragment-to-Fragment communication is done through the associated Activity. Two Fragments should never communicate directly.Using findFragmentById() or findFragmentByTag()

• Activity 和 Fragment 之间的通信

  Activity 也可以使用 findFragmentById() 或 findFragmentByTag()，通过从 FragmentManager 获取对 Fragment 的引用来调用Fragment中的方法

Fragment 管理器：FragmentManager

调用 commit() 不会立即执行事务，而是在 Activity 的 UI 线程（“主”线程）可以执行该操作时再安排其在线程上运行。不过，如有必要，您也可以从 UI 线程调用 executePendingTransactions() 以立即执行 commit() 提交的事务。通常不必这样做，除非其他线程中的作业依赖该事务。
注意：您只能在 Activity 保存其状态（用户离开 Activity）之前使用 commit() 提交事务。如果您试图在该时间点后提交，则会引发异常。 这是因为如需恢复 Activity，则提交后的状态可能会丢失。 对于丢失提交无关紧要的情况，请使用 commitAllowingStateLoss()。