Fragment （一）

1，简介

Fragement（碎片）允许将Activity拆分成多个完全独立封装的可重用组件，每个组件有它自己的生命周期和UI布局，由此可见，Fragement依赖于Activity,它的生命周期直接被其所属的宿主activity的生命周期影响。

形象的理解Fragement,手机屏幕如下图所示:

Fragement 具有以下优点：

• 组件重用，多个Activity可重用同一个Fragement;
• 为不同屏幕大小的设备创建动态的灵活的UI,在Activity运行过程中，可以添加、移除或者替换Fragment（add()、remove()、replace()）

2，生命周期

Fragement的生命周期镜像它的宿主Activity的生命周期事件。若Activity进入active-resumed状态的时候，添加或者移除一个Fragement就会影响它自己的生命周期。

Fragement生命周期方面如下：

/**
 * Listing 4-4: Fragment skeleton code
 * Listing 4-5: Fragment lifecycle event handlers
 */
package com.paad.fragments;

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class MySkeletonFragment extends Fragment {

  // Called when the Fragment is attached to its parent Activity.
  @Override
  public void onAttach(Activity activity) {
    super.onAttach(activity);
    // Get a reference to the parent Activity.
  }

  // Called to do the initial creation of the Fragment.
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // Initialize the Fragment.
  }

  // Called once the Fragment has been created in order for it to
  // create its user interface.
  @Override
  public View onCreateView(LayoutInflater inflater, 
                           ViewGroup container,
                           Bundle savedInstanceState) {
    // Create, or inflate the Fragment's UI, and return it. 
    // If this Fragment has no UI then return null.
    return inflater.inflate(R.layout.my_fragment, container, false);
  }



  // Called once the parent Activity and the Fragment's UI have 
  // been created.
  @Override
  public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    // Complete the Fragment initialization �particularly anything
    // that requires the parent Activity to be initialized or the 
    // Fragment's view to be fully inflated.
  }

  // Called at the start of the visible lifetime.
  @Override
  public void onStart(){
    super.onStart();
    // Apply any required UI change now that the Fragment is visible.
  }

  // Called at the start of the active lifetime.
  @Override
  public void onResume(){
    super.onResume();
    // Resume any paused UI updates, threads, or processes required
    // by the Fragment but suspended when it became inactive.
  }

  // Called at the end of the active lifetime.
  @Override
  public void onPause(){
    // Suspend UI updates, threads, or CPU intensive processes
    // that don't need to be updated when the Activity isn't
    // the active foreground activity.
    // Persist all edits or state changes
    // as after this call the process is likely to be killed.
    super.onPause();
  }

  // Called to save UI state changes at the
  // end of the active lifecycle.
  @Override
  public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save UI state changes to the savedInstanceState.
    // This bundle will be passed to onCreate, onCreateView, and
    // onCreateView if the parent Activity is killed and restarted.
    super.onSaveInstanceState(savedInstanceState);
  }

  // Called at the end of the visible lifetime.
  @Override
  public void onStop(){
    // Suspend remaining UI updates, threads, or processing
    // that aren't required when the Fragment isn't visible.
    super.onStop();
  }

  // Called when the Fragment's View has been detached.
  @Override
  public void onDestroyView() {
    // Clean up resources related to the View.
    super.onDestroyView();
  }

  // Called at the end of the full lifetime.
  @Override
  public void onDestroy(){
    // Clean up any resources including ending threads,
    // closing database connections etc.
    super.onDestroy();
  }

  // Called when the Fragment has been detached from its parent Activity.
  @Override
  public void onDetach() {
    super.onDetach();
  }
}

生命周期要点：

• 开始于绑定到它的你Activity，结束于从父Activity分离，对应于onAttach 和onDetach事件，通常情况下，onAttach用来获取一下Fragement的父Activity的引用，为进一步初始化做准备；
• 不能依赖调用onDestroy方法来销毁它，因为此方面不定会被执行；
• 与Activity一样，应该使用onCreate方法来初始化Fragement,在其生命周期内创建的作用域对象，且确保只创建一次；
• 在onCreateView和onDestroyView上初始化和销毁UI；
• 当Fragement暂停或停止时，保存所有的UI状态和持久化所有的数据。

3，使用Fragement

最佳实践是使用容器View来创建布局，将Fragement在运行时放入容器内

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="horizontal"
  android:layout_width="match_parent"
  android:layout_height="match_parent">

    <FrameLayout
        android:id="@+id/ui_container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1" >
    </FrameLayout>

  <FrameLayout
    android:id="@+id/details_container"
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:layout_weight="3"    

    android:visibility =”gone”    --隐藏
  />
</LinearLayout>

在运行时使用Fragement transaction来动态填充布局，从而当配置改变时，能确保一致性，框架代码如下：

package com.paad.fragments;

import android.app.Activity;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;

public class MyFragmentActivity extends Activity {

  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Inflate the layout containing the Fragment containers
    setContentView(R.layout.fragment_container_layout);
    
    FragmentManager fm = getFragmentManager();

    // Check to see if the Fragment back stack has been populated
    // If not, create and populate the layout.
    DetailsFragment detailsFragment = 
      (DetailsFragment)fm.findFragmentById(R.id.details_container);
    
    if (detailsFragment == null) {
       FragmentTransaction ft = fm.beginTransaction(); 
       ft.add(R.id.details_container, new DetailsFragment());
       ft.add(R.id.ui_container, new MyListFragment());
       ft.commit();
     }
  }
}

package com.paad.weatherstation;

import com.paad.fragments.R;

import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class DetailsFragment extends Fragment {
  
  public DetailsFragment() {
  }

  // Called once the Fragment has been created in order for it to
  // create its user interface.
  @Override
  public View onCreateView(LayoutInflater inflater, 
                           ViewGroup container,
                           Bundle savedInstanceState) {
    // Create, or inflate the Fragment's UI, and return it. 
    // If this Fragment has no UI then return null.
    return inflater.inflate(R.layout.details_fragment, container, false);
  }
}

package com.paad.weatherstation;

import com.paad.fragments.R;

import android.annotation.SuppressLint;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class MyListFragment extends Fragment {
  
  public MyListFragment() {
  }

  // Called once the Fragment has been created in order for it to
  // create its user interface.
  @Override
  public View onCreateView(LayoutInflater inflater, 
                           ViewGroup container,
                           Bundle savedInstanceState) {
    // Create, or inflate the Fragment's UI, and return it. 
    // If this Fragment has no UI then return null.
    return inflater.inflate(R.layout.list_fragment, container, false);
  }
}

Fragement 和Activity之间的接口：

• 任何Fragement中可使用getActivity方法返回对父Activity的引用；
• 在fragement需要和它的主Activity共享事件的地方，最好在Fragement中创建回调接口，而主Activity必须实现它来监听Fragement中的改变；

package com.paad.fragments;

import android.app.Activity;
import android.app.Fragment;

/**
 * MOVED TO PA4AD_Ch04_Seasons
 */
public class SeasonFragment extends Fragment {

  public interface OnSeasonSelectedListener {
    public void onSeasonSelected(Season season);
  }
    
  private OnSeasonSelectedListener onSeasonSelectedListener;
  private Season currentSeason;

  @Override
  public void onAttach(Activity activity) {
    super.onAttach(activity);
      
    try {
      onSeasonSelectedListener = (OnSeasonSelectedListener)activity;
    } catch (ClassCastException e) {
      throw new ClassCastException(activity.toString() + 
                " must implement OnSeasonSelectedListener");
    }
  }

  private void setSeason(Season season) {
    currentSeason = season;
    onSeasonSelectedListener.onSeasonSelected(season);
  }
  
}

若理解有难道，可以了解一下JAVA编程思想中的上塑造型，内部类，接口方面的知识