Fragment生命周期

前言

本篇只介绍Fragment的生命周期

Activity和Fragment生命周期

二者生命周期对比

image.png

Activity与Fragment生命周期关系
创建过程：

image

销毁过程：

image

Fragment切换时生命周期

取自第二行代码：

碎片的生命周期.png

对部分生命周期进行说明：
onAttach:当碎片和活动建立关联的时候调用
onCreateView:为碎片创建视图（加载布局）时调用
onActivityCreated:确保与碎片相关联的活动一定已经创建完毕的时候调用
onDestroyView:当与碎片关联的视图被移除的时候调用
onDetach:当碎片和活动解除关联的时候调用。

举例说明fragment替换时的生命周期执行过程

RightFragment代码（AnotherRightFragment代码与之基本一致）

public class RightFragment extends Fragment {

    private static final String TAG = "RightFragment";

    public RightFragment() {
       
    }
    
    public static RightFragment newInstance() {
        RightFragment fragment = new RightFragment();
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(TAG, "onCreate: ");
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        Log.d(TAG, "onCreateView: ");
        View view = inflater.inflate(R.layout.right_fragment,container,false);
        return view;
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        Log.d(TAG, "onAttach: ");
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Log.d(TAG, "onActivityCreated: ");
    }

    @Override
    public void onStart() {
        super.onStart();
        Log.d(TAG, "onStart: ");

    }

    @Override
    public void onResume() {
        super.onResume();
        Log.d(TAG, "onResume: ");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.d(TAG, "onPause: ");
    }

    @Override
    public void onStop() {
        super.onStop();
        Log.d(TAG, "onStop: ");
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        Log.d(TAG, "onDestroyView: ");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy: ");
    }

    @Override
    public void onDetach() {
        super.onDetach();
        Log.d(TAG, "onDetach: ");
    }
}

Main2Activity代码（在布局中有一个FrameLayout来做相应的fragment替换）

public class Main2Activity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                replaceFragment(new AnotherRightFragment());
            }
        });
        replaceFragment(new RightFragment());
    }

    private void replaceFragment(Fragment fragment) {
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction transaction = fm.beginTransaction();
        transaction.replace(R.id.right_layout,fragment);
        transaction.addToBackStack(null);
        transaction.commit();
    }
}

注意代码中在transaction做commit之前的transaction.addToBackStack(null);

启动页面时的打印信息：

05-13 19:12:43.094 25674-25674/com.dhasa.restore D/RightFragment: onAttach: 
    onCreate: 
05-13 19:12:43.095 25674-25674/com.dhasa.restore D/RightFragment: onCreateView: 
05-13 19:12:43.099 25674-25674/com.dhasa.restore D/RightFragment: onActivityCreated: 
    onStart: 
05-13 19:12:43.100 25674-25674/com.dhasa.restore D/RightFragment: onResume: 

点击按钮，让AnotherRightFragment替换RightFramgent时：

05-13 19:15:05.316 25674-25674/com.dhasa.restore D/AnotherRightFragment: onAttach: 
    onCreate: 
05-13 19:15:05.316 25674-25674/com.dhasa.restore D/RightFragment: onPause: 
    onStop: 
    onDestroyView: 
05-13 19:15:05.318 25674-25674/com.dhasa.restore D/AnotherRightFragment: onCreateView: 
05-13 19:15:05.321 25674-25674/com.dhasa.restore D/AnotherRightFragment: onActivityCreated: 
    onStart: 
    onResume: 

RightFragment只调用了onPause、onStop和onDestroyView，只是移除了视图，并没有对Fragment进行销毁。

点击安卓手机的back回退键：

05-13 19:17:19.418 25674-25674/com.dhasa.restore D/AnotherRightFragment: onPause: 
    onStop: 
    onDestroyView: 
05-13 19:17:19.423 25674-25674/com.dhasa.restore D/AnotherRightFragment: onDestroy: 
    onDetach: 
05-13 19:17:19.424 25674-25674/com.dhasa.restore D/RightFragment: onCreateView: 
05-13 19:17:19.428 25674-25674/com.dhasa.restore D/RightFragment: onActivityCreated: 
    onStart: 
    onResume: 

AnotherRightFragment执行了onPause->onStop->onDestroyView->onDestroy->onDetach，然后RightFragment恢复，执行了onCreateView->onActivityCreated->onStart->onResume

现在我们修改Main2Activity中replaceFragment方法中的代码

private void replaceFragment(Fragment fragment) {
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction transaction = fm.beginTransaction();
        transaction.replace(R.id.right_layout,fragment);
        //transaction.addToBackStack(null);
        transaction.commit();
}

将transaction.addToBackStack(null);代码注释掉

其他打印输出一致，在用AnotherRightFragment替换RightFragment时打印如下：

05-13 19:23:44.779 30558-30558/com.dhasa.restore D/AnotherRightFragment: onAttach: 
    onCreate: 
05-13 19:23:44.780 30558-30558/com.dhasa.restore D/RightFragment: onPause: 
    onStop: 
    onDestroyView: 
05-13 19:23:44.781 30558-30558/com.dhasa.restore D/RightFragment: onDestroy: 
    onDetach: 
05-13 19:23:44.782 30558-30558/com.dhasa.restore D/AnotherRightFragment: onCreateView: 
05-13 19:23:44.785 30558-30558/com.dhasa.restore D/AnotherRightFragment: onActivityCreated: 
    onStart: 
    onResume: 

RightFragment调用了onPause->onStop->onDestroyView->onDestroy->onDetach，对Fragment进行销毁并与Activity分离。此时按返回键，该Activity页面直接finish返回。