添加到返回堆栈时如何保持片段状态?

问题描述

我编写了一个在两个片段之间切换的虚拟活动.当您从 FragmentA 转到 FragmentB 时，FragmentA 被添加到后堆栈中.但是，当我返回到 FragmentA(通过按回)时，会创建一个全新的 FragmentA 并且它所处的状态丢失了.我觉得我和这个问题一样，但我已经包含了一个完整的代码示例来帮助解决这个问题:

I've written up a dummy activity that switches between two fragments. When you go from FragmentA to FragmentB, FragmentA gets added to the back stack. However, when I return to FragmentA (by pressing back), a totally new FragmentA is created and the state it was in is lost. I get the feeling I'm after the same thing as this question, but I've included a complete code sample to help root out the issue:

public class FooActivity extends Activity {
  @Override public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    final FragmentTransaction transaction = getFragmentManager().beginTransaction();
    transaction.replace(android.R.id.content, new FragmentA());
    transaction.commit();
  }

  public void nextFragment() {
    final FragmentTransaction transaction = getFragmentManager().beginTransaction();
    transaction.replace(android.R.id.content, new FragmentB());
    transaction.addToBackStack(null);
    transaction.commit();
  }

  public static class FragmentA extends Fragment {
    @Override public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
      final View main = inflater.inflate(R.layout.main, container, false);
      main.findViewById(R.id.next_fragment_button).setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
          ((FooActivity) getActivity()).nextFragment();
        }
      });
      return main;
    }

    @Override public void onSaveInstanceState(Bundle outState) {
      super.onSaveInstanceState(outState);
      // Save some state!
    }
  }

  public static class FragmentB extends Fragment {
    @Override public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
      return inflater.inflate(R.layout.b, container, false);
    }
  }
}

添加了一些日志消息:

07-05 14:28:59.722 D/OMG     ( 1260): FooActivity.onCreate
07-05 14:28:59.742 D/OMG     ( 1260): FragmentA.onCreateView
07-05 14:28:59.742 D/OMG     ( 1260): FooActivity.onResume
<Tap Button on FragmentA>
07-05 14:29:12.842 D/OMG     ( 1260): FooActivity.nextFragment
07-05 14:29:12.852 D/OMG     ( 1260): FragmentB.onCreateView
<Tap 'Back'>
07-05 14:29:16.792 D/OMG     ( 1260): FragmentA.onCreateView

它从不调用 FragmentA.onSaveInstanceState 并且在您回击时创建一个新的 FragmentA.但是，如果我在 FragmentA 上并锁定屏幕，FragmentA.onSaveInstanceState 确实会被调用.太奇怪了......我期望添加到后台堆栈中的片段不需要重新创建是错误的吗?以下是 docs 的说明:

It's never calling FragmentA.onSaveInstanceState and it creates a new FragmentA when you hit back. However, if I'm on FragmentA and I lock the screen, FragmentA.onSaveInstanceState does get called. So weird...am I wrong in expecting a fragment added to the back stack to not need re-creation? Here's what the docs say:

然而，如果您在删除片段时确实调用了 addToBackStack()，然后片段停止并在用户导航时恢复回来了.

推荐答案

如果你从后栈返回一个片段，它不会重新创建片段而是重新使用相同的实例并以 onCreateView() 在fragment生命周期中，参见Fragment生命周期.

If you return to a fragment from the back stack it does not re-create the fragment but re-uses the same instance and starts with onCreateView() in the fragment lifecycle, see Fragment lifecycle.

所以如果你想存储状态，你应该使用实例变量，不要依赖onSaveInstanceState().

So if you want to store state you should use instance variables and not rely on onSaveInstanceState().

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