如何在后台堆栈中正确保存 Fragments 的实例状态？

要正确保存 Fragment 的实例状态，您应该执行以下操作：

1. 在片段中，通过覆盖 onSaveInstanceState() 保存实例状态并在 onActivityCreated() 中恢复：

class MyFragment extends Fragment {

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        ...
        if (savedInstanceState != null) {
            //Restore the fragment's state here
        }
    }
    ...
    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        
        //Save the fragment's state here
    }

}

2.还有重点，在activity中，你必须将fragment的实例保存在onSaveInstanceState()中，并在onCreate()中恢复。

class MyActivity extends Activity {

    private MyFragment 

    public void onCreate(Bundle savedInstanceState) {
        ...
        if (savedInstanceState != null) {
            //Restore the fragment's instance
            mMyFragment = getSupportFragmentManager().getFragment(savedInstanceState, "myFragmentName");
            ...
        }
        ...
    }
    
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
            
        //Save the fragment's instance
        getSupportFragmentManager().putFragment(outState, "myFragmentName", mMyFragment);
    }

}

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这是一个非常古老的答案。

我不再为 Android 编写代码，因此无法保证最新版本的功能，也不会对其进行任何更新。

这就是我现在使用的方式......它非常复杂，但至少它处理了所有可能的情况。万一有人感兴趣。

public final class MyFragment extends Fragment {
    private TextView vstup;
    private Bundle savedState = null;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.whatever, null);
        vstup = (TextView)v.findViewById(R.id.whatever);

        /* (...) */

        /* If the Fragment was destroyed inbetween (screen rotation), we need to recover the savedState first */
        /* However, if it was not, it stays in the instance from the last onDestroyView() and we don't want to overwrite it */
        if(savedInstanceState != null && savedState == null) {
            savedState = savedInstanceState.getBundle(App.STAV);
        }
        if(savedState != null) {
            vstup.setText(savedState.getCharSequence(App.VSTUP));
        }
        savedState = null;

        return v;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        savedState = saveState(); /* vstup defined here for sure */
        vstup = null;
    }

    private Bundle saveState() { /* called either from onDestroyView() or onSaveInstanceState() */
        Bundle state = new Bundle();
        state.putCharSequence(App.VSTUP, vstup.getText());
        return state;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        /* If onDestroyView() is called first, we can use the previously savedState but we can't call saveState() anymore */
        /* If onSaveInstanceState() is called first, we don't have savedState, so we need to call saveState() */
        /* => (?:) operator inevitable! */
        outState.putBundle(App.STAV, (savedState != null) ? savedState : saveState());
    }

    /* (...) */

}

或者，始终可以将数据显示在变量中的被动 View 中，并仅使用 View 来显示它们，从而使两者保持同步。不过，我不认为最后一部分很干净。

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在最新的支持库中，这里讨论的解决方案都不再需要了。您可以使用 FragmentTransaction 随意使用 Activity 的片段。只需确保可以使用 id 或标签来识别您的片段。

只要您不尝试在每次调用 onCreate() 时重新创建它们，片段就会自动恢复。相反，您应该检查 savedInstanceState 是否不为空，并在这种情况下找到对已创建片段的旧引用。

这是一个例子：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    if (savedInstanceState == null) {
        myFragment = MyFragment.newInstance();
        getSupportFragmentManager()
                .beginTransaction()
                .add(R.id.my_container, myFragment, MY_FRAGMENT_TAG)
                .commit();
    } else {
        myFragment = (MyFragment) getSupportFragmentManager()
                .findFragmentByTag(MY_FRAGMENT_TAG);
    }
...
}

但是请注意，在恢复片段的隐藏状态时当前存在 bug。如果您在活动中隐藏片段，则在这种情况下您需要手动恢复此状态。

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我只想提供我想出的解决方案，它可以处理我从 Vasek 和 devconsole 派生的这篇文章中提出的所有案例。此解决方案还可以处理手机多次旋转而片段不可见的特殊情况。

这是我存储包以供以后使用，因为 onCreate 和 onSaveInstanceState 是在片段不可见时进行的唯一调用

MyObject myObject;
private Bundle savedState = null;
private boolean createdStateInDestroyView;
private static final String SAVED_BUNDLE_TAG = "saved_bundle";

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (savedInstanceState != null) {
        savedState = savedInstanceState.getBundle(SAVED_BUNDLE_TAG);
    }
}

由于在特殊的旋转情况下不调用destroyView，我们可以确定如果它创建了我们应该使用它的状态。

@Override
public void onDestroyView() {
    super.onDestroyView();
    savedState = saveState();
    createdStateInDestroyView = true;
    myObject = null;
}

这部分是一样的。

private Bundle saveState() { 
    Bundle state = new Bundle();
    state.putSerializable(SAVED_BUNDLE_TAG, myObject);
    return state;
}

现在这是棘手的部分。在我的 onActivityCreated 方法中，我实例化了“myObject”变量，但旋转发生在 onActivity 并且 onCreateView 不会被调用。因此，当方向旋转不止一次时，myObject 在这种情况下将为空。我通过重用保存在 onCreate 中的同一个包作为输出包来解决这个问题。

    @Override
public void onSaveInstanceState(Bundle outState) {

    if (myObject == null) {
        outState.putBundle(SAVED_BUNDLE_TAG, savedState);
    } else {
        outState.putBundle(SAVED_BUNDLE_TAG, createdStateInDestroyView ? savedState : saveState());
    }
    createdStateInDestroyView = false;
    super.onSaveInstanceState(outState);
}

现在无论您想恢复状态，只需使用 savedState 包

  @Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    ...
    if(savedState != null) {
        myObject = (MyObject) savedState.getSerializable(SAVED_BUNDLE_TAG);
    }
    ...
}

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感谢 DroidT，我做到了：

我意识到如果 Fragment 不执行 onCreateView()，它的视图就不会被实例化。因此，如果返回堆栈上的片段没有创建它的视图，我会保存最后存储的状态，否则我会使用我想要保存/恢复的数据构建我自己的包。

1）扩展这个类：

import android.os.Bundle;
import android.support.v4.app.Fragment;

public abstract class StatefulFragment extends Fragment {

    private Bundle savedState;
    private boolean saved;
    private static final String _FRAGMENT_STATE = "FRAGMENT_STATE";

    @Override
    public void onSaveInstanceState(Bundle state) {
        if (getView() == null) {
            state.putBundle(_FRAGMENT_STATE, savedState);
        } else {
            Bundle bundle = saved ? savedState : getStateToSave();

            state.putBundle(_FRAGMENT_STATE, bundle);
        }

        saved = false;

        super.onSaveInstanceState(state);
    }

    @Override
    public void onCreate(Bundle state) {
        super.onCreate(state);

        if (state != null) {
            savedState = state.getBundle(_FRAGMENT_STATE);
        }
    }

    @Override
    public void onDestroyView() {
        savedState = getStateToSave();
        saved = true;

        super.onDestroyView();
    }

    protected Bundle getSavedState() {
        return savedState;
    }

    protected abstract boolean hasSavedState();

    protected abstract Bundle getStateToSave();

}

2）在你的片段中，你必须有这个：

@Override
protected boolean hasSavedState() {
    Bundle state = getSavedState();

    if (state == null) {
        return false;
    }

    //restore your data here

    return true;
}

3）比如可以在onActivityCreated中调用hasSavedState：

@Override
public void onActivityCreated(Bundle state) {
    super.onActivityCreated(state);

    if (hasSavedState()) {
        return;
    }

    //your code here
}

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final FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.hide(currentFragment);
ft.add(R.id.content_frame, newFragment.newInstance(context), "Profile");
ft.addToBackStack(null);
ft.commit();

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