当 Activity 刚刚将 fragment 添加到返回堆栈时，我遇到了来自 Fragment.isRemoving() 的不一致返回值。 第一次 fragment 因配置更改而被临时销毁，isRemoving()返回true。如果 fragment 被第二次临时销毁，isRemoving() 返回 false!

我的代码:

public class MainActivityFragment extends Fragment {
    private static final String TAG = "MainActivityFragment";
    private static final String LEVEL = "MainActivityFragment.LEVEL";

    public MainActivityFragment() {
    }

    public static MainActivityFragment newInstance(int n) {
        MainActivityFragment f = new MainActivityFragment();
        f.setArguments(new Bundle());
        f.getArguments().putInt(LEVEL, n);
        return f;
    }

    private int getLevel() {
        return (getArguments() == null) ? 0 : getArguments().getInt(LEVEL);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);

        Button button = (Button) rootView.findViewById(R.id.button);

        button.setText(String.valueOf(getLevel()));

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                getActivity().getSupportFragmentManager()
                        .beginTransaction()
                        .replace(R.id.fragment, MainActivityFragment.newInstance(getLevel() + 1))
                        .addToBackStack(null)
                        .commit();
            }
        });

        return rootView;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.i(TAG, String.valueOf(getLevel()) + ": onCreate");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(TAG, String.valueOf(getLevel()) + ": onDestroy");
        Log.i(TAG, String.valueOf(getLevel()) + ": isChangingConfigurations() == " + getActivity().isChangingConfigurations());
        Log.i(TAG, String.valueOf(getLevel()) + ": isRemoving() == " + isRemoving());
    }

日志(以#开头的行是我的评论):

# Start Activity
I/MainActivityFragment: 0: onCreate
# Click button in fragment 0 to add it to back stack and replace it with fragment 1
I/MainActivityFragment: 1: onCreate
# Rotate the device
I/MainActivityFragment: 0: onDestroy
I/MainActivityFragment: 0: isChangingConfigurations() == true
I/MainActivityFragment: 0: isRemoving() == true # ???????
I/MainActivityFragment: 1: onDestroy
I/MainActivityFragment: 1: isChangingConfigurations() == true
I/MainActivityFragment: 1: isRemoving() == false
I/MainActivityFragment: 0: onCreate
I/MainActivityFragment: 1: onCreate
# Rotate the device a second time
I/MainActivityFragment: 0: onDestroy
I/MainActivityFragment: 0: isChangingConfigurations() == true
I/MainActivityFragment: 0: isRemoving() == false # Correct result
I/MainActivityFragment: 1: onDestroy
I/MainActivityFragment: 1: isChangingConfigurations() == true
I/MainActivityFragment: 1: isRemoving() == false
I/MainActivityFragment: 0: onCreate
I/MainActivityFragment: 1: onCreate
# Click button in fragment 1 to add it to back stack and replace it with fragment 2
I/MainActivityFragment: 2: onCreate
# Rotate the device
I/MainActivityFragment: 0: onDestroy
I/MainActivityFragment: 0: isChangingConfigurations() == true
I/MainActivityFragment: 0: isRemoving() == false # Ok, correct
I/MainActivityFragment: 1: onDestroy
I/MainActivityFragment: 1: isChangingConfigurations() == true
I/MainActivityFragment: 1: isRemoving() == true # WHY????
I/MainActivityFragment: 2: onDestroy
I/MainActivityFragment: 2: isChangingConfigurations() == true
I/MainActivityFragment: 2: isRemoving() == false
I/MainActivityFragment: 0: onCreate
I/MainActivityFragment: 1: onCreate
I/MainActivityFragment: 2: onCreate

这是 Android 中的错误还是我理解错了？

更新:我在 onDestroy 中添加了对 Fragment.dump() 的调用，得到以下结果:

fragment 放入回栈之前:

mFragmentId=#7f0c006b mContainerId=#7f0c006b mTag=null
mState=2 mIndex=0 mWho=android:fragment:0 mBackStackNesting=0
mAdded=true mRemoving=false mResumed=false mFromLayout=false mInLayout=false
mHidden=false mDetached=false mMenuVisible=true mHasMenu=false
mRetainInstance=false mRetaining=false mUserVisibleHint=true
mFragmentManager=FragmentManager{336d670b in HostCallbacks{387c69e8}}
mHost=android.support.v4.app.FragmentActivity$HostCallbacks@387c69e8
mSavedViewState={2131492979=android.view.AbsSavedState$1@6adf801}
  Child FragmentManager{2b6916a6 in null}}:
    FragmentManager misc state:
    mHost=null
    mContainer=null
    mCurState=0 mStateSaved=true mDestroyed=true

fragment放入回栈并第一次销毁后:

mFragmentId=#7f0c006b mContainerId=#7f0c006b mTag=null
mState=1 mIndex=0 mWho=android:fragment:0 mBackStackNesting=1
mAdded=false mRemoving=true mResumed=false mFromLayout=false mInLayout=false
mHidden=false mDetached=false mMenuVisible=true mHasMenu=false
mRetainInstance=false mRetaining=false mUserVisibleHint=true
mFragmentManager=FragmentManager{34638ae1 in HostCallbacks{2db8e006}}
mHost=android.support.v4.app.FragmentActivity$HostCallbacks@2db8e006
mSavedViewState={2131492979=android.view.AbsSavedState$1@6adf801}
Child FragmentManager{169d66c7 in null}}:
  FragmentManager misc state:
    mHost=null
    mContainer=null
    mCurState=0 mStateSaved=true mDestroyed=true

第二次销毁:

mFragmentId=#7f0c006b mContainerId=#7f0c006b mTag=null
mState=1 mIndex=0 mWho=android:fragment:0 mBackStackNesting=1
mAdded=false mRemoving=false mResumed=false mFromLayout=false mInLayout=false
mHidden=false mDetached=false mMenuVisible=true mHasMenu=false
mRetainInstance=false mRetaining=false mUserVisibleHint=true
mFragmentManager=FragmentManager{23beb2bc in HostCallbacks{c0f9245}}
mHost=android.support.v4.app.FragmentActivity$HostCallbacks@c0f9245
mSavedFragmentState=Bundle[{android:view_state={2131492979=android.view.AbsSavedState$1@6adf801}}]
mSavedViewState={2131492979=android.view.AbsSavedState$1@6adf801}

第一个(尚未入栈)和第二个(入栈)之间的区别是:

1. mState=2 (ACTIVITY_CREATED) 与 mState=1 (CREATED)
2. mBackStackNesting=0 与 mBackStackNesting=1
3. mAdded=true 与 mAdded=false
4. mRemoving=false 与 mRemoving=true(显然)

第二次(第一次销毁)和第三次(第二次+次销毁)的区别是:

1. mRemoving=true 与 mRemoving=false
2. mSavedFragmentState=null 与 mSavedFragmentState=Bundle[...]
3. 有子 FragmentManager 与没有子 FragmentManager

但是，我不知道如何解释这些结果。

我开始认为 isRemoving 不是我需要的(我真正需要的是相当于 Activity.isFinishing 的东西，但对于 fragment 。我需要知道“这个 fragment 将永远再次被重用”，所以我可以取消后台任务。现在我正在使用 isRemoving() && !getActivity().isChangingConfigurations() 但是我不确定这是正确的解决方案)。

最佳答案

原来不是很正确的答案

我不确定这是错误还是设计使然，但 fragment 仅在 FragmentManager.removeFragment 中设置为删除支持v4库v23.1.1的方法。

这很可能会有所不同，具体取决于您是否使用支持库以及版本，但对于您在 GitHub 存储库中的代码，这就是原因。

只有在删除已放置在后堆栈上的 fragment 时才会调用此方法。

这里是完整的方法供引用:

public void removeFragment(Fragment fragment, int transition, int transitionStyle) {
    if (DEBUG) Log.v(TAG, "remove: " + fragment + " nesting=" + fragment.mBackStackNesting);
    final boolean inactive = !fragment.isInBackStack();
    if (!fragment.mDetached || inactive) {
        if (mAdded != null) {
            mAdded.remove(fragment);
        }
        if (fragment.mHasMenu && fragment.mMenuVisible) {
            mNeedMenuInvalidate = true;
        }
        fragment.mAdded = false;
        fragment.mRemoving = true;
        moveToState(fragment, inactive ? Fragment.INITIALIZING : Fragment.CREATED,
                transition, transitionStyle, false);
    }
}

“如何知道这个 fragment 永远不会被再次使用”问题的可能答案

要回答您关于如何知道可以在 fragment 中取消后台任务的问题，通常这些 fragment 使用 setRetainInstance(true)

这样，当设备的方向改变时，相同的 Fragment 将被重复使用，并且可以保留任何正在进行的后台操作。

当保留实例为真时， fragment 的 onDestroy()在方向更改期间也不会调用方法，因此您可以将取消逻辑放在那里以了解 fragment 是否会永久消失。

isRemoving 工作原理的更好答案基于查看源代码

看到这个答案已被接受，我觉得我应该从我的原始答案中修正一些不准确的地方。我说“只有在删除已放置在后堆栈上的 fragment 时才调用此方法”，这并不完全正确。替换一个 fragment 也调用了该方法并正确地将 isRemoving 设置为 true 作为一个示例。

现在通过分析您的日志来回答您关于为什么 isRemoving 在轮换中出现不一致的问题。我的附加评论以##开头

# Start Activity
# Click button in fragment 0 to add it to back stack and replace it with fragment 1
## FragmentManager.removeFragment is called on fragment 0 setting mRemoving to true
I/MainActivityFragment: 1: onCreate
# Rotate the device
I/MainActivityFragment: 0: onDestroy
I/MainActivityFragment: 0: isChangingConfigurations() == true
I/MainActivityFragment: 0: isRemoving() == true ## To emphasize, this is true because as soon as you replaced fragment 0 it was set to true in the FragmentManager.removeFragment method.
I/MainActivityFragment: 1: onDestroy
I/MainActivityFragment: 1: isChangingConfigurations() == true
I/MainActivityFragment: 1: isRemoving() == false ## fragment 1 is never actually removed so mRemoving is false.
I/MainActivityFragment: 0: onCreate
I/MainActivityFragment: 1: onCreate

# Rotate the device a second time
## after rotating the device the first time your same fragments are not reused but new instances are created. This resets all the internal state of the fragments so mRemoving is false for all fragments.
I/MainActivityFragment: 0: onDestroy
I/MainActivityFragment: 0: isChangingConfigurations() == true
I/MainActivityFragment: 0: isRemoving() == false # Correct result
I/MainActivityFragment: 1: onDestroy
I/MainActivityFragment: 1: isChangingConfigurations() == true
I/MainActivityFragment: 1: isRemoving() == false
I/MainActivityFragment: 0: onCreate
I/MainActivityFragment: 1: onCreate
# Click button in fragment 1 to add it to back stack and replace it with fragment 2
## fragment 1 now has mRemoving set to true in FragmentManager.removeFragment
I/MainActivityFragment: 2: onCreate
# Rotate the device
I/MainActivityFragment: 0: onDestroy
I/MainActivityFragment: 0: isChangingConfigurations() == true
I/MainActivityFragment: 0: isRemoving() == false ## still false from prior rotation
I/MainActivityFragment: 1: onDestroy
I/MainActivityFragment: 1: isChangingConfigurations() == true
I/MainActivityFragment: 1: isRemoving() == true ## true because mRemoving was set to true in FragmentManager.removeFragment.
I/MainActivityFragment: 2: onDestroy
I/MainActivityFragment: 2: isChangingConfigurations() == true
I/MainActivityFragment: 2: isRemoving() == false
I/MainActivityFragment: 0: onCreate
I/MainActivityFragment: 1: onCreate
I/MainActivityFragment: 2: onCreate

如果您再次旋转设备，所有 fragment 都会从 isRemoving() 返回 false。

有趣的是，即使使用相同的 fragment 实例，您仍然可能得到相同的输出。 Fragment 中有一个方法类名为 initState有以下评论:

Called by the fragment manager once this fragment has been removed, so that we don't have any left-over state if the application decides to re-use the instance. This only clears state that the framework internally manages, not things the application sets.

在旋转期间为每个 fragment 调用一次此方法，它所做的一件事就是将 mRemoving 重置为 false。

关于android - fragment 回栈和 isRemoving()，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/34649126/