片段onResume从后堆栈

我正在使用兼容性包在Android 2.2上使用Fragments。使用片段并将片段之间的过渡添加到后台时，我想实现活动的onResume相同的行为，即，每当片段从弹出窗口中移到“前景”（用户可见）时，回栈，我想在片段中激活某种回调（例如，对共享UI资源执行某些更改）。

我看到片段框架中没有内置的回调。为了达到这个目的，有什么好的做法吗？

由于缺少更好的解决方案，我为我准备了这个工作：假设我有1个活动（MyActivity），并且相互替换的片段很少（一次只能看到一个）。

在MyActivity中，添加以下侦听器：

getSupportFragmentManager().addOnBackStackChangedListener(getListener());

（如您所见，我正在使用兼容软件包）。

getListener实现：

private OnBackStackChangedListener getListener()
    {
        OnBackStackChangedListener result = new OnBackStackChangedListener()
        {
            public void onBackStackChanged() 
            {                   
                FragmentManager manager = getSupportFragmentManager();

                if (manager != null)
                {
                    MyFragment currFrag = (MyFragment) manager.findFragmentById(R.id.fragmentItem);

                    currFrag.onFragmentResume();
                }                   
            }
        };

        return result;
    }

MyFragment.onFragmentResume()按下“返回”后将被调用。一些注意事项：

1. 假设您已将所有交易添加到后台（使用 FragmentTransaction.addToBackStack()）
2. 每次更改堆栈时都会激活它（您可以将其他内容存储在后堆栈中，例如动画），因此您可能会收到同一片段实例的多个调用。

我对建议的解决方案做了一些更改。这样对我更好地工作：

private OnBackStackChangedListener getListener() {
    OnBackStackChangedListener result = new OnBackStackChangedListener() {
        public void onBackStackChanged() {
            FragmentManager manager = getSupportFragmentManager();
            if (manager != null) {
                int backStackEntryCount = manager.getBackStackEntryCount();
                if (backStackEntryCount == 0) {
                    finish();
                }
                Fragment fragment = manager.getFragments()
                                           .get(backStackEntryCount - 1);
                fragment.onResume();
            }
        }
    };
    return result;
}

之后，popStackBack()您可以onHiddenChanged(boolean hidden)在片段中使用以下回调：

Android Developers的以下部分介绍了一种通信机制，该机制创建活动的事件回调。引用其中的一行：

做到这一点的一种好方法是在片段内定义一个回调接口，并要求主机活动实现它。当活动通过接口收到回调时，它可以根据需要与布局中的其他片段共享信息。

编辑： 该片段有一个onStart(...)当用户可见该片段时调用的。同样，onResume(...)当可见且正在运行时。这些与他们的活动对应物相关。简而言之：使用onResume()

在我的活动中onCreate（）

getSupportFragmentManager().addOnBackStackChangedListener(getListener());

使用此方法捕获特定的Fragment并调用onResume（）

private FragmentManager.OnBackStackChangedListener getListener()
    {
        FragmentManager.OnBackStackChangedListener result = new FragmentManager.OnBackStackChangedListener()
        {
            public void onBackStackChanged()
            {
                Fragment currentFragment = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
                if (currentFragment instanceof YOURFRAGMENT) {
                    currentFragment.onResume();
                }
            }
        };

        return result;
    }

有所改进，并纳入了管理器解决方案。

要记住的事情。FragmentManager不是单例，它仅管理Activity中的片段，因此在每个Activity中它都是新的。另外，到目前为止，该解决方案并未考虑使用ViewPager来调用setUserVisibleHint（）方法来帮助控制Fragment的可见性。

处理此问题时，可以随意使用以下类（使用Dagger2注入）。致电活动：

//inject FragmentBackstackStateManager instance to myFragmentBackstackStateManager
FragmentManager fragmentManager = getSupportFragmentManager(); 
myFragmentBackstackStateManager.apply(fragmentManager);

FragmentBackstackStateManager.java：

@Singleton
public class FragmentBackstackStateManager {

    private FragmentManager fragmentManager;

    @Inject
    public FragmentBackstackStateManager() {
    }

    private BackstackCallback backstackCallbackImpl = new BackstackCallback() {
        @Override
        public void onFragmentPushed(Fragment parentFragment) {
            parentFragment.onPause();
        }

        @Override
        public void onFragmentPopped(Fragment parentFragment) {
            parentFragment.onResume();
        }
    };

    public FragmentBackstackChangeListenerImpl getListener() {
        return new FragmentBackstackChangeListenerImpl(fragmentManager, backstackCallbackImpl);
    }

    public void apply(FragmentManager fragmentManager) {
        this.fragmentManager = fragmentManager;
        fragmentManager.addOnBackStackChangedListener(getListener());
    }
}

FragmentBackstackChangeListenerImpl.java：

public class FragmentBackstackChangeListenerImpl implements FragmentManager.OnBackStackChangedListener {

    private int lastBackStackEntryCount = 0;
    private final FragmentManager fragmentManager;
    private final BackstackCallback backstackChangeListener;

    public FragmentBackstackChangeListenerImpl(FragmentManager fragmentManager, BackstackCallback backstackChangeListener) {
        this.fragmentManager = fragmentManager;
        this.backstackChangeListener = backstackChangeListener;
        lastBackStackEntryCount = fragmentManager.getBackStackEntryCount();
    }

    private boolean wasPushed(int backStackEntryCount) {
        return lastBackStackEntryCount < backStackEntryCount;
    }

    private boolean wasPopped(int backStackEntryCount) {
        return lastBackStackEntryCount > backStackEntryCount;
    }

    private boolean haveFragments() {
        List<Fragment> fragmentList = fragmentManager.getFragments();
        return fragmentList != null && !fragmentList.isEmpty();
    }


    /**
     * If we push a fragment to backstack then parent would be the one before => size - 2
     * If we pop a fragment from backstack logically it should be the last fragment in the list, but in Android popping a fragment just makes list entry null keeping list size intact, thus it's also size - 2
     *
     * @return fragment that is parent to the one that is pushed to or popped from back stack
     */
    private Fragment getParentFragment() {
        List<Fragment> fragmentList = fragmentManager.getFragments();
        return fragmentList.get(Math.max(0, fragmentList.size() - 2));
    }

    @Override
    public void onBackStackChanged() {
        int currentBackStackEntryCount = fragmentManager.getBackStackEntryCount();
        if (haveFragments()) {
            Fragment parentFragment = getParentFragment();

            //will be null if was just popped and was last in the stack
            if (parentFragment != null) {
                if (wasPushed(currentBackStackEntryCount)) {
                    backstackChangeListener.onFragmentPushed(parentFragment);
                } else if (wasPopped(currentBackStackEntryCount)) {
                    backstackChangeListener.onFragmentPopped(parentFragment);
                }
            }
        }

        lastBackStackEntryCount = currentBackStackEntryCount;
    }
}

BackstackCallback.java：

public interface BackstackCallback {
    void onFragmentPushed(Fragment parentFragment);

    void onFragmentPopped(Fragment parentFragment);
}

如果将片段放回堆栈，Android只会破坏其视图。片段实例本身不会被杀死。一种简单的启动方法应该是监听onViewCreated事件，在该事件中放置“ onResume（）”逻辑。

boolean fragmentAlreadyLoaded = false;
    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onViewCreated(view, savedInstanceState);

        if (savedInstanceState == null && !fragmentAlreadyLoaded) {
            fragmentAlreadyLoaded = true;

            // Code placed here will be executed once
        }

        //Code placed here will be executed even when the fragment comes from backstack
    }

如果片段已附加到活动中，则可以调用onResume（）来获得正确的答案。或者，您可以使用onAttach和onDetach

该片段的onResume（）可以正常工作...

public class listBook extends Fragment {

    private String listbook_last_subtitle;
...

    @Override
       public void onCreate(Bundle savedInstanceState) {

        String thisFragSubtitle = (String) getActivity().getActionBar().getSubtitle();
        listbook_last_subtitle = thisFragSubtitle;
       }
...

    @Override
        public void onResume(){
            super.onResume();
            getActivity().getActionBar().setSubtitle(listbook_last_subtitle);
        }
...

public abstract class RootFragment extends Fragment implements OnBackPressListener {

 @Override
 public boolean onBackPressed() {
  return new BackPressImpl(this).onBackPressed();
 }

 public abstract void OnRefreshUI();

}


public class BackPressImpl implements OnBackPressListener {

 private Fragment parentFragment;

 public BackPressImpl(Fragment parentFragment) {
  this.parentFragment = parentFragment;
 }

 @Override
 public boolean onBackPressed() {
  ((RootFragment) parentFragment).OnRefreshUI();
 }
}

最后是RootFragment的Frament，以查看效果

我的解决方法是在“片段”中获取操作栏的当前标题，然后再将其设置为新标题。这样，一旦片段被弹出，我就可以改回到那个标题。

@Override
public void onResume() {
    super.onResume();
    // Get/Backup current title
    mTitle = ((ActionBarActivity) getActivity()).getSupportActionBar()
            .getTitle();
    // Set new title
    ((ActionBarActivity) getActivity()).getSupportActionBar()
        .setTitle(R.string.this_fragment_title);
}

@Override
public void onDestroy() {
    // Set title back
    ((ActionBarActivity) getActivity()).getSupportActionBar()
        .setTitle(mTitle);

    super.onDestroy();
}

我用枚举FragmentTags定义了我所有的片段类。

TAG_FOR_FRAGMENT_A(A.class),
TAG_FOR_FRAGMENT_B(B.class),
TAG_FOR_FRAGMENT_C(C.class)

通过 FragmentTags.TAG_FOR_FRAGMENT_A.name()作为片段标签。

现在

@Override
public void onBackPressed(){
   FragmentManager fragmentManager = getFragmentManager();
   Fragment current
   = fragmentManager.findFragmentById(R.id.fragment_container);
    FragmentTags fragmentTag = FragmentTags.valueOf(current.getTag());

  switch(fragmentTag){
    case TAG_FOR_FRAGMENT_A:
        finish();
        break;
   case TAG_FOR_FRAGMENT_B:
        fragmentManager.popBackStack();
        break;
   case default: 
   break;
}