避免在执行选项卡切换时重新创建相同的视图 [英] Avoid recreating same view when perform tab switching

问题描述

当前，我有2个 Fragments ，可以通过 ActionBar 的选项卡进行切换.

  getSupportActionBar().setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);ActionBar.Tab newTab = getSupportActionBar().newTab();newTab.setText("history");newTab.setTabListener(new TabListenerHistoryFragment>(this，"history"，HistoryFragment.class)); 

---

  @Overridepublic void onTabSelected(标签"标签，FragmentTransaction ft){//检查片段是否已经初始化如果(mFragment == null){//如果没有，请实例化并将其添加到活动中mFragment = Fragment.instantiate(mActivity，mClass.getName());mFragment.setRetainInstance(true);ft.add(android.R.id.content，mFragment，mTag);} 别的 {//如果存在，只需附加它以显示它ft.attach(mFragment);}} 

我意识到第一次启动我的 Activity (此活动包含2个片段)，将按以下顺序调用 Fragment s的方法.

onCreate->onCreateView->onStart

当我执行Tab切换，然后再将Tab切换回相同的Fragment时，将再次调用以下方法.

onCreateView->onStart

当切换回Tab时，我只希望保留相同的GUI视图状态.

• 我希望图表继续放大到上一级.
• 我希望图表水平滚动保持在先前的水平.
• 我希望我的列表继续滚动以保持在上一级.
• ...

我知道在切换Tab时可以使用以下方法保存/恢复简单变量

android片段-如何将片段中的视图状态保存到另一个片段上

但是，这不是我想要的，因为很难在整堆原始值中描述我的GUI状态.

我尝试以下方法.当然，它不会起作用，因为我遇到了以下运行时错误.

 公共类HistoryFragment扩展Fragment {视图view = null;@Override公共视图onCreateView(LayoutInflater充气器，ViewGroup容器，捆绑保存的实例状态){如果(this.view！= null){返回this.view;}this.view = inflater.inflate(R.layout.history_activity，container，false);}} 

java.lang.IllegalStateException:指定的子代已经有一个父代.您必须先在孩子的父母上调用removeView().

我意识到以下演示示例可以在进行Tab切换时保留其片段GUI状态(例如，列表的垂直滚动位置).但是我想，也许是因为他们正在使用ListFragment吗?我发现它们没有执行任何特殊处理来保留GUI状态.

• com.example.android.apis.app.FragmentTabs
• com.example.android.apis.app.LoaderCursor.CursorLoaderListFragment

我可以知道，在执行选项卡切换时如何避免重新创建相同的视图?

解决方案

我遇到了同样的问题，并尝试遵循错误消息中的建议.我尝试了以下代码，并且对我有用.

  public View onCreateView(LayoutInflater充气器，ViewGroup容器，捆绑状态){如果(mMyView == null){mMyView =新的MyView(getActivity());} 别的 {((ViewGroup)mMyView.getParent()).removeView(mMyView);}返回mPuzzleView;} 

Current, I have 2 Fragments, which is switch-able through ActionBar's tab.

    getSupportActionBar().setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    ActionBar.Tab newTab = getSupportActionBar().newTab();
    newTab.setText("history");
    newTab.setTabListener(new TabListenerHistoryFragment>(this, "history",
        HistoryFragment.class));

---

 @Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
    // Check if the fragment is already initialized
    if (mFragment == null) {

        // If not, instantiate and add it to the activity
        mFragment = Fragment.instantiate(mActivity, mClass.getName());
        mFragment.setRetainInstance(true);
        ft.add(android.R.id.content, mFragment, mTag);
    } else {
        // If it exists, simply attach it in order to show it
        ft.attach(mFragment);
    }        
}

I realize the first time of my Activity (This activity is holding 2 fragments) being launched, Fragments' methods will be called in the following sequence.

onCreate -> onCreateView -> onStart

When I perform Tab switching, and then Tab switching back to the same Fragment, the following methods will be called again.

onCreateView -> onStart

I just wish to retain the same GUI view state, when Tab is being switched back.

• I want my chart continue to be zoomed into previous level.
• I want my chart horizontal scroll stay at previous level.
• I want my list continue scroll stay at previous level.
• ...

I know that I can save/restore simple variables using the following method when Tab switching

android fragment- How to save states of views in a fragment when another fragment is pushed on top of it

But, that is not something I want, as my GUI state is pretty difficult to describe within whole bunch of primitive values.

I try the following approach. Of course it won't work, as I am getting the following runtime error.

public class HistoryFragment extends Fragment {
    View view = null;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (this.view != null) {
            return this.view;
        }
        this.view = inflater.inflate(R.layout.history_activity, container, false); 
    }
}

java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.

I realize the following demo example is able to preserve its fragment GUI state (For instance, the position of vertical scroll of list) when there is Tab switching. But I guess, perhaps it is because they are using ListFragment? As I do not find they perform any special handling to preserve GUI state.

• com.example.android.apis.app.FragmentTabs
• com.example.android.apis.app.LoaderCursor.CursorLoaderListFragment

May I know, how I can avoid from recreating same view when perform tab switching？

解决方案

I had the same problem, and tried to follow the suggestion in the error message. I tried the following code, and it worked for me.

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle state)  {
     if (mMyView == null) {
         mMyView = new MyView(getActivity());
     } else {
         ((ViewGroup) mMyView.getParent()).removeView(mMyView);
     }

     return mPuzzleView; 
}

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