我们真的应该在 onActivityCreated 中调用 getLoaderManager().initLoader，这会导致 onLoadFinished 被调用两次

Question

Google 建议我们在 Fragment 的 onActivityCreated 中调用 getLoaderManager().initLoader(0, null, this);

http://developer.android.com/reference/android/content/AsyncTaskLoader.html

但是，这会产生以下问题：onLoadFinished 将在配置更改（轮换）期间被调用两次

我们可以模拟如下问题。

代码

package org.yccheok.gui;

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.AsyncTaskLoader;
import android.support.v4.content.Loader;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.actionbarsherlock.app.SherlockFragment;

public class HomeMenuFragment extends SherlockFragment implements LoaderManager.LoaderCallbacks<HomeMenuFragment.Infos> {
    private static class InfosLoader extends AsyncTaskLoader<Infos> {

        private Infos infos = null;

        public InfosLoader(Context context) {
            super(context);
        }

        @Override
        public Infos loadInBackground() {
            Log.i(TAG, "loadInBackground");

            this.infos = Infos.newInstance();
            return infos;
        }

        /**
         * Handles a request to cancel a load.
         */
        @Override 
        public void onCanceled(Infos infos) {
            super.onCanceled(infos);
        }

        /**
         * Handles a request to stop the Loader.
         * Automatically called by LoaderManager via stopLoading.
         */
        @Override 
        protected void onStopLoading() {
            // Attempt to cancel the current load task if possible.
            cancelLoad();
        }

        /**
         * Handles a request to start the Loader.
         * Automatically called by LoaderManager via startLoading.
         */
        @Override        
        protected void onStartLoading() {
            if (this.infos != null) {
                Log.i(TAG, "deliverResult");
                deliverResult(this.infos);
            }

            if (takeContentChanged() || this.infos == null) {
                Log.i(TAG, "forceLoad");
                forceLoad();
            }
        }

        /**
         * Handles a request to completely reset the Loader.
         * Automatically called by LoaderManager via reset.
         */
        @Override 
        protected void onReset() {
            super.onReset();

            // Ensure the loader is stopped
            onStopLoading();

            // At this point we can release the resources associated with 'apps'
            // if needed.
            this.infos = null;
        }        
    }

    static class Infos {

        private Infos() {
        }

        public static Infos newInstance() {
            return new Infos();
        }
    }

    @Override
    public void onActivityCreated (Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Log.i(TAG, "onActivityCreated");
        // Prepare the loader.  Either re-connect with an existing one,
        // or start a new one.
        getLoaderManager().initLoader(0, null, this);
    }

    @Override
    public Loader<Infos> onCreateLoader(int arg0, Bundle arg1) {
        return new InfosLoader(this.getSherlockActivity());
    }

    @Override
    public void onLoadFinished(Loader<Infos> arg0, Infos arg1) {
        Log.i(TAG, "onLoadFinished! -> " + arg1);
    }

    @Override
    public void onLoaderReset(Loader<Infos> arg0) {
    }

    public void reloadAfterOpenFromCloud() {
        this.getLoaderManager().getLoader(0).onContentChanged();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.home_menu, container, false);
        return v;
    }

    private static final String TAG = HomeMenuFragment.class.getSimpleName();
}

记录

I/HomeMenuFragment(14776): onActivityCreated
I/HomeMenuFragment(14776): forceLoad
I/HomeMenuFragment(14776): loadInBackground
I/HomeMenuFragment(14776): onLoadFinished! -> org.yccheok.gui.HomeMenuFragment$Infos@4195ad58

[Rotation happens right here]

I/HomeMenuFragment(14776): onActivityCreated
I/HomeMenuFragment(14776): onLoadFinished! -> org.yccheok.gui.HomeMenuFragment$Infos@4195ad58
I/HomeMenuFragment(14776): onLoadFinished! -> org.yccheok.gui.HomeMenuFragment$Infos@4195ad58

根据Android: LoaderCallbacks.OnLoadFinished called twice，建议的解决方案之一是在onResume 中调用initLoader。

@Override
public void onActivityCreated (Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    Log.i(TAG, "onActivityCreated");
    //getLoaderManager().initLoader(0, null, this);
}

@Override
public void onResume()
{
    super.onResume();
    Log.i(TAG, "onResume");
    // Prepare the loader.  Either re-connect with an existing one,
    // or start a new one.
    getLoaderManager().initLoader(0, null, this);
}

这里是日志。在我们将initLoader 移动到onResume 之后，现在看起来OK。

记录

I/HomeMenuFragment(15468): onActivityCreated
I/HomeMenuFragment(15468): onResume
I/HomeMenuFragment(15468): forceLoad
I/HomeMenuFragment(15468): loadInBackground
I/HomeMenuFragment(15468): onLoadFinished! -> org.yccheok.gui.HomeMenuFragment$Infos@4195aed0


I/HomeMenuFragment(15468): onActivityCreated
I/HomeMenuFragment(15468): onResume
I/HomeMenuFragment(15468): onLoadFinished! -> org.yccheok.gui.HomeMenuFragment$Infos@4195aed0

我想知道

1. 为什么建议的解决方案有效？
2. 这是一个错误吗？我们是否应该向 Google 提交有关此行为的错误？也许有人向 Google 提交了罚单，但我找不到。

Answer 1

建议的解决方案为何有效

如果我们在onActivityCreated() 中调用getLoaderManager()，那么我们会初始化变量Fragment.mLoaderManager。

因此，我们在FragmentActivity.onStart() 上调用了mLoaderManager.doReportStart() Fragment.performStart()：

  void performStart() {
    if (mChildFragmentManager != null) {
        mChildFragmentManager.noteStateNotSaved();
        mChildFragmentManager.execPendingActions();
    }
    mCalled = false;
    onStart();
    if (!mCalled) {
        throw new SuperNotCalledException("Fragment " + this
                + " did not call through to super.onStart()");
    }
    if (mChildFragmentManager != null) {
        mChildFragmentManager.dispatchStart();
    }
    if (mLoaderManager != null) {
        mLoaderManager.doReportStart();
    }
}

这是onLoadFinished()第一次调用的原因。

稍后在FragmentActivity.onStart() 中，我们调用了lm.finishRetain()（参见代码 sn-p）：

 if (mAllLoaderManagers != null) {
     LoaderManagerImpl loaders[] = new LoaderManagerImpl[mAllLoaderManagers.size()];
     mAllLoaderManagers.values().toArray(loaders);
     if (loaders != null) {
         for (int i=0; i<loaders.length; i++) {
             LoaderManagerImpl lm = loaders[i];
             lm.finishRetain();
             lm.doReportStart();
         }
     }
 }

这是onLoadFinished()第二次调用的原因。

---

好的。现在考虑我们在onResume()中调用getLoaderManager().initLoader(0, null, this)的情况：

如果我们这样做，在onActivityCreated() 之后我们既没有mLoaderManager.doReportStart() 也没有lm.finishRetain()，而是在initLoader() 期间有onLoadFinished() 调用：

public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
    if (mCreatingLoader) {
        throw new IllegalStateException("Called while creating a loader");
    }

    LoaderInfo info = mLoaders.get(id);

    if (DEBUG) Log.v(TAG, "initLoader in " + this + ": args=" + args);

    if (info == null) {
        // Loader doesn't already exist; create.
        info = createAndInstallLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        if (DEBUG) Log.v(TAG, "  Created new loader " + info);
    } else {
        if (DEBUG) Log.v(TAG, "  Re-using existing loader " + info);
        info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;
    }

    if (info.mHaveData && mStarted) {
        // If the loader has already generated its data, report it now.
        info.callOnLoadFinished(info.mLoader, info.mData);
    }

    return (Loader<D>)info.mLoader;
}

在这个sn-p中可以看到info.callOnLoadFinished()调用：

if (info.mHaveData && mStarted) {
     // If the loader has already generated its data, report it now.
     info.callOnLoadFinished(info.mLoader, info.mData);
}

我认为很清楚:)

Answer 2

尝试从您的onStartLoading 中删除传递结果。当 initLoader 为已经返回的加载程序调用时，LoaderManager 已经返回现有值。