android - 保留 fragment 泄漏

Question

我正在使用带有保留 fragment 的简单 Activity ，该 fragment 包含 Activity 使用的一些数据。保留的 fragment 使用加载器从内容提供者获取数据。在配置更改(屏幕旋转)时， Activity 被重新创建并且旧实例被泄漏，如 LeakCanary 库所报告的那样(保留 fragment -> 加载程序管理器 -> 旧 Activity )。这与 support-v4 23.0.0 库(以及以前的版本)一起复制。复制泄漏的保留 fragment 的 Activity 示例(此处没有有用的代码，仅用于演示泄漏):

package com.leaksample;

import android.database.Cursor;
import android.os.Bundle;
import android.provider.MediaStore;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.CursorLoader;
import android.support.v4.content.Loader;
import android.support.v7.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    private ModelFragment mModelFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        FragmentManager fm = getSupportFragmentManager();
        mModelFragment = (ModelFragment) fm.findFragmentByTag(ModelFragment.TAG);
        if (mModelFragment == null) {
            mModelFragment = new ModelFragment();
            fm.beginTransaction()
                    .add(mModelFragment, ModelFragment.TAG)
                    .commit();
            fm.executePendingTransactions();
        }
    }

    public static class ModelFragment extends Fragment implements LoaderManager.LoaderCallbacks<Cursor> {
        private static final String TAG = ModelFragment.class.getSimpleName();

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setRetainInstance(true);
            getLoaderManager().initLoader(0, null, this);
        }

        @Override
        public Loader<Cursor> onCreateLoader(int id, Bundle args) {
            return new CursorLoader(getActivity(), MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
                    new String[]{MediaStore.Images.Media.DATA}, null, null, null);
        }

        @Override
        public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
        }

        @Override
        public void onLoaderReset(Loader<Cursor> loader) {
        }
    }
}

来自 LeakCanary 的堆栈:

08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ In com.leaksample:1.0:1.
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * com.leaksample.MainActivity has leaked:
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * GC ROOT thread java.lang.Thread.<Java Local> (named 'Binder_1')
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * references android.view.ViewRootImpl.mContext
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * references com.leaksample.MainActivity.mModelFragment
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * references com.leaksample.MainActivity$ModelFragment.mLoaderManager
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * references android.support.v4.app.LoaderManagerImpl.mHost
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * references android.support.v4.app.FragmentActivity$HostCallbacks.this$0
08-29 19:52:34.129  15271-16632/com.leaksample D/LeakCanary﹕ * leaks com.leaksample.MainActivity instance

也许我做错了什么，忘记调用一些close 或release 方法？我认为将保留的 fragment 与加载程序一起使用是一种常见的模式，在这里不应该是内存泄漏。

Answer 1

您在 ModelFragment 中对 getLoaderManager() 的调用最终会创建一个新的 LoaderManagerImpl 实例，其中包含对 FragmentActivity 的引用。见下文:

// FragmentActivity.java

LoaderManagerImpl getLoaderManager(String who, boolean started, boolean create) {
        if (mAllLoaderManagers == null) {
            mAllLoaderManagers = new SimpleArrayMap<String, LoaderManagerImpl>();
        }
        LoaderManagerImpl lm = mAllLoaderManagers.get(who);
        if (lm == null) {
            if (create) {
                lm = new LoaderManagerImpl(who, this, started);
                mAllLoaderManagers.put(who, lm);
            }
        } else {
            lm.updateActivity(this);
        }
        return lm;
    }

从 FragmentActivity 的进一步检查来看，似乎当 Fragment 保留其实例时，加载程序没有被删除，并且仍然持有对被销毁 Activity 的引用。

@Override
    public final Object onRetainNonConfigurationInstance() {
        ...
            for (int i=0; i<N; i++) {
                LoaderManagerImpl lm = loaders[i];
                if (lm.mRetaining) {
                    retainLoaders = true;
                } else {
                    lm.doDestroy();
                    mAllLoaderManagers.remove(lm.mWho);
                }
            }
    ...
    }

解决此问题的一种方法是更改​​ ModelFragment，使 retainInstance 为 false。