Question

当用户单击主页按钮时，我想取消运行AsyncTask（在AsyncTaskLoader中）。这是我到目前为止创建的内容：

package cz.davidliska.android.loaders;

import java.text.SimpleDateFormat;
import java.util.Date;

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.AsyncTaskLoader;
import android.support.v4.content.Loader;
import android.util.Log;

public class MainActivity extends FragmentActivity implements LoaderManager.LoaderCallbacks<Date> {
    private static final String TAG = "loader";
    private Date date;

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

        LoaderManager.enableDebugLogging(true);
        getSupportLoaderManager().initLoader(0, savedInstanceState, this);
    }

    private static class DateLoader extends AsyncTaskLoader<Date> {
        public static final String STATE_DATE_LOADER = "dateloader.state";
        private Date mDate;

        public DateLoader(Context context, Bundle savedInstanceState) {
            super(context);
            if (savedInstanceState != null) {
                long timeStamp = savedInstanceState.getLong(STATE_DATE_LOADER);
                if (timeStamp != 0L) mDate = new Date(timeStamp);
            }
        }

        @Override
        protected void onStartLoading() {
            Log.d(TAG, "Loader.onStartLoading()");
            if (mDate != null) {
                deliverResult(mDate);
            } else {
                forceLoad();
            }
        }

        @Override
        public void deliverResult(Date data) {
            super.deliverResult(data);
            Log.d(TAG, "Loader.deliverResult()");
            mDate = data;
        }

        @Override
        protected void onStopLoading() {
            super.onStopLoading();
            cancelLoad(); // try to cancel AsyncTask
            Log.d(TAG, "Loader.onStopLoading()");
        }


        @Override
        protected void onForceLoad() { // overridden in AsyncTaskLoader
            super.onForceLoad();
            Log.d(TAG, "Loader.onForceLoad()");
        }

        @Override
        protected void onReset() {
            super.onReset();
            mDate = null;
            Log.d(TAG, "Loader.onForceLoad()");
        }

        @Override
        public void onContentChanged() {
            super.onContentChanged();
            Log.d(TAG, "Loader.onContentChanged()");
        }

        @Override
        protected void onAbandon() {
            super.onAbandon();
            Log.d(TAG, "Loader.onContentChanged()");
        }

        @Override
        public Date loadInBackground() { // AsyncTaskLoader only
            Log.d(TAG, "Loader.loadInBackground()");
            try {
                // there will be some HttpClient.execute()
                while(true) {
                    Thread.sleep(100);
                    Log.d(TAG, "Loader.loadInBackground() still running");
                }

            } catch (InterruptedException e) {
                Log.d(TAG, "Loader.loadInBackground() interupted");
            }
            Log.d(TAG, "Loader.loadInBackground() finished");
            return new Date();
        }

        @Override
        public void onCanceled(Date data) { // AsyncTaskLoader only
            super.onCanceled(data);
            Log.d(TAG, "Loader.onCanceled()");
        }

        @Override
        protected Date onLoadInBackground() { // AsyncTaskLoader only
            Log.d(TAG, "Loader.onContentChanged()");
            return super.onLoadInBackground();
        }

        @Override
        public void abandon() {
            Log.d(TAG, "Loader.abandon()");
            super.abandon();
        }

        @Override
        public boolean cancelLoad() {
            Log.d(TAG, "Loader.cancelLoad()");
            return super.cancelLoad();
        }

        @Override
        public void forceLoad() {
            Log.d(TAG, "Loader.forceLoad()");
            super.forceLoad();
        }

        @Override
        public boolean isAbandoned() {
            Log.d(TAG, "Loader.isAbandoned()");
            return super.isAbandoned();
        }

        @Override
        public boolean isReset() {
            Log.d(TAG, "Loader.isReset()");
            return super.isReset();
        }

        @Override
        public boolean isStarted() {
            Log.d(TAG, "Loader.isStarted()");
            return super.isStarted();
        }

        @Override
        public void reset() {
            Log.d(TAG, "Loader.reset()");
            super.reset();
        }

        @Override
        public void stopLoading() {
            Log.d(TAG, "Loader.stopLoading()");
            super.stopLoading();
        }

        @Override
        public boolean takeContentChanged() {
            Log.d(TAG, "Loader.takeContentChanged()");
            return super.takeContentChanged();
        }

    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        if (date != null)
            outState.putLong(DateLoader.STATE_DATE_LOADER, date.getTime());
    }

    public Loader<Date> onCreateLoader(int id, Bundle args) {
        Log.d(TAG, "LoaderCallback.onCreateLoader()");
        if (args != null) Log.d(TAG, "bundle: " + args.getLong(DateLoader.STATE_DATE_LOADER));
        return new DateLoader(this, args);
    }

    public void onLoadFinished(Loader<Date> loader, Date data) {
        Log.d(TAG, "LoaderCallback.onLoadFinished(): " + new SimpleDateFormat("dd/MM/yyyy").format(data));
        date = data;
    }

    public void onLoaderReset(Loader<Date> loader) {
        Log.d(TAG, "LoaderCallback.onLoaderReset()");
    }
}

所以在onStopLoading（）中我调用了cancelLoad（），我认为它应该取消当前任务，但是AsyncTaskLoader中的AsyncTask仍在运行（而loadInBackground（）中的循环仍在进行中。）

问题可能在“java.android.support.v4.content.AsyncTaskLoader.java”中的cancelLoad（）方法中，其中mTask.cancel（boolean）在参数中使用“false”调用：

public boolean cancelLoad() {
    ...
    boolean cancelled = mTask.cancel(false);

是否有机会取消在AsyncTaskLoader中运行AsyncTask？

Answer 1

如果你的意思是＆＃34;有没有机会取消在android.content.AsyncTaskLoader中运行AsyncTask？＆＃34;答案是肯定的：你只需要添加一些＆＃34;取消点＆＃34;在你的loadInBackground方法中检查是否已发出取消请求（isLoadInBackgroundCanceled（）== true），然后返回或抛出OperationCanceledException。

您正在使用的AsyncTaskLoader的支持库版本似乎并未在此时完全实现取消（至少不在飞行途中，并且框架的粗略比较 Loader的支持版本似乎建议取消可能根本不受支持......）。

http://developer.android.com/reference/android/content/Loader.html http://developer.android.com/reference/android/support/v4/content/Loader.html

我想到了两种缓解问题的方法：

创建Loader的两个实现（一个使用API级别11及更高版本的框架，一个没有旧设备的取消功能）
创建一个android.support.v4.content.Loader子类并自己处理每个AsyncTask和取消请求

Answer 2

public boolean cancelLoad() {
    ...
    boolean cancelled = mTask.cancel(false);
}



        @Override
        public Date loadInBackground() { // AsyncTaskLoader only
            Log.d(TAG, "Loader.loadInBackground()");

            try {
                // there will be some HttpClient.execute()
                while(true) {
                    Thread.sleep(100);
                    Log.d(TAG, "Loader.loadInBackground() still running");

                   if(cancelled)                 <---------
                      return new Date();         <----------
                }

            } catch (InterruptedException e) {
                Log.d(TAG, "Loader.loadInBackground() interupted");
            }
            Log.d(TAG, "Loader.loadInBackground() finished");
            return new Date();
        }

无法取消在AsyncTaskLoader中运行AsyncTask

2 个答案: