从Android中的AsyncTask返回一个值

Question

一个简单的问题:是否可以返回值AsyncTask？

//AsyncTask is a member class
private class MyTask extends AsyncTask<Void, Void, Void>{

    protected Void doInBackground(Void... params) {
         //do stuff
         return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        //do stuff
        //how to return a value to the calling method?
    }
}

然后在我的Activity/ Fragment:

// The task is started from activity
myTask.execute()
// something like this?
myvalue = myTask.getvalue() 

编辑:这是很久以前我不熟悉Java的问题,现在我对它更好了,我会做一个简短的总结:

异步任务的重点是任务是asynchronous,这意味着在您调用execute()任务之后,任务开始在自己的线程上运行.从asynctask返回一个值是没有意义的,因为原始调用线程已经继续做其他事情(因此任务是异步的).

想想时间:在某个时间点,您启动了一个与主线程并行运行的任务.当并行运行任务完成时,主线程上也经过了时间.并行任务无法及时返回以将值返回给主线程.

我是从C来的,所以我对此并不了解.但似乎很多人都有同样的问题,所以我想我会稍微澄清一下.

Answer 1

这onPostExecute()是为了什么.它在UI线程上运行,您可以将结果从那里传送到屏幕(或您需要的任何其他地方).在最终结果可用之前不会调用它.如果您想提供中间结果,请查看onProgressUpdate()

Answer 2

为什么不调用一个处理值的方法？

public class MyClass extends Activity {

    private class myTask extends AsyncTask<Void, Void, Void> {

        //initiate vars
        public myTask() {
            super();
            //my params here
        }

        protected Void doInBackground(Void... params) {
            //do stuff
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            //do stuff
            myMethod(myValue);
        }
    }

    private myHandledValueType myMethod(Value myValue) {
        //handle value 
        return myHandledValueType;
    }
}

Answer 3

最简单的方法是将调用对象传递给异步任务(如果您愿意,可以在构建它时):

public class AsyncGetUserImagesTask extends AsyncTask<Void, Void, Void> {

    private MyImagesPagerFragment mimagesPagerFragment;
    private ArrayList<ImageData> mImages = new ArrayList<ImageData>();

    public AsyncGetUserImagesTask(MyImagesPagerFragment imagesPagerFragment) {
        this.mimagesPagerFragment = imagesPagerFragment;
    }

    @Override
    public Void doInBackground(Void... records) {
        // do work here
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        mimagesPagerFragment.updateAdapter(mImages);
    }
}

并且在调用类(您的活动或片段)中执行任务:

public class MyImagesPagerFragment extends Fragment {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        AsyncGetUserImagesTask mGetImagesTask = new AsyncGetUserImagesTask(this);
        mGetImagesTask.execute();
    }

然后onPostExecuteMethod将调用您喜欢的原始类上的任何方法,例如:

    public void updateAdapter(List<ImageData> images) {
        mImageAdapter.setImages(images);
        mImageAdapter.notifyDataSetChanged();
    }
}

Answer 4

代码示例:Activity使用AsyncTask在后台线程中获取值,然后AsyncTask通过调用processValue将结果返回给Activity:

public class MyClass extends Activity {
  private void getValue() {
      new MyTask().execute();
  }

  void processValue(Value myValue) {
     //handle value 
     //Update GUI, show toast, etc..
  }

  private class MyTask extends AsyncTask<Void, Void, Value> {
    @Override
    protected Value doInBackground(Void... params) {
      //do stuff and return the value you want 
      return Value;
    }

    @Override
    protected void onPostExecute(Value result) {
      // Call activity method with results
      processValue(result);
    }
  }
}

Answer 5

你可以尝试这个:myvalue = new myTask().execute().get(); 减去它会冻结进程,直到asyncron不会完成;

Answer 6

您需要使用"协议"来委派或提供数据AsynTask.

代表和数据来源

委托是一个对象,当该对象遇到程序中的事件时,该对象代表另一个对象或与另一个对象协调.(Apple定义)

协议是定义一些委托某些行为的方法的接口.

DELEGATE :从后台线程中的对象中截取事件

AsynkTask:

public final class TaskWithDelegate extends AsyncTask<..., ..., ...> {
    //declare a delegate with type of protocol declared in this task
    private TaskDelegate delegate;

    //here is the task protocol to can delegate on other object
    public interface TaskDelegate {
        //define you method headers to override
        void onTaskEndWithResult(int success);
        void onTaskFinishGettingData(Data result);
    }

    @Override
    protected Integer doInBackground(Object... params) {
        //do something in background and get result
        if (delegate != null) {
            //return result to activity
            delegate.onTaskFinishGettingData(result);
        }   
    }

    @Override
    protected void onPostExecute(Integer result) {
        if (delegate != null) {
            //return success or fail to activity
            delegate.onTaskEndWithResult(result);
        }   
    }
}

活动:

public class DelegateActivity extends Activity implements TaskDelegate {
    void callTask () {
            TaskWithDelegate task = new TaskWithDelegate;
        //set the delegate of the task as this activity
        task.setDelegate(this);
    }

    //handle success or fail to show an alert...
    @Override
    void onTaskEndWithResult(int success) {

    }

    //handle data to show them in activity...
    @Override
    void onTaskFinishGettingData(Data result) {

    }
}

编辑:如果您在doInBackground中调用委托,并且委托尝试编辑某个视图,那将崩溃,因为视图只能由主线程操纵.

额外

DATASOURCE:在后台线程中为对象提供数据

的AsyncTask:

// this refers to Activity
this.runOnUiThread(new Runnable() {
    @Override
    public void run() {
    // Here you can edit views when task notify some changes from background thread
        textView.setText(someValue);
    }
});

活动:

public final class TaskWithDataSource extends AsyncTask<..., ..., ...> {
    //declare a datasource with type of protocol declared in this task
    private TaskDataSource dataSource;
    private Object data;

    //here is the task protocol to can provide data from other object
    public interface TaskDataSource {
        //define you method headers to override
        int indexOfObject(Object object);
        Object objectAtIndex(int index);
    }

    @Override
    protected void onPreExecute(Integer result) {
        if (dataSource != null) {
            //ask for some data
            this.data = dataSource.objectAtIndex(0);
        }   
    }

    @Override
    protected Integer doInBackground(Object... params) {
        //do something in background and get result
        int index;
        if (dataSource != null) {
            //ask for something more
            index = dataSource.indexOfObject(this.data);
        }   
    }
}