Android应用：两次调用AsyncTask？

Question

I'm using AsyncTask and some pretty common Android code to get the contents of a remote webpage. Based on that returned content, I can then call another page.

http://developer.android.com/reference/android/os/AsyncTask.html

My debugging lines should print like this:

 1> StartA() 
 2> onPreExecute
 3> doInBackground 
 4> onPostExecute    Note: Code here will call EndA()
 5> EndA()
 6> 
 7> StartB() 
 8> onPreExecute 
 9> doInBackground 
10> onPostExecute     Note:  Code here will call EndB() 
11> EndB()

Is that impossible to do? I get all of the above to work... EXCEPT I get one addtional call to EndB() appearing between lines 8 and 9.

I can't for the life of me figure out why. Nothing looks like it should call EndB() twice. And it definitely shouldn't get called BEFORE 9 and 10.

private void StartA()
{
    Debug("StartA()");

    g_GetWhat = 1;
    DownloadWebPageTask task = new DownloadWebPageTask();
task.execute(new String[] { "http://google.com" });

}

private void EndA()
{
    Debug("EndA()");

    StartB();
}

private void StartB()
{
    Debug("StartB()");

    g_GetWhat = 2;
    DownloadWebPageTask task = new DownloadWebPageTask();
task.execute(new String[] { "http://yahoo.com" });

}

private void EndB()
{
    Debug("EndB()");
}

///////////////////////////////////////////////////

private class DownloadWebPageTask extends AsyncTask<String, Void, String> 
{
protected void onPreExecute()   
    {
        Debug("onPreExecute()");

    }

protected String doInBackground(String... urls) 
     {
    Debug("doInBackground()");
}

protected void onPostExecute(String result) 
{

    Debug("onPostExecute()"); 
    if(g_GetWhat == 1)  { EndA(); }
    if(g_GetWhat == 2)  { EndB(); }

}
}

Answer 1

You can only execute an AsyncTask instance once. You are actually creating two instances, but you should call it like this anyways so that it can never be recalled:

new DownloadWebPageTask().execute(new String[] { "http://yahoo.com" });
new DownloadWebPageTask().execute(new String[] { "http://google.com" });

instead of like this:

DownloadWebPageTask task = new DownloadWebPageTask();
task.execute(new String[] { "http://google.com" });

I think your running into the problem here:

private void EndA()
{
    Debug("EndA()");

    StartB();
}

Your value for g_GetWhat is getting changed as soon as StartB begins. So when execution returns from EndA() the next if statement evaluates to true since g_GetWhat 's value has changed.

if(g_GetWhat == 1)  { EndA(); }
if(g_GetWhat == 2)  { EndB(); }

The value for g_GetWhat is actually 2, which is why you see the result you are seeing. You should pass g_GetWhat into your AsyncTask when you call it and make it an instance variable of the task.

Answer 2

如果您需要在后台同时进行多个工作，那么您应该在Android中使用Service类（而不是IntentService，因为调用已入队并且不会同时运行）。