问题
I want to cancel running AsyncTask (in AsyncTaskLoader) when the user clicks the home button. Here is what I created so far:
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()");
}
}
So in onStopLoading() i am calling cancelLoad(), which i think should cancel current task, but AsyncTask in AsyncTaskLoader is still running (while loop in loadInBackground() is still in progress).
The problem is maybe in cancelLoad() method in "java.android.support.v4.content.AsyncTaskLoader.java", where mTask.cancel(boolean) is called with "false" in arguments:
public boolean cancelLoad() {
...
boolean cancelled = mTask.cancel(false);
Is there any chance to cancel running AsyncTask in AsyncTaskLoader?
回答1:
If you mean "Is there any chance to cancel running AsyncTask in android.content.AsyncTaskLoader?" the answer is yes: you just need to add some "cancel points" in your loadInBackground method and check whether a cancellation request has been issued (isLoadInBackgroundCanceled() == true), then either return or throw an OperationCanceledException).
The support library version of AsyncTaskLoader you are using though doesn't seem to fully implement cancellation at this time (not in mid-flight at least, and a cursory comparison of the framework and of the support version of Loader seems to suggest that cancellation might not be supported at all...).
http://developer.android.com/reference/android/content/Loader.html http://developer.android.com/reference/android/support/v4/content/Loader.html
Two ways to alleviate the problem come to my mind:
- create two implementations of your Loader (one using the framework for API level 11 and above, and one without the cancel feature for older devices)
- create an android.support.v4.content.Loader subclass and handle each AsyncTask and cancellation request yourself
回答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();
}
来源：https://stackoverflow.com/questions/11612530/cannot-cancel-running-asynctask-in-asynctaskloader