如何启动和停止后台线程iOS

Question

I am developing an iPhone app in which, I want start background thread for downloading image from server. while downloading the image if user wants to cancel a download then on click of button he can cancel downloading means I want to stop execution of background thread.

is it possible to do ? in iPhone

Answer 1

you can start a continue the background download with this code.

taskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^(void) {
    }];

//code to download .

[[UIApplication sharedApplication] endBackgroundTask:taskId];

you can call

[[UIApplication sharedApplication] endBackgroundTask:taskId];

to stop the background task.

But this download will continue only for few minutes . Then app will crash if it continue downloading.

Answer 2

If you want to perform an operation in the background, then you'll need to wrap it in a background task. It's also very important that you call endBackgroundTask when you're finished - otherwise the app will be killed after its allotted time has expired.

Something like this should work for you:

Declare a property:

@property(assign, nonatomic) UIBackgroundTaskIdentifier backgroundFetchTask;

Then:

- (void) fetchImageInBackground
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    [self beginBackgroundFetchTask];

    NSURLResponse * response = nil;
    NSError  * error = nil;
    NSData * responseData = [NSURLConnection sendSynchronousRequest: request returningResponse: &response error: &error];

        // Store your image

        [self endBackgroundFetchTask];
    });
}

- (void) beginBackgroundFetchTask
{
    self.backgroundUpdateTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
        [self endBackgroundFetchTask];
    }];
}

- (void) endBackgroundFetchTask
{
    [[UIApplication sharedApplication] endBackgroundTask: self.backgroundUpdateTask];
    self.backgroundFetchTask = UIBackgroundTaskInvalid;
}

Answer 3

You can use AFNetworking for this purpose. https://github.com/AFNetworking/AFNetworking

AFImageRequestOperation exactly does this.

Example to download image:

AFImageRequestOperation * _operation = [AFImageRequestOperation imageRequestOperationWithRequest:request success:^(UIImage *image) {
    // image is your image
    _operation = nil;
}];

[_operation start];

Example to cancel thread:

[_operation cancel];

So in your case -> declare variable AFImageRequestOperation *_operation; in the @interface or @implementation and use it to download your images in background.

I would do a singlton class MyImageBackgrounfDownloader