iOS App進入Background Process也可不中斷(二)

之前一次已經說過iOS App進入Background Process也可不中斷連線下載的方法，
不過上次的方法有一個壞處,
就是用戶再開啟App，App是不會更新畫面，
只會出Default Image要直到Callback完成，
所以今天看了一下Help Document，
終於找到最好的方法了

UIBackgroundTaskIdentifier bgTask;
if(bgTask == UIBackgroundTaskInvalid)
{
    UIApplication* app = [UIApplication sharedApplication];

    // 開啟了BackgroundTask就要以令以下的queue在Background/Foreground Task都可以運行
    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        NSLog(@"System Expiration End Background Task");
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    // Start the long-running task and return immediately.
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    // Do the work associated with the task.
    for (int i = 0; i < 1000; i++)
    {
        NSLog(@"Sleep %d", i);
        // 返回Main Process更新UI
        dispatch_async(dispatch_get_main_queue(), ^{
            UIButton* btn = (UIButton*)[viewController.view viewWithTag:100];
            [btn setTitle:[NSString stringWithFormat:@"Sleep %d", i] forState:UIControlStateNormal];
        }); 
        [NSThread sleepForTimeInterval:5];
    }

    NSLog(@"Completed State End Background Task");
    [app endBackgroundTask:bgTask];
    bgTask = UIBackgroundTaskInvalid;
    });
}