比较GCD与performSelectorInBackground:dispatch_async不在后台

Question

Grand Central Dispatch很棒,减少了代码量,但为什么我不能在后台线程上运行？
我做了一个示例应用程序来显示我的意思(没有评论的工作):

- (IBAction)performSel {
    [self performSelectorInBackground:@selector(doStuff) withObject:nil];
    [NSThread sleepForTimeInterval:3];
    [[self.view.subviews lastObject] removeFromSuperview];
}

- (IBAction)gcd {
    dispatch_async(dispatch_queue_create("myGCDqueue", NULL), ^(void) {
    //dispatch_sync(dispatch_queue_create("myGCDqueue", NULL), ^(void) {
    //dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
    //dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
    //dispatch_async(dispatch_get_main_queue(), ^(void) {
    //dispatch_sync(dispatch_get_main_queue(), ^(void) {
        [self doStuff]; // only for readability, will move the code on success
    });
    [NSThread sleepForTimeInterval:3];
    [[self.view.subviews lastObject] removeFromSuperview];
}

- (void)doStuff {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];

    UIView *abortingView = [[UIView alloc]initWithFrame: self.view.bounds];
    abortingView.backgroundColor = [UIColor whiteColor];
    abortingView.alpha = 0.7;
    [self.view insertSubview:abortingView atIndex:10];
    [abortingView release];

    [pool drain];
}

这[NSThread sleepForTimeInterval:3];是模拟默认的UI功能.例如,如果有人从一个导航视图切换到另一个导航视图.
只需在基于新视图的应用程序中复制代码,创建两个按钮并连接它们.

Answer 1

UIKit类只能从应用程序的主线程中使用.(从iOS4开始,绘图到图形上下文是线程安全的.)你不能在后台线程中使用UIKit东西.

因此,在这种情况下,您只能使用dispatch_async(dispatch_get_main_queue(),block).

dispatch_async(dispatch_get_main_queue(), ^(void) {

它将在主线程的runloop中调用主线程上的块.

dispatch_async(dispatch_queue_create("myGCDqueue", NULL), ^(void) {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {

它将在后台线程中调用该块.你不能使用它,因为你想在块中使用UIKit.并且要小心dispatch_async(dispatch_queue_create(,它可能会导致内存泄漏,您必须释放由dispatch_queue_create创建的串行队列).

dispatch_sync(dispatch_queue_create("myGCDqueue", NULL), ^(void) {
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {

dispatch_sync一直等到块完成.

dispatch_sync(dispatch_get_main_queue(), ^(void) {

它导致DEADLOCK.