比较GCD与PerformSelectorInbackground：dispatch_async不在后台
Grand Central Dispatch 很棒，可以减少代码量，但为什么我不能在后台线程上运行某些内容？
我制作了一个示例应用程序来展示我的意思（没有任何注释工作）：
- (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];
}
[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。
只需将代码复制到基于新视图的应用程序中，创建两个按钮并将它们连接起来。
Grand Central Dispatch is great and reduces the amount of code but why I cannot run something on a background thread?
I have made a sample application to show what I mean (none of the commented work):
- (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];
}
the [NSThread sleepForTimeInterval:3]; is to simulate a default UI functionality. For example if someone is switching from one navigation view to another.
Simply copy the code in a new view based application, create two buttons and connect them.
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UIKit 类只能在应用程序的主线程中使用。 （从 iOS4 开始，绘制到图形上下文是线程安全的。）您不能在后台线程中使用 UIKit 内容。
因此，在这种情况下只能使用dispatch_async(dispatch_get_main_queue(), block)。
它将在主线程的运行循环中调用主线程上的块。
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，你必须释放由dispatch_queue_create创建的串行队列。dispatch_sync
会等待块完成。
这会导致死锁。
UIKit classes should be used only from an application’s main thread. (From iOS4, drawing to a graphics context is thread-safe.) You can't use UIKit stuff in a background thread.
Thus, you can only use dispatch_async(dispatch_get_main_queue(), block) in this situation.
It will invoke the block on the main thread in the runloop of the main thread.
It will invoke the block in a background thread. You can't use it because you want to use UIKit in the block. And be careful dispatch_async(dispatch_queue_create(, it might cause memory leak, you have to release the serial queue that is created by dispatch_queue_create.
dispatch_sync waits until the block is done.
It causes DEADLOCK.