- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;

这个方法是单线程的，也就是说只有当前调用次方法的函数执行完毕后，selector方法才会被调用。

比如：

- (void)changeText:(NSString *)string

{

label.text = string;

NSLog(@"changeText:(NSString *)string");

}

- (void)changePopoverSize

{

[self performSelector:@selector(changeText:) withObject:@"Happy aha" afterDelay:1];

NSLog(@"changePopoverSize#####end");

sleep(5);

NSLog(@"changePopoverSize-----end");

}

执行结果（注意时间）：

2012-08-17 17:14:06.697 awrbv[1973:f803] changePopoverSize#####end

2012-08-17 17:14:11.698 awrbv[1973:f803] changePopoverSize-----end

2012-08-17 17:14:11.701 awrbv[1973:f803] changeText:(NSString *)string

如果要想多线程的话，可以是使用

- (void)performSelectorInBackground:(SEL)aSelector withObject:(id)arg

或者

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait;

代码如下：

- (void)changeText:(NSString *)string

{

label.text = string;

NSLog(@"changeText:(NSString *)string");

}

- (void)changePopoverSize

{

[self performSelectorOnMainThread:@selector(changeText:) withObject:@"Happy aha111" waitUntilDone:YES];

NSLog(@"changePopoverSize#####end");

sleep(5);

NSLog(@"changePopoverSize-----end");

}

执行结果如下：

2012-08-17 17:19:29.618 awrbv[2024:f803] changeText:(NSString *)string

2012-08-17 17:19:29.619 awrbv[2024:f803] changePopoverSize#####end

2012-08-17 17:19:34.620 awrbv[2024:f803] changePopoverSize-----end

可以看出，如果waitUntilDone:YES那么等changeText执行完毕后再往下执行

如果waitUntilDone:NO的话，结果如下：

2012-08-17 17:21:12.135 awrbv[2049:f803] changePopoverSize#####end

2012-08-17 17:21:17.137 awrbv[2049:f803] changePopoverSize-----end

2012-08-17 17:21:17.139 awrbv[2049:f803] changeText:(NSString *)string