performSelector 和 cancelPreviousPerformRequestsWithTarget
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NSObject中的performSelector:withObject:afterDelay:方法将会在当前线程的Run Loop中根据afterDelay参数创建一个Timer,如果没有调用有inModes参数的方法,该Timer会运行在当前Run Loop的默认模式中,也就是NSDefaultRunLoopMode定义的模式中。
performSelector:withObject:afterDelay:方法的使用看起来还是很简单的。这里讲另外一个辅助函数,NSObject中静态的cancelPreviousPerformRequestsWithTarget方法。该方法就是专门用来取消取消performSelector:withObject:afterDelay:方法所创建的Selector source(内部上就是一个Run Loop的Timer source)。因此该方法和performSelector:withObject:afterDelay:方法一样,只限于当前Run Loop中。
我们可以利用cancelPreviousPerformRequestsWithTarget直接取消一个对象在当前Run Loop中的所有未执行的performSelector:withObject:afterDelay:方法所产生的Selector Sources,如下代码:
- (void)viewDidLoad{ [super viewDidLoad]; [self performSelector:@selector(test:) withObject:nil afterDelay:1]; [self performSelector:@selector(test:) withObject:@"mgen" afterDelay:2]; [NSObject cancelPreviousPerformRequestsWithTarget:self]; }- (void)test:(id)obj{ NSLog(@"调用成功: %@", obj);}
不会有任何输出,因为两个调用都被取消了。
如果想取消单独一个的话,需使用cancelPreviousPerformRequestsWithTarget:selector:object:方法,注意selector和object参数需要一一对应。如下代码:
- (void)viewDidLoad{ [super viewDidLoad]; [self performSelector:@selector(test:) withObject:[NSNumber numberWithInt:26] afterDelay:1]; [self performSelector:@selector(test:) withObject:[NSNumber numberWithInt:17] afterDelay:2]; [self performSelector:@selector(test:) withObject:[NSNumber numberWithInt:17] afterDelay:3]; [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(test:) object:[NSNumber numberWithInt:17]]; }- (void)test:(id)obj{ NSLog(@"调用成功: %@", obj);}
只会输出:
调用成功: 26
其他两个Selector都被取消了。
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