objective-c 定时运行和取消

[self performSelector:@selector(handleKeyPressAndHold:)
                 withObject:nil afterDelay:0.5];

0.5秒之后运行handleKeyPressAndHold函数

如果0.5内要取消运行，
[NSObject cancelPreviousPerformRequestsWithTarget:self 
                                             selector:@selector(handleKeyPressAndHold:)
                                               object:nil];