cancelPreviousPerformRequestsWithTarget

在ios官方代碼中：

/**************** Delayed perform ******************/

@interface NSObject (NSDelayedPerforming)

- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;

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

+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(id)anArgument;

+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;

@end

performSelector（延遲）執行某個方法

cancelPreviousPerformRequestsWithTarget 取消先前執行的請求根據目標

通常，我們在一個類中延遲perform某個方法的話，由於時間稍長，我們就直接退出界面，那這時我們最好在disappear的時候cancel掉這個perfrom。