iOS进阶——可取消的block

+ (id)performBlock:(void (^)())aBlock onQueue:(dispatch_queue_t)queue afterDelay:(NSTimeInterval)delay

{

    NSParameterAssert(aBlock != nil);

    __block BOOL cancelled = NO;

    void (^wrapper)(BOOL) = ^(BOOL cancel){

        if (cancel) {

            cancelled = YES;

            return;

        }

        if (!cancelled) {

            aBlock();

        }

    };

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delay * NSEC_PER_SEC)), queue, ^{

        wrapper(NO);

    });

    return [wrapper copy];

}



+ (void)cancelBlock:(id)aBlock

{

    NSParameterAssert(aBlock != nil);

    void (^wrapper)(BOOL) = aBlock;

    wrapper(YES);

}