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);
}