iOS 防止按钮多次点击&延迟的执行与取消

//这里是关键，点击按钮后先取消之前的操作，再进行需要进行的操作
        [[self class] cancelPreviousPerformRequestsWithTarget:self selector:@selector(readEnglish) object:nil];
        [self performSelector:@selector(readEnglish)withObject:nil afterDelay:0.75f];