移除一個view的所有子view

    // 方法一
    if (self.subviews.count > 0) {
        for (UIView *subView in self.subviews) {
            [subView removeFromSuperview];
        }
    }
    
    // 方法二
    [self.subviews makeObjectsPerformSelector:@selector(removeFromSuperview)];
    
    // 方法三
    [self.subviews enumerateObjectsUsingBlock:^(__kindof UIView * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
        [obj removeFromSuperview];
    }];