iOS 11上 navigationItem使用图片后因图片过大而被拉伸

在iOS 11之前，我们自定义的导航按钮一般是这样

NSMutableArray * items = [[NSMutableArray alloc] init];
NSInteger i = 0;
    for (NSString * imageName in imageNames) {
        UIButton * btn = [UIButton buttonWithType:UIButtonTypeCustom];
        [btn setImage:[UIImage imageNamed:imageName] forState:UIControlStateNormal];
        btn.frame = CGRectMake(0, 0, 30, 30);
        [btn addTarget:target action:action forControlEvents:UIControlEventTouchUpInside];
        
        if (isLeft) {
            [btn setContentEdgeInsets:UIEdgeInsetsMake(0, -5, 0, 5)];
        }else{
            [btn setContentEdgeInsets:UIEdgeInsetsMake(0, 5, 0, -5)];
        }
        
        btn.tag = [tags[i++] integerValue];
        UIBarButtonItem * item = [[UIBarButtonItem alloc] initWithCustomView:btn];
        [items addObject:item];
    }
    if (isLeft) {
        self.navigationItem.leftBarButtonItems = items;
    } else {
        self.navigationItem.rightBarButtonItems = items;
    }

但是我们到iOS 11后，发现按钮使用图片后会出现变形拉伸等问题，除非使用正确大小的图片，显然，这个正确大小的图片是不好轻易控制的。

那么这个问题为什么会出现？
这是因为iOS 11的UIBarButtonItem使用了自动布局。

那么我们怎么解决呢？
如果是在xcode9，我们可以给这个图像的按钮一个宽高约束。

NSMutableArray * items = [[NSMutableArray alloc] init];
    NSInteger i = 0;
    for (NSString * imageName in imageNames) {
        UIButton * btn = [UIButton buttonWithType:UIButtonTypeCustom];
        [btn setImage:[UIImage imageNamed:imageName] forState:UIControlStateNormal];
        btn.frame = CGRectMake(0, 0, 30, 30);
        [btn addTarget:target action:action forControlEvents:UIControlEventTouchUpInside];
        
        if (isLeft) {
            [btn setContentEdgeInsets:UIEdgeInsetsMake(0, -5, 0, 5)];
        }else{
            [btn setContentEdgeInsets:UIEdgeInsetsMake(0, 5, 0, -5)];
        }
        
        btn.tag = [tags[i++] integerValue];
        UIBarButtonItem * item = [[UIBarButtonItem alloc] initWithCustomView:btn];
        [items addObject:item];
        
        //iOS 11图片约束，针对此约束最好做一个iOS 11的版本判断，让他只在iOS 11上实行
        [btn mas_makeConstraints:^(MASConstraintMaker *make) {
            make.size.mas_equalTo(CGSizeMake(30, 30));
        }];
    }
    if (isLeft) {
        self.navigationItem.leftBarButtonItems = items;
    } else {
        self.navigationItem.rightBarButtonItems = items;
    }