今天做UI用VFL适配在View上添加一个图片想让指定宽高的图片居中显示，我用下面的代码想着能实现可是出来的效果并没有居中。

UIImageView *headView=[[UIImageView alloc]initWithImage:[UIImage imageNamed:@"MR WU.png"]];    [headView setTranslatesAutoresizingMaskIntoConstraints:NO];        [self.view addSubview:headView];    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-90-[headView(100)]-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(headView)]];    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[headView(100)]-|" options:NSLayoutFormatAlignAllCenterX metrics:nil views:NSDictionaryOfVariableBindings(headView)]];

后来在网上查了下终于找到解决方法

self.view.backgroundColor=[UIColor whiteColor];    UIImageView *headView=[[UIImageView alloc]initWithImage:[UIImage imageNamed:@"MR WU.png"]];    [headView setTranslatesAutoresizingMaskIntoConstraints:NO];    [self.view addSubview:headView];    NSDictionary* views = NSDictionaryOfVariableBindings(headView);    //设置高度    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-90-[headView(100)]" options:0 metrics:nil views:views]];    //设置宽度    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"[headView(100)]" options:0 metrics:nil views:views]];    //水平居中    [self.view addConstraint:[NSLayoutConstraint constraintWithItem:headView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1 constant:0]];

同理垂直居中也是