VFL子视图居中

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: 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: metrics:nil views:views]];
    //设置宽度
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat: metrics:nil views:views]];
    //水平居中
    [self.view addConstraint:[NSLayoutConstraint constraintWithItem:headView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier: constant:]];

同理垂直居中也是