Autolayout之VFL

         前面说过给控件一个一个的添加约束代码量很多，写得手软，在这里我们可以用Visual Format Langage,大大的减少了代码量如下图：

                   

 同样的界面用VFL只需要几句话即可如下:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    UIButton *TopLeftBtn=[UIButton buttonWithType:UIButtonTypeRoundedRect];
    [TopLeftBtn setTitle:@"TopLeftBtn" forState:UIControlStateNormal];
    [TopLeftBtn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    [TopLeftBtn setBackgroundColor:[UIColor greenColor]];
    [TopLeftBtn setTranslatesAutoresizingMaskIntoConstraints:NO];
    [self.view addSubview:TopLeftBtn];
    
    
    UIButton *TopRightBtn=[UIButton buttonWithType:UIButtonTypeRoundedRect];
    [TopRightBtn setTitle:@"TopRightBtn" forState:UIControlStateNormal];
    [TopRightBtn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    [TopRightBtn setBackgroundColor:[UIColor redColor]];
    [TopRightBtn setTranslatesAutoresizingMaskIntoConstraints:NO];
    [self.view addSubview:TopRightBtn];
    
    UIButton *BottomBtn=[UIButton buttonWithType:UIButtonTypeRoundedRect];
    [BottomBtn setTitle:@"BottomBtn" forState:UIControlStateNormal];
    [BottomBtn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    [BottomBtn setBackgroundColor:[UIColor blueColor]];
    [BottomBtn setTranslatesAutoresizingMaskIntoConstraints:NO];
    [self.view addSubview:BottomBtn];

    NSDictionary *metrics = @{@"pad":@10};
    //metrics就是下面VFL语言中的10可以用pad来代替如下：H:|-pad-[TopLeftBtn(TopRightBtn)]-pad-[TopRightBtn]-pad-|
    
    NSDictionary *dict=NSDictionaryOfVariableBindings(TopLeftBtn,TopRightBtn,BottomBtn);
    NSString *vfl0=@"H:|-[TopLeftBtn(TopRightBtn)]-[TopRightBtn]-|";//其中水平方向H:可以去掉，默认表示水平，|-表示TopLeftBtn的左边离父视图的距离，默认为20像素，[TopLeftBtn(TopRightBtn)]表示TopLeftBtn控件这里表示TopLeftBtn按钮,（TopRightBtn）表示TopLeftBtn与TopRightBtn的宽相同,也可以写成[TopLeftBtn(==TopRightBtn)],[TopLeftBtn(TopRightBtn)]与[TopRightBtn]之间的-默认间距为8,-|表示TopRightBtn离父视图右边的间距为20,当然我们也可以自定义间距在需要定义的的地方加－10,如:@"H:|-10-[TopLeftBtn(TopRightBtn)]-10-[TopRightBtn]-10-|
    NSString *vfl1=@"|-[BottomBtn]-|";
    //表示BottomBtn距父视图的左边和右边的间距都是默认的20
    
    //垂直关系（V:）
    NSString *vfl2 = @"V:|-[TopLeftBtn(BottomBtn)]-[BottomBtn]-|";
    NSString *vfl3 = @"V:|-[TopRightBtn(==BottomBtn)]-[BottomBtn]-|";
    
    //V:必须存在表示垂直方向其他的一样，这里说一哈垂直方向|-表示的间距为0,当然在这里也可加入权级也叫抗压缩等级如下：@"V:|-[TopRightBtn(==100@750)]-[BottomBtn]-|",这里的@750代表权级
    
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vfl0 options:0 metrics:metrics views:dict]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vfl1 options:0 metrics:metrics views:dict]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vfl2 options:0 metrics:metrics views:dict]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:vfl3 options:0 metrics:metrics views:dict]];
}