iOS开发中利用VFL方法添加约束的基本操作步骤，便于自己查阅

UIButton *b1 = [UIButton buttonWithType:UIButtonTypeSystem];

    [b1 setTitle:@"确定" forState:UIControlStateNormal];

    b1.backgroundColor=[UIColor lightGrayColor];

    [self.view addSubview:b1];

    UIButton *b2 = [UIButton buttonWithType:UIButtonTypeSystem];

    [b2 setTitle:@"取消" forState:UIControlStateNormal];

    b2.backgroundColor=[UIColor lightGrayColor];

    [self.view addSubview:b2];

    UIButton *b3 = [UIButton buttonWithType:UIButtonTypeSystem];

    [b3 setTitle:@"重试" forState:UIControlStateNormal];

    b3.backgroundColor=[UIColor lightGrayColor];

    [self.view addSubview:b3];

    //预备步骤:关闭自动翻译约束功能

    b1.translatesAutoresizingMaskIntoConstraints = NO;

    b2.translatesAutoresizingMaskIntoConstraints = NO;

    b3.translatesAutoresizingMaskIntoConstraints = NO;

    // 此函数可以自动生成一个Dictionary

    // 将传入的对象引用作为value,将引用名变成字符串作为key

    // 如 传入的是 (b1,b2,b3)

    // 则生成的字典如下:

    // {@"b1":b1,@"b2":b2,@"b3":b3}

    NSDictionary *diction1 = NSDictionaryOfVariableBindings(b1,b2,b3);

    // 为vfl式子中的一些特殊含义的数字做一个名称对照表

    NSDictionary *diction2 = @{@"top":@20,@"left":@20,@"right":@20,@"spacing":@10};

    //1.准备一个VFL

    NSString *hVFL = @"|-left-[b1]-spacing-[b2(b1)]-spacing-[b3(b1)]-right-|";

    //2.创建约束

    NSArray *cs1 = [NSLayoutConstraint constraintsWithVisualFormat:hVFL options:NSLayoutFormatAlignAllCenterY metrics:diction2 views:diction1];

    //3.将约束添加到父视图上

    [self.view addConstraints:cs1];

    //创建竖直方向的约束

    NSString *vVFL = @"V:|-top-[b1]";

    NSArray *cs2 = [NSLayoutConstraint constraintsWithVisualFormat:vVFL options:0 metrics:diction2 views:diction1];

    [self.view addConstraints:cs2];