• (void)viewDidLoad {
  [super viewDidLoad];

  UIView *view = [[UIView alloc] init];
  view.backgroundColor = [UIColor redColor];

  //用Constraints（layout）来布局;哪个视图（view）使用layout布局哪个控件就必须写这句话
  [view setTranslatesAutoresizingMaskIntoConstraints:NO];
  [self.view addSubview:view];

  //****************** 给视图添加约束的四个步骤 ***************************
  //被添加的约束对象的superView
  //第一个参数:表示被添加约束的对象；
  //第二个参数:表示被约束的方向；是一个枚举值；
  /*
  NSLayoutAttributeLeft = 1,
  NSLayoutAttributeRight,
  NSLayoutAttributeTop,
  NSLayoutAttributeBottom,
  NSLayoutAttributeLeading,
  NSLayoutAttributeTrailing,
  NSLayoutAttributeWidth,
  NSLayoutAttributeHeight,
  NSLayoutAttributeCenterX,
  NSLayoutAttributeCenterY,
  */
  //第三个参数: 写零

  //第四个参数:参照的对象
  //第五个参数:与参照对象的方向
  //第六个参数:比例,指的是view的宽和高的比例
  //第七个参数:两个视图的距离
  [self.view addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeLeft relatedBy:0 toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:30]];

//确定y轴
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeTop relatedBy:0 toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:30]];

//******* 第一种约束方法 ********
//单独给约束对象宽度，是不需要给参照对象的；
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeWidth relatedBy:0 toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:40]];

//******* 第二种约束方法 *******
//用一个view的宽度来约束另一个view的宽度;约束是相互制约的
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeWidth relatedBy:0 toItem:view attribute:NSLayoutAttributeHeight multiplier:1 constant:0]];



UIView *view1 = [[UIView alloc] init];
view1.backgroundColor = [UIColor blueColor];;
[view1 setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.view addSubview:view1];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeLeft relatedBy:0 toItem:view attribute:NSLayoutAttributeRight multiplier:1 constant:40]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeCenterY relatedBy:0 toItem:view attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeWidth relatedBy:0 toItem:view attribute:NSLayoutAttributeWidth multiplier:1 constant:0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeHeight relatedBy:0 toItem:view attribute:NSLayoutAttributeHeight multiplier:1 constant:0]];

}