1、 //代码约束 NSLayoutConstraint

UIView *blue = [UIView new];

blue.backgroundColor = [UIColor blueColor];

blue.translatesAutoresizingMaskIntoConstraints = NO;

[self.view addSubview:blue];

UIView *red = [UIView new];

red.backgroundColor = [UIColor yellowColor];

red.translatesAutoresizingMaskIntoConstraints = NO;

[self.view addSubview:red];

//高度

NSLayoutConstraint *blueHeight = [NSLayoutConstraint constraintWithItem:blue attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:40];

[blue addConstraint:blueHeight];

NSLayoutConstraint *redHeight = [NSLayoutConstraint constraintWithItem:red attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:blue attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0];

[self.view addConstraint:redHeight];

//上

NSLayoutConstraint *blueTop = [NSLayoutConstraint constraintWithItem:blue attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:20];

[self.view addConstraint:blueTop];

NSLayoutConstraint *redTop = [NSLayoutConstraint constraintWithItem:red attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:blue attribute:NSLayoutAttributeBottom multiplier:1.0 constant:20];

[self.view addConstraint:redTop];

//左

NSLayoutConstraint *blueLeft = [NSLayoutConstraint constraintWithItem:blue attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20];

[self.view addConstraint:blueLeft];

NSLayoutConstraint *redLeft = [NSLayoutConstraint constraintWithItem:red attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];

[self.view addConstraint:redLeft];

//右

NSLayoutConstraint *blueRight = [NSLayoutConstraint constraintWithItem:blue attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20];

[self.view addConstraint:blueRight];

NSLayoutConstraint *redwidth = [NSLayoutConstraint constraintWithItem:red attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20];

[self.view addConstraint:redwidth];

2.VFL-