代码实现AutoLayout

添加约束的规则（1）

在创建约束之后，需要将其添加到作用的view上
在添加时要注意目标view需要遵循以下规则：
1）对于两个同层级view之间的约束关系，添加到它们的父view上

添加约束的规则（2）

2）对于两个不同层级view之间的约束关系，添加到他们最近的共同父view上

添加约束的规则（3）

3）对于有层次关系的两个view之间的约束关系，添加到层次较高的父view上

NSLayoutConstraint

一个NSLayoutConstraint对象就代表一个约束

创建约束对象的常用方法
+(id)constraintWithItem:(id)view1 attribute:(NSLayoutAttribute)attr1 relatedBy:(NSLayoutRelation)relation toItem:(id)view2 attribute:(NSLayoutAttribute)attr2 multiplier:(CGFloat)multiplier constant:(CGFloat)c;
view1 ：要约束的控件
attr1 ：约束的类型（做怎样的约束）
relation ：与参照控件之间的关系 相等 小于 或者大于  这个控件的某个属性大于等于或者小于等于另外一个控件的某个属性 
obj1.property1 =（obj2.property2 * multiplier）+ constant value

view2 ：参照的控件
attr2 ：约束的类型（做怎样的约束）
multiplier ：乘数
c ：常量

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
  
    UIView *blueView = [[UIView alloc] init];
    blueView.backgroundColor = [UIColor blueColor];
    // 不要将AutoresizingMask转为Autolayout的约束
    blueView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:blueView];
    
    UIView *redView = [[UIView alloc] init];
    redView.backgroundColor = [UIColor redColor];
    // 不要将AutoresizingMask转为Autolayout的约束
    redView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:redView];// 添加宽度约束:100
    
    /************************** 蓝色 **************************/
    // 添加高度约束:40
    NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:40];
    [blueView addConstraint:heightConstraint];
    
    // 添加左边约束：blueView的左边距离父控件左边有20的间距
    NSLayoutConstraint *leftConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20];
    [self.view addConstraint:leftConstraint];
    
    // 添加右边约束：blueView的右边距离父控件右边有20的间距
    NSLayoutConstraint *rightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20];
    [self.view addConstraint:rightConstraint];
    
    // 添加顶部约束：blueView的顶部距离父控件顶部有20的间距
    NSLayoutConstraint *topConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:20];
    [self.view addConstraint:topConstraint];
    
    /************************** 红色 **************************/
    // 添加高度约束:蓝色等高
    NSLayoutConstraint *heightConstraint2 = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0];
    [self.view addConstraint:heightConstraint2];
    
    // 添加左边约束：redView的左边 == 父控件的中心x
    NSLayoutConstraint *leftConstraint2 = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];
    [self.view addConstraint:leftConstraint2];
    
    // 添加顶部约束：redView的顶部距离blueView的底部有20的间距
    NSLayoutConstraint *topConstraint2 = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeBottom multiplier:1.0 constant:20];
    [self.view addConstraint:topConstraint2];
    
    // 添加右边约束：redView的右边 == blueView的右边
    NSLayoutConstraint *rightConstraint2 = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeRight multiplier:1.0 constant:0];
    [self.view addConstraint:rightConstraint2];
}
/**
 * 宽度高度为100，在父控件中垂直水平居中
 */
- (void)test2
{
    UIView *blueView = [[UIView alloc] init];
    blueView.backgroundColor = [UIColor blueColor];
    // 不要将AutoresizingMask转为Autolayout的约束
    blueView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:blueView];
    
    // 添加宽度约束:父控件的一半
    NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];
    [self.view addConstraint:widthConstraint];
    
    // 添加高度约束:父控件的一半
    NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeHeight multiplier:0.5 constant:0];
    [self.view addConstraint:heightConstraint];
    
    // 水平居中
    NSLayoutConstraint *centerXConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];
    [self.view addConstraint:centerXConstraint];
    
    // 垂直居中
    NSLayoutConstraint *centerYConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0];
    [self.view addConstraint:centerYConstraint];
}

/**
 * 宽度高度为100，粘着父控件的右上角
 */
- (void)test1
{
    UIView *blueView = [[UIView alloc] init];
    blueView.backgroundColor = [UIColor blueColor];
    // 不要将AutoresizingMask转为Autolayout的约束
    blueView.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:blueView];
    
    // 添加宽度约束:100
    NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:100];
    [blueView addConstraint:widthConstraint];
    
    // 添加高度约束:100
    NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:100];
    [blueView addConstraint:heightConstraint];
    
    // 添加右边约束：blueView的右边距离父控件右边有10的间距
    NSLayoutConstraint *rightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-10];
    [self.view addConstraint:rightConstraint];
    
    // 添加顶部约束：blueView的顶部距离父控件顶部有10的间距
    NSLayoutConstraint *topConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:10];
    [self.view addConstraint:topConstraint];
}



- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end