代碼實現Auto Layout的步驟：

1>利用NSLayoutConstraint類創建具體的約束對象

2>添加約束對象到相應的view上

-(void)addConstraint:(NSLayoutConstraint *)constraint;

-(void)addConstraints:(NSArray *)constraints;

注1：代碼實現Auto Layout的注意要先禁止autoresizing功能，設置view的下面屬性為NO

view.translatesAutoresizingMaskIntoConstraints= NO;

注2：添加約束之前，一定要保證相關控件都已經在各自的父控件上

注3：不用再給view設置frame

 

 

一個NSLayoutConstraint對象就代表一個約束

創建約束對象的常用方法

+(id)constraintWithItem:(id)view1attribute:(NSLayoutAttribute)attr1 relatedBy:(NSLayoutRelation)relationtoItem:(id)view2 attribute:(NSLayoutAttribute)attr2multiplier:(CGFloat)multiplier constant:(CGFloat)c;

view1：要約束的控件

attr1：約束的類型（做怎樣的約束）

relation：與參照控件之間的關系

view2：參照的控件

attr2：約束的類型（做怎樣的約束）

multiplier：乘數

c：常量

注：自動布局有個核心公式

obj1.property1 =（obj2.property2 * multiplier）+ constant value

 

如讓一個View居中顯示代碼實現如下

    // 1.添加控件
    UIView *blueView = [[UIView alloc] init];
    blueView.backgroundColor = [UIColor blueColor];
    [self.view addSubview:blueView];
    
    // 2.添加約束
    // 關閉autoresizing
    blueView.translatesAutoresizingMaskIntoConstraints = NO;
    
    /**
     寬高:100
     位置:在父控件中居中
     */
    // blueView的width == 100
    NSLayoutConstraint *width = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth 
        relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100];
    [blueView addConstraint:width];
    
    // blueView的height == 100
    NSLayoutConstraint *height = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight 
        relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100];
    [blueView addConstraint:height];
    
    // blueView的CenterX == self.view的CenterX * 1.0 + 0.0
    NSLayoutConstraint *centerX = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterX 
        relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0];
    [self.view addConstraint:centerX];
    
    // blueView的CenterY == self.view的CenterY * 1.0 + 0.0
    NSLayoutConstraint *centerY = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterY 
        relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0];
    
    [self.view addConstraint:centerY];

3、補充

 

3.1 UILabel使用Auto Layout

 

 

3.2 使用Auto Layout制作動畫

 

 

在修改了約束之後，只要執行下面代碼，就能做動畫效果

[UIViewanimateWithDuration:1.0 animations:^{

[添加了約束的viewlayoutIfNeeded];

}];

@interface ViewController ()

@property (weak, nonatomic) IBOutlet NSLayoutConstraint *leftMargin;
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *width;
@property (weak, nonatomic) IBOutlet UIView *redView;

@end

@implementation ViewController

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    self.leftMargin.constant = 100;
    self.width.constant = 200;
    
    [UIView animateWithDuration:2.0 animations:^{
        [self.view layoutIfNeeded];
        [self.redView layoutIfNeeded];
    }];
}
@end