为小白学iOS约束打造 -- NSLayoutConstraint

原理如下：

代码自己将其拷贝到viewcontroller.m参看

#import "ViewController.h"

#define WIDTH [UIScreen mainScreen].bounds.size.width

#define HEIGHT [UIScreen mainScreen].bounds.size.height

@interface ViewController ()

@property (nonatomic, strong) UIView *myView1;

@property (nonatomic, strong) UIView *myView2;

@property (nonatomic, strong) UIView *myView3;

@end

@implementation ViewController

- (UIView *)myView1

{

    if(!_myView1)

    {

        self.myView1 = [[UIView alloc]init];

        [self.myView1 setBackgroundColor:[UIColor blueColor]];

        [self.myView1 setTranslatesAutoresizingMaskIntoConstraints:NO];

        [self.view addSubview:self.myView1];

    }

    return _myView1;

}

- (UIView *)myView2

{

    if(!_myView2)

    {

        self.myView2 = [[UIView alloc]init];

        [self.myView2 setBackgroundColor:[UIColor blueColor]];

        [self.myView2 setTranslatesAutoresizingMaskIntoConstraints:NO];

        [self.view addSubview:self.myView2];

    }

    return _myView2;

}

- (UIView *)myView3

{

    if(!_myView3)

    {

        self.myView3 = [[UIView alloc]init];

        [self.myView3 setBackgroundColor:[UIColor blueColor]];

        [self.myView3 setTranslatesAutoresizingMaskIntoConstraints:NO];

        [self.view addSubview:self.myView3];

    }

    return _myView3;

}

- (void)viewDidLoad {

    [super viewDidLoad];

    [self.view setBackgroundColor:[UIColor orangeColor]];

    [self.view addSubview:self.myView1];

    NSLayoutConstraint *myView1Top =

            [NSLayoutConstraint constraintWithItem:self.myView1 //给谁加约束

                                         attribute:NSLayoutAttributeTop //给哪个边加约束

                                         relatedBy:NSLayoutRelationEqual //什么关系这里是等于

                                            toItem:self.view //相对于哪个视图

                                         attribute:NSLayoutAttributeTop //相对于这个视图的哪个边

                                        multiplier:1 //多少倍

                                          constant:20]; //距离多少右正左负

    NSLayoutConstraint *myView1left = [NSLayoutConstraint constraintWithItem:self.myView1 attribute:NSLayoutAttributeLeft relatedBy:0 toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:10];

    [myView1Top setActive:YES];

    [myView1left setActive:YES];

    NSLayoutConstraint *myView1Width = [NSLayoutConstraint constraintWithItem:self.myView1 attribute:NSLayoutAttributeWidth relatedBy:0 toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:(WIDTH-10)/2.0];

    NSLayoutConstraint *myView1Height = [NSLayoutConstraint constraintWithItem:self.myView1 attribute:NSLayoutAttributeHeight relatedBy:0 toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:(HEIGHT-10)/2.0];

    [myView1Height setActive:YES];

    [myView1Width setActive:YES];

    NSLayoutConstraint *myView2Bottom = [NSLayoutConstraint constraintWithItem:self.myView2 attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.myView1 attribute:NSLayoutAttributeHeight multiplier:1 constant:0];

    NSLayoutConstraint *myView2Top = [NSLayoutConstraint constraintWithItem:self.myView2 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:20];

    NSLayoutConstraint *myView2Left = [NSLayoutConstraint constraintWithItem:self.myView2 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.myView1 attribute:NSLayoutAttributeRight multiplier:1 constant:10];

    NSLayoutConstraint *myView2Right = [NSLayoutConstraint constraintWithItem:self.myView2 attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1 constant:-10];

    [myView2Bottom setActive:YES];

    [myView2Top setActive:YES];

    [myView2Left setActive:YES];

    [myView2Right setActive:YES];

    

    NSLayoutConstraint *myView3Bottom = [NSLayoutConstraint constraintWithItem:self.myView3 attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1 constant:-20];

    NSLayoutConstraint *myView3Top = [NSLayoutConstraint constraintWithItem:self.myView3 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.myView1 attribute:NSLayoutAttributeBottom multiplier:1 constant:10];

    NSLayoutConstraint *myView3Left = [NSLayoutConstraint constraintWithItem:self.myView3 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:10];

    NSLayoutConstraint *myView3Right = [NSLayoutConstraint constraintWithItem:self.myView3 attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1 constant:-10];

    [myView3Bottom setActive:YES];

    [myView3Top setActive:YES];

    [myView3Left setActive:YES];

    [myView3Right setActive:YES];

}