PureLayout

@property (nonatomic, strong) UIView *blueView;
@property (nonatomic, strong) UIView *redView;
@property (nonatomic, strong) UIView *yellowView;
@property (nonatomic, strong) UIView *greenView;
@property (nonatomic, assign) BOOL didSetupConstraints;  // 判断是否存在约束条件

- (void)loadView
{
    self.view = [UIView new];
    self.view.backgroundColor = [UIColor colorWithWhite:0.1 alpha:1.0];
    
    [self.view addSubview:self.blueView];
    [self.view addSubview:self.redView];
    [self.view addSubview:self.yellowView];
    [self.view addSubview:self.greenView];
    [self.view setNeedsUpdateConstraints]; //  设置新的约束天剑
}

- (UIView *)blueView
{
    if (!_blueView) {
        _blueView = [UIView newAutoLayoutView];
        _blueView.backgroundColor = [UIColor blueColor];
    }
    return _blueView;
}

- (UIView *)redView
{
    if (!_redView) {
        _redView = [UIView newAutoLayoutView];
        _redView.backgroundColor = [UIColor redColor];
    }
    return _redView;
}

- (UIView *)yellowView
{
    if (!_yellowView) {
        _yellowView = [UIView newAutoLayoutView];
        _yellowView.backgroundColor = [UIColor yellowColor];
    }
    return _yellowView;
}

- (UIView *)greenView
{
    if (!_greenView) {
        _greenView = [UIView newAutoLayoutView];
        _greenView.backgroundColor = [UIColor greenColor];
    }
    return _greenView;
}

- (void)updateViewConstraints   //更新约束条件
{
   // 如果没有自动约束条件
    if (!self.didSetupConstraints) {
        // Blue view is centered on screen,(Centers the view in its superview.) with size {50 pt, 50 pt}

        //设置蓝色的 view 在父试图的中心, 
        [self.blueView autoCenterInSuperview];
        //设置蓝色的 view 的宽和高(50.0 50.0)
        [self.blueView autoSetDimensionsToSize:CGSizeMake(50.0, 50.0)];
        
        // Red view is positioned at the bottom right corner of the blue view, with the same width, and a height of 40 pt
        // 设置红色 view 的顶部是蓝色 view 的底部
        [self.redView autoPinEdge:ALEdgeTop toEdge:ALEdgeBottom ofView:self.blueView];
        //设置红色 view 的左边是蓝色 view 的右边
        [self.redView autoPinEdge:ALEdgeLeft toEdge:ALEdgeRight ofView:self.blueView];
        //设置红色 view 的宽度就是蓝色 view 的宽度
        [self.redView autoMatchDimension:ALDimensionWidth toDimension:ALDimensionWidth ofView:self.blueView];
        //设置红色 view 的高度为40.0
        [self.redView autoSetDimension:ALDimensionHeight toSize:40.0];
        
        // Yellow view is positioned 10 pt below the red view, extending across the screen with 20 pt insets from the edges,
        // and with a fixed height of 25 pt
        //设置黄色 veiw 的顶部距离红色 view 距离为10.0
        [self.yellowView autoPinEdge:ALEdgeTop toEdge:ALEdgeBottom ofView:self.redView withOffset:10.0];
        //设置黄色 view 的高度为25.0
        [self.yellowView autoSetDimension:ALDimensionHeight toSize:25.0];
        //设置黄色 view 距离父试图左边的距离为20.0
        [self.yellowView autoPinEdgeToSuperviewEdge:ALEdgeLeft withInset:20.0];
        //设置黄色 view 距离父试图右边的距离为20.0
        [self.yellowView autoPinEdgeToSuperviewEdge:ALEdgeRight withInset:20.0];
        
        // Green view is positioned 10 pt below the yellow view, aligned to the vertical axis of its superview,
        // with its height twice the height of the yellow view and its width fixed to 150 pt
        //设置绿色 view 的顶部距离黄色 view 的底部距离为10.0
        [self.greenView autoPinEdge:ALEdgeTop toEdge:ALEdgeBottom ofView:self.yellowView withOffset:10.0];
        //设置绿色 view 相对父试图竖向居中
        [self.greenView autoAlignAxisToSuperviewAxis:ALAxisVertical];
        //设置绿色的 view 的高度是黄色 view 高度的2.0倍
        [self.greenView autoMatchDimension:ALDimensionHeight toDimension:ALDimensionHeight ofView:self.yellowView withMultiplier:2.0];
        //设置绿色 view 的宽度为150.0
        [self.greenView autoSetDimension:ALDimensionWidth toSize:150.0];
        
        //设置已经添加过约束了
        self.didSetupConstraints = YES;
    }
    
    [super updateViewConstraints];
}