Autolayout自动布局2

需求：控件水平居中，垂直居中，宽度高度均为父控件的一半

    // 1.创建控件    UIView *blueView = [[UIView alloc] init];    [self.view addSubview:blueView];    blueView.backgroundColor = [UIColor blueColor];    //不要将AutoresizingMask转为Autolayout的约束    blueView.translatesAutoresizingMaskIntoConstraints = NO;        // 2.添加约束    //宽度约束：父控件的一半    NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:blueView.superview attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];    [blueView.superview addConstraint:widthConstraint];        //高度约束：父控件的一半    NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:blueView.superview attribute:NSLayoutAttributeHeight multiplier:0.5 constant:0];    [blueView.superview addConstraint:heightConstraint];        //水平居中    NSLayoutConstraint *centerXConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:blueView.superview attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];    [blueView.superview addConstraint:centerXConstraint];        //垂直居中    NSLayoutConstraint *centerYConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:blueView.superview attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0];    [blueView.superview addConstraint:centerYConstraint];