- (void)viewDidLoad
{
    [super viewDidLoad];
    redBox = [[UIView alloc] init];
    [redBox setBackgroundColor:[UIColor redColor]];
    [redBox setTranslatesAutoresizingMaskIntoConstraints:NO];
    [self.view addSubview:redBox];

    //H:
    widthConstraint = [NSLayoutConstraint constraintWithItem:redBox
                                                   attribute:NSLayoutAttributeWidth
                                                   relatedBy:NSLayoutRelationEqual
                                                      toItem:self.view
                                                   attribute:NSLayoutAttributeWidth
                                                  multiplier:0.5
                                                    constant:0.0];
    [self.view addConstraint:widthConstraint];

    //More constraints... 
}

我想动画一下redBox宽度的增加.

我试过这个：

widthConstraint.constant = 100;

[UIView animateWithDuration:1
                 animations:^{
                     [self.view layoutIfNeeded];
                 }
 ];

这会将视图的宽度增加100并对其进行动画处理,但我需要将其增加一个与其超视图宽度成比例的量.换句话说,我想将widthConstraint的乘数设置为0.8.

但是NSLayoutConstraint’s multiplier is readonly！这是否意味着每次我想要对其乘数进行动画处理时,我必须删除,修改和重新添加widthConstraint,还是有更好的方法？