ios - 使用约束将按钮移向目标，没有反应？

Question

我希望红色按钮动画朝向第二个按钮的领先位置:

一些例子展示了如何用数字改变“常量”，但我想自动放在第二个按钮的前导位置。

我试过了，但是红色按钮没有移动，不过动画日志被正确调用了:

- (void)updateConstr{

    NSLayoutConstraint *newLeading = [NSLayoutConstraint
                                                  constraintWithItem:self.redB
                                                  attribute:NSLayoutAttributeLeading
                                                  relatedBy:NSLayoutRelationEqual
                                                  toItem:self.secondButton
                                                  attribute:NSLayoutAttributeLeading
                                                  multiplier:1.0
                                                  constant:0.0f];

    self.leadingConstraint = newLeading;//is an IBOutlet pointing on the constraint (see the image)
    [self.redB setNeedsUpdateConstraints];
    [UIView animateWithDuration:0.5 animations:^{
        [self.redB layoutIfNeeded];
        NSLog(@"animations");//is called, but the red button does not move
    }];
}

- (IBAction)firstAction:(id)sender { //after a click on button "one"
    NSLog(@"firstAction");
    [self updateConstr];
}

Answer 1

必须这样做:

- (void)updateConstr{

    NSLayoutConstraint *newLeading = [NSLayoutConstraint
                                      constraintWithItem:self.redB
                                      attribute:NSLayoutAttributeLeading
                                      relatedBy:NSLayoutRelationEqual
                                      toItem:self.secondButton
                                      attribute:NSLayoutAttributeLeading
                                      multiplier:1.0
                                      constant:0.0f];

    [self.redB.superview removeConstraint: self.leadingConstraint];
    [self.redB.superview addConstraint: newLeading];
    self.leadingConstraint = newLeading;

    [UIView animateWithDuration:0.5 animations:^{
        [self.redB layoutIfNeeded];
    }];
}