您需要创建约束的IBOutlet。

You need to create an IBOutlet of your constraint.

然后在代码中设置约束的常量值：

Then you set the constant value of your constraint in code:

labelWidthConstraint.constant = newValue

如果你想要它的动画，你可以这样做：

If you want it animated you can do something like this:

labelWidthConstraint.constant = newValue
UIView.animate(withDuration: 0.3, animations: { 
    view.layoutIfNeeded()
}

目标-C

Objective-C

self.labelWidthConstraint.constant = newValue;
[UIView animateWithDuration:0.3 animations:^{        
    [self.view layoutIfNeeded];
}];