ios – 使用自动布局的基于百分比的保证金

从逻辑上讲,我只需要将左边距值设置为父宽度的某个百分比值,但此刻,我无法将该逻辑转换为自动布局.

这是我目前的代码：

var view = UIView();
view.backgroundColor = UIColor.redColor();
view.frame = CGRectMake(0,320,400);

var sview = UIView();
sview.setTranslatesAutoresizingMaskIntoConstraints(false);
sview.backgroundColor = UIColor.yellowColor();
//sview.frame = CGRectMake(0,50,50);
view.addSubview(sview);

var dict = Dictionary<String,UIView>()
dict["Box"] = sview;

var con1 = NSLayoutConstraint(item: sview,attribute: NSLayoutAttribute.Top,relatedBy: NSLayoutRelation.Equal,toItem: view,multiplier: 1.0,constant: 20.0);
view.addConstraint(con1);

var con2 = NSLayoutConstraint(item: sview,attribute: NSLayoutAttribute.Left,attribute: NSLayoutAttribute.Width,multiplier: 0.75,constant: 0.0);
view.addConstraint(con2);

var con3 = NSLayoutConstraint(item: sview,constant: 0.0);
view.addConstraint(con3);

var con4 = NSLayoutConstraint(item: sview,attribute: NSLayoutAttribute.Height,constant: 0.0);
view.addConstraint(con4);

这是代码返回错误的位置：

var con2 = NSLayoutConstraint(item: sview,constant: 0.0);
view.addConstraint(con2);

错误：

* Terminating app due to uncaught exception ‘NSinvalidargumentexception’,reason: ‘* +[NSLayoutConstraint
constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]:
Invalid pairing of layout attributes’

有没有人知道如何实现这一目标？我只想让左边距为父视图宽度的0.75％.

谢谢.