Ios 具有不同对齐子视图的视图

我想将蓝色和紫色视图与屏幕中心对齐，将绿色视图与屏幕左侧对齐：

这是我的密码：

view.backgroundColor = UIColor.orangeColor()

//Stackview: 
let stackView   = UIStackView()
stackView.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(stackView)

stackView.topAnchor.constraintEqualToAnchor(self.view.topAnchor).active = true
stackView.leftAnchor.constraintEqualToAnchor(self.view.leftAnchor).active = true
stackView.rightAnchor.constraintEqualToAnchor(self.view.rightAnchor).active = true

stackView.axis  = UILayoutConstraintAxis.Vertical
stackView.alignment = .Center

//Blue view: 
let blueBox = UIView()
stackView.addArrangedSubview(blueBox)
blueBox.backgroundColor = UIColor.blueColor()
blueBox.heightAnchor.constraintEqualToConstant(140).active = true
blueBox.widthAnchor.constraintEqualToConstant(140).active = true

//Inner stackview that contains the green view: 
let greenBoxContainer = UIStackView()
let greenBox = UIView()
stackView.addArrangedSubview(greenBoxContainer)
greenBoxContainer.addArrangedSubview(greenBox)
greenBoxContainer.alignment = .Leading

//Green view:
greenBox.backgroundColor = UIColor.greenColor()
greenBox.widthAnchor.constraintEqualToConstant(120).active = true
greenBox.heightAnchor.constraintEqualToConstant(120).active = true

//Purple view: 
let purpleView = UIView()
stackView.addArrangedSubview(purpleView)
purpleView.backgroundColor = UIColor.purpleColor()
purpleView.heightAnchor.constraintEqualToConstant(50.0).active = true
purpleView.widthAnchor.constraintEqualToConstant(50.0).active = true

重复一下，如何将绿色视图的左边缘与屏幕的左边缘对齐

我试过这个：

greenBoxContainer.widthAnchor.constraintEqualToAnchor(stackView.widthAnchor).active = true

---

但它只能将绿色视图延伸到整个屏幕的长度

一种方法是添加间隔视图：

let greenBox = UIView()
stackView.addArrangedSubview(greenBoxContainer)
greenBoxContainer.addArrangedSubview(greenBox)

let spacer = UIView()
greenBoxContainer.addArrangedSubview(spacer)
spacer.rightAnchor.constraintEqualToAnchor(greenBoxContainer.rightAnchor).active = true

greenBoxContainer.widthAnchor.constraintEqualToAnchor(stackView.widthAnchor).active = true
greenBox.backgroundColor = UIColor.greenColor()
greenBox.widthAnchor.constraintEqualToConstant(120).active = true
greenBox.heightAnchor.constraintEqualToConstant(120).active = true

请注意，我不再设置

greenBoxContainer

的对齐方式，因此它默认为

.Fill

。因此，

间隔片的右边缘

与内部stackview的右边缘齐平，占据了所有宽度，只留下足够的空间来满足其宽度约束

但这是一种解决方法，我希望能够在不必创建间隔视图的情况下指定路线