界面生成器不会拉伸自定义视图的子视图

我已经创建了带有一个内部UILabel的自定义视图(FAQItemView),它被约束到superview的四个面。以下是此视图的源代码:

import UIKit

@IBDesignable class FAQItemView: UIView {
    var questionLabel: UILabel = UILabel()

   override init(frame: CGRect) {
        super.init(frame: frame)
        setup()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setup()
    }

    func setup() {
        translatesAutoresizingMaskIntoConstraints = false
        addSubview(questionLabel)
        questionLabel.translatesAutoresizingMaskIntoConstraints = false
        questionLabel.textColor = UIColor.black
        questionLabel.textAlignment = .center
        questionLabel.numberOfLines = 0
        questionLabel.topAnchor.constraint(equalTo: self.topAnchor).isActive = true
        questionLabel.leadingAnchor.constraint(equalTo: self.leadingAnchor).isActive = true
        questionLabel.trailingAnchor.constraint(equalTo: self.trailingAnchor).isActive = true
        questionLabel.bottomAnchor.constraint(equalTo: self.bottomAnchor).isActive = true
        questionLabel.text = "question"
        questionLabel.backgroundColor = UIColor.green
    }
}

我在Interface Builder中添加了FAQItemView,并将其宽度限制为200px。在这种情况下,FAQItemView的内部标签应延伸至FAQItemView的大小。运行应用程序时一切正常,但在Interface Builder中,标签以其默认(固有)大小定位在容器的左侧。

示例项目位于 https://www.dropbox.com/s/u2923l8exqtg3ir/testapp1.zip?dl=0 . 此处FAQItemView具有红色背景,内部标签具有绿色背景。在运行时,红色不可见,因为标签具有绿色背景,但在Interface Builder中,红色也可见(在具有绿色背景的标签的右侧)

有人能说出我做错了什么吗?

提前谢谢。

UPD: Screenshot of view in interface builder