無法將我的UILabel居中在我的自定義視圖上

Question

我正在創建一個自來水戰爭遊戲，並將自定義視圖添加到我的應用程序，redView和blueView。這兩個視圖都佔據了屏幕的一半。我試圖在兩個視圖中的每一個上定位一個標籤，以便跟蹤分數（標籤稱爲「topviewLabel」和「bottomViewLabel」），特別是在中心，但標籤顯示在其各自視圖的左上角並不會正確居中。我以編程方式創建視圖並設置它們的約束，然後將標籤添加到redView或blueView，我以編程方式完成所有這些操作。我會留下下面的代碼，讓我知道我做錯了什麼。

import UIKit 

class ViewController: UIViewController { 

override func viewDidLoad() { 
    super.viewDidLoad() 

    let redView = UIView() 
    redView.backgroundColor = UIColor(red: 206/255, green: 0/255, blue: 0/255, alpha: 1) 
    redView.isMultipleTouchEnabled = true 
    redView.translatesAutoresizingMaskIntoConstraints = false 
    view.addSubview(redView) 

    let redviewConstraints: [NSLayoutConstraint] = [ 
     NSLayoutConstraint(item: redView, attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0), 
     NSLayoutConstraint(item: redView, attribute: .right, relatedBy: .equal, toItem: self.view, attribute: .right, multiplier: 1, constant: 0), 
     NSLayoutConstraint(item: redView, attribute: .top, relatedBy: .equal, toItem: self.view, attribute: .top, multiplier: 1, constant: 0), 
     NSLayoutConstraint(item: redView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: view.frame.height/2) 
    ] 

    view.addConstraints(redviewConstraints) 

    let blueView = UIView() 
    blueView.backgroundColor = UIColor(red: 0/255, green: 0/255, blue: 99/255, alpha: 1) 
    blueView.isMultipleTouchEnabled = true 
    blueView.translatesAutoresizingMaskIntoConstraints = false 
    view.addSubview(blueView) 

    let blueViewConstraints: [NSLayoutConstraint] = [ 
     NSLayoutConstraint(item: blueView, attribute: .left, relatedBy: .equal, toItem: self.view, attribute: .left, multiplier: 1, constant: 0), 
     NSLayoutConstraint(item: blueView, attribute: .right, relatedBy: .equal, toItem: self.view, attribute: .right, multiplier: 1, constant: 0), 
     NSLayoutConstraint(item: blueView, attribute: .bottom, relatedBy: .equal, toItem: self.view, attribute: .bottom, multiplier: 1, constant: 0), 
     NSLayoutConstraint(item: blueView, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: view.frame.height/2) 
    ] 

    view.addConstraints(blueViewConstraints) 


    let topViewLabel = UILabel() 
    topViewLabel.frame = CGRect(x: redView.frame.size.width/2, y: redView.frame.size.height/2, width: 80, height: 80) 
    topViewLabel.text = "30" 
    topViewLabel.textAlignment = .center 
    topViewLabel.font.withSize(30) 
    topViewLabel.textColor = .white 
    topViewLabel.backgroundColor = UIColor.blue 
    topViewLabel.center = redView.center 
    redView.addSubview(topViewLabel) 
    redView.bringSubview(toFront: topViewLabel) 

    let bottomViewLabel = UILabel() 
    bottomViewLabel.frame = CGRect(x: blueView.frame.width/2, y: blueView.frame.height/2, width: 80, height: 80) 
    bottomViewLabel.center = blueView.center 
    bottomViewLabel.text = "20" 
    bottomViewLabel.backgroundColor = UIColor.red 
    blueView.addSubview(bottomViewLabel) 
    blueView.bringSubview(toFront: bottomViewLabel) 

} 

Answer 1

作爲保利斯說，在viewDidLoad你的子視圖沒有設置其框架。所以，當，例如，您嘗試設置您的標籤框架

topViewLabel.frame = CGRect(x: redView.frame.size.width/2, y: redView.frame.size.height/2, width: 80, height: 80) 

redview.frame具有高度和寬度0

這些參數將只在layoutSubviews正確計算。

因此，您可以在viewDidLayoutSubviews（）方法 或（我會這樣做）中使用約束來設置標籤幀。像

let topViewLabelConstraints: [NSLayoutConstraint] = [ 
    NSLayoutConstraint(item: topViewLabel, attribute: .centerX, relatedBy: .equal, toItem: redView, attribute: .centerX, multiplier: 1, constant: 0), 
    NSLayoutConstraint(item: topViewLabel, attribute: .centerY, relatedBy: .equal, toItem: redView, attribute: .centerY, multiplier: 1, constant: 0) 
] 
redView.addConstraints(topViewLabelConstraints) 

應該做的伎倆。

Answer 2

一個UIView沒有添加約束後的正確的幀大小的權利，你需要添加：

blueView.setNeedsLayout() 
blueView.layoutIfNeeded() 

現在blueView的框架設置。