Swift-通过Runtime为UITextView添加占位文字

OC版在这

• 纯代码使用

self.tv.placeholder = "我是占位文字"
self.tv.placeholderColor = UIColor.yellow

• Xib使用

  
  image.png

import Foundation
import UIKit

extension UITextView {
    
    private struct RuntimeKey {
        static let hw_placeholderLabelKey = UnsafeRawPointer.init(bitPattern: "hw_placeholderLabelKey".hashValue)
        /// ...其他Key声明
    }
    /// 占位文字
    @IBInspectable public var placeholder: String {
        get {
            return self.placeholderLabel.text ?? ""
        }
        set {
            self.placeholderLabel.text = newValue
        }
    }
    
    /// 占位文字颜色
    @IBInspectable public var placeholderColor: UIColor {
        get {
            return self.placeholderLabel.textColor
        }
        set {
            self.placeholderLabel.textColor = newValue
        }
    }
    
    private var placeholderLabel: UILabel {
        get {
            var label = objc_getAssociatedObject(self, UITextView.RuntimeKey.hw_placeholderLabelKey!) as? UILabel
            if label == nil { // 不存在是 创建 绑定
                if (self.font == nil) { // 防止没大小时显示异常 系统默认设置14
                    self.font = UIFont.systemFont(ofSize: 14)
                }
                label = UILabel.init(frame: self.bounds)
                label?.numberOfLines = 0
                label?.font = self.font
                label?.textColor = UIColor.lightGray
                self.addSubview(label!)
                self.setValue(label!, forKey: "_placeholderLabel")
                objc_setAssociatedObject(self, UITextView.RuntimeKey.hw_placeholderLabelKey!, label!, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
                self.sendSubviewToBack(label!)
            }
            return label!
        }
        set {
            objc_setAssociatedObject(self, UITextView.RuntimeKey.hw_placeholderLabelKey!, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}