轻松实现UITextView 占位符（placeholder）

轻松让UITextView实现placeholder, 不需要一系列的隐藏显示逻辑。

• 在UITextView extension中新建一个设置placeholder的方法

import UIKit

extension UITextView {
    func setPlaceholder(text: String) {
        let placeholderLabel = UILabel()
        placeholderLabel.numberOfLines = 0
        placeholderLabel.font = font
        placeholderLabel.textColor = UIColor(white: 120/255, alpha: 1)
        placeholderLabel.text = text
        placeholderLabel.sizeToFit()
        addSubview(placeholderLabel)
        setValue(placeholderLabel, forKeyPath: "_placeholderLabel")
    }
}

我的理解是，Apple把“_placeholderLabel”这个key预留了。

• 使用UITextView, 调用setPlaceholder()给placeholder赋值

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        setupTextView()
    }

    private func setupTextView() {
        let textView = UITextView()
        textView.font = UIFont.systemFont(ofSize: 20.0)
        textView.layer.borderColor = UIColor(white: 229/255, alpha: 1).cgColor
        textView.layer.borderWidth = 1.0
        textView.layer.masksToBounds = true
        textView.textContainer.lineFragmentPadding = 15.0
        textView.setPlaceholder(text: "请输入你想说的话。")
        view.addSubview(textView)
        
        textView.translatesAutoresizingMaskIntoConstraints = false
        textView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20.0).isActive = true
        textView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -20.0).isActive = true
        textView.topAnchor.constraint(equalTo: view.topAnchor, constant: 100.0).isActive = true
        textView.heightAnchor.constraint(equalToConstant: 200.0).isActive = true
    }
}

不需要在UITextView delegate或者给UITextView text赋值时候做隐藏显示等逻辑处理了。