UITextView 设置 PlaceHolder-白红宇

在日常开发中时常会遇到为 TextView 设置placeholder 的需求。基本的思想是这个样子的： 1.在 UITextView 的左上角上放一个 UILabel，让 UILabel 的 text 来作为 UITextView的 placeholder； 2.为 UITextView添加一个通知NotificationCenter，监听 UITextView 中值得变化； 3.UITextView 的值发生变化就把UILabel 隐藏或者显示。
来贴代码：（代码中有很详细的注释，如果使用的话可以直接新建一个文件把代码粘贴过去就可以了）
import UIKit// @IBDesignable 可以让自定义的这个 view 在 Xib 中使用@IBDesignablepublic class YHYPlaceholderTextView: UITextView {		// 默认的占位字的颜色    private struct Constants {        static let defaultiOSPlaceholderColor = UIColor(red: 179/255, green: 179/255, blue: 179/255, alpha: 0.2)    }		// 占位 label    public let placeholderLabel: UILabel = UILabel()		// 使用 AutoLayout 的约束集合    private var placeholderLabelConstraints = [NSLayoutConstraint]()		// @IBInspectable xib 中可以设置这些属性  这个属性是 text    @IBInspectable public var placeholder: String = "" {        didSet {            placeholderLabel.text = placeholder        }    }		// placeholder 的颜色，有默认值 defaultiOSPlaceholderColor    @IBInspectable public var placeholderColor: UIColor = YHYPlaceholderTextView.Constants.defaultiOSPlaceholderColor {        didSet {            placeholderLabel.textColor = placeholderColor        }    }		// textView 的字体大小重写， 默认placeholder 的字体大小和 textView一样    override public var font: UIFont! {        didSet {            if placeholderFont == nil {                placeholderLabel.font = font            }        }    }		//	placeholder 的字体大小    public var placeholderFont: UIFont? {        didSet {            let font = (placeholderFont != nil) ? placeholderFont : self.font            placeholderLabel.font = font        }    }		// 对齐方式    override public var textAlignment: NSTextAlignment {        didSet {            placeholderLabel.textAlignment = textAlignment        }    }		    override public var text: String! {        didSet {            textDidChange()        }    }        override public var attributedText: NSAttributedString! {        didSet {            textDidChange()        }    }		// 设置 UIlabel 的位置约束    override public var textContainerInset: UIEdgeInsets {        didSet {            updateConstraintsForPlaceholderLabel()        }    }		// 初始化的时候 添加通知，和作为placeholder的UILabel    override public init(frame: CGRect, textContainer: NSTextContainer?) {        super.init(frame: frame, textContainer: textContainer)        commonInit()    }		// 初始化的时候 添加通知，和作为placeholder的UILabel    required public init?(coder aDecoder: NSCoder) {        super.init(coder: aDecoder)        commonInit()    }		// 初始化的时候 添加通知，和作为placeholder的UILabel    private func commonInit() {		        NotificationCenter.default.addObserver(self,            selector: #selector(textDidChange),            name: NSNotification.Name.UITextViewTextDidChange,            object: nil)                placeholderLabel.font = font        placeholderLabel.textColor = placeholderColor        placeholderLabel.textAlignment = textAlignment        placeholderLabel.text = placeholder        placeholderLabel.numberOfLines = 0        placeholderLabel.backgroundColor = UIColor.clear        placeholderLabel.translatesAutoresizingMaskIntoConstraints = false        addSubview(placeholderLabel)		// 添加 UILabel 的约束        updateConstraintsForPlaceholderLabel()    }		// AutoLayout 添加约束    private func updateConstraintsForPlaceholderLabel() {		// 水平方向上 placeholderLabel 距离 textView 的左边距离为 textContainerInset.left + textContainer.lineFragmentPadding        var newConstraints = NSLayoutConstraint.constraints(withVisualFormat: "H:|-(\(textContainerInset.left + textContainer.lineFragmentPadding))-[placeholder]",            options: [],            metrics: nil,            views: ["placeholder": placeholderLabel])				// 竖直方向上 placeholderLabel 距离 textView 的上边距离为textContainerInset.top        newConstraints += NSLayoutConstraint.constraints(withVisualFormat: "V:|-(\(textContainerInset.top))-[placeholder]",            options: [],            metrics: nil,            views: ["placeholder": placeholderLabel])				// placeholderLabel 的宽和 self 的宽的比例约束 注意是有 constant的        newConstraints.append(NSLayoutConstraint(            item: placeholderLabel,            attribute: .width,            relatedBy: .equal,            toItem: self,            attribute: .width,            multiplier: 1.0,            constant: -(textContainerInset.left + textContainerInset.right + textContainer.lineFragmentPadding * 2.0)            ))        removeConstraints(placeholderLabelConstraints)        addConstraints(newConstraints)        placeholderLabelConstraints = newConstraints    }		// 根据 textView 的编辑情况，隐藏或者显示作为placeholder的UILabel    @objc private func textDidChange() {        placeholderLabel.isHidden = !text.isEmpty    }        public override func layoutSubviews() {        super.layoutSubviews()        placeholderLabel.preferredMaxLayoutWidth = textContainer.size.width - textContainer.lineFragmentPadding * 2.0    }		// 移除通知    deinit {        NotificationCenter.default.removeObserver(self,            name: NSNotification.Name.UITextViewTextDidChange,            object: nil)    }    }复制代码