PlaceHolderedTextView.swift

//
//  PlaceHolderedTextView.swift
//  Musubi
//
//  Created by はるふ on 2016/11/29.
//  Copyright © 2016年 はるふ. All rights reserved.
//

import UIKit

@IBDesignable
class PlaceHolderedTextView: UITextView {
    
    // MARK: stored properties
    
    @IBInspectable var placeHolder = ""
    @IBInspectable var placeHolderColor = UIColor.lightGray
    
    private lazy var placeHolderLayer: CATextLayer = {
        let layer = CATextLayer()
        layer.fontSize = self.font?.pointSize ?? UIFont.systemFontSize
        layer.frame = CGRect(x: self.textContainerInset.left + self.textContainer.lineFragmentPadding, y: self.textContainerInset.top, width: self.frame.width, height: layer.fontSize+10)
        layer.string = self.placeHolder
        layer.foregroundColor = self.placeHolderColor.cgColor
        layer.contentsScale = UIScreen.main.scale
        layer.alignmentMode = kCAAlignmentLeft
        self.layer.addSublayer(layer)
        return layer
    }()
    
    private func updateLayer() {
        DispatchQueue.main.async { [weak self] in
            self?.placeHolderLayer.isHidden = !(self?.text?.isEmpty ?? true)
        }
    }
    
    func onChangedText(_ notification: NSNotification?) {
        updateLayer()
    }
    
    // MARK: Observer
    
    private func addObserver() {
        updateLayer()
        NotificationCenter.default.addObserver(self, selector: #selector(self.onChangedText(_:)), name: NSNotification.Name.UITextViewTextDidChange, object: self)
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        addObserver()
    }
    
    override init(frame: CGRect, textContainer: NSTextContainer?) {
        super.init(frame: frame, textContainer: textContainer)
        addObserver()
    }
    
    // iOS 9.0以降なら不要
    // http://qiita.com/mono0926/items/754c5d2dbe431542c75e
    deinit {
        NotificationCenter.default.removeObserver(self)
    }
}