Add a placeholder property to UITextView.

UITextView+placeholder.swift

//
//  UITextView+placeholder.swift
//
//  Created by Li-Heng Hsu on 02/06/2018.
//  Copyright © 2018 narrativesaw. All rights reserved.
//

import UIKit

private let defaultPlaceholderColor = UIColor.gray

extension UITextView {
    
    private var placeholderLabel: PlaceholderLabel {
        if let label = subviews.compactMap( { $0 as? PlaceholderLabel }).first {
            return label
        } else {
            let label = PlaceholderLabel(frame: bounds)
            label.autoresizingMask = [.flexibleBottomMargin, .flexibleWidth]
            label.font = font
            label.textColor = defaultPlaceholderColor
            label.numberOfLines = 0
            label.insetsProvider = self
            textStorage.delegate = label
            insertSubview(label, at: 0)
            return label
        }
    }
    
    @IBInspectable
    var placeholder: String {
        get {
            return placeholderLabel.text ?? ""
        }
        set {
            placeholderLabel.text = newValue
        }
    }
    
    @IBInspectable
    var placeholderColor: UIColor {
        get {
            return placeholderLabel.textColor
        }
        set {
            placeholderLabel.textColor = newValue
        }
    }
    
}

extension UITextView: PlaceholderLabelInsetsProviding {
    
    fileprivate func insets(for label: PlaceholderLabel) -> UIEdgeInsets {
        let padding = textContainer.lineFragmentPadding
        var insets = textContainerInset
        insets.left += padding
        insets.right += padding
        return insets
    }
    
}

private protocol PlaceholderLabelInsetsProviding: AnyObject {
    func insets(for label: PlaceholderLabel) -> UIEdgeInsets
}

private class PlaceholderLabel: UILabel {
    
    weak var insetsProvider: PlaceholderLabelInsetsProviding?
    
    override func drawText(in rect: CGRect) {
        var rect = rect
        if let insets = insetsProvider?.insets(for: self) {
            rect = UIEdgeInsetsInsetRect(rect, insets)
        }
        rect.size = sizeThatFits(CGSize(width: rect.width, height: .greatestFiniteMagnitude))
        super.drawText(in: rect)
    }
    
}
    
extension PlaceholderLabel: NSTextStorageDelegate {
    
    func textStorage(_ textStorage: NSTextStorage, didProcessEditing editedMask: NSTextStorageEditActions, range editedRange: NSRange, changeInLength delta: Int) {
        if editedMask.contains(.editedCharacters) {
            isHidden = textStorage.length != 0
        }
    }
    
}

I fixed the auto resizing part and moved the nested class out to make the code structure flatter.