環境

Swift3
Xcode8
Mac Os Sierra

コピペで動くはず

Outletはストーリーボードから


class AddTextViewController: UIViewController, UITextViewDelegate, UITextFieldDelegate {
    
    
    @IBOutlet weak var TitleText: UITextField!
    @IBOutlet weak var DetailText: UITextView!
    

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        TitleText.text = "タイトル"
        TitleText.textColor = UIColor.lightGray
        TitleText.delegate = self
        
        DetailText.text = "本文"
        DetailText.textColor = UIColor.lightGray
        DetailText.delegate = self
    }
    
    func textViewDidBeginEditing(_ textView: UITextView) {
        
        print("反応")
        if DetailText.textColor == UIColor.lightGray {
            DetailText.text = nil
            DetailText.textColor = UIColor.black
        }
    }
    
    
    func textViewDidEndEditing(_ textView: UITextView) {
        if DetailText.text.isEmpty {
            DetailText.text = "本文"
            DetailText.textColor = UIColor.lightGray
        }
    }
    
    func textFieldDidBeginEditing(_ textField: UITextField) {
        if TitleText.textColor == UIColor.lightGray {
            TitleText.text = nil
            TitleText.textColor = UIColor.black
        }
    }
    
    func textFieldDidEndEditing(_ textField: UITextField) {
        if (TitleText.text?.isEmpty)! {
            TitleText.text = "タイトル"
            TitleText.textColor = UIColor.lightGray
        }
    }
}

実行

Step1

UITextViewDelegateをデリゲート

class MyViewController: UIViewController, UITextViewDelegate {

step2

viewWillAppearに下記のように書く bodyTextViewは自分のIBOutletです

 
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        
        bodyTextView.text = "Placeholder"
        bodyTextView.textColor = UIColor.lightGray
        
        
    }

step3

textViewDidBeginEditingとtextViewDidEndEditingを実装

 
    
    func textViewDidBeginEditing(_ textView: UITextView) {
        if bodyTextView.textColor == UIColor.lightGray {
            bodyTextView.text = nil
            bodyTextView.textColor = UIColor.black
        }
    }
    
    
    func textViewDidEndEditing(_ textView: UITextView) {
        if bodyTextView.text.isEmpty {
            bodyTextView.text = "Placeholder"
            bodyTextView.textColor = UIColor.lightGray
        }
    }

注意点

ちなみに、これだと、ストーリーボードに最初からくっついているtextFieldのPlaceholderとは動きが異なるため、textFieldとは併用ができません。なので、textFieldと併用する場合は、textFieldも今回のTextViewと同じように実装しないといけません。

step4

UITextField をデリゲート

class MyViewController: UIViewController, UITextFieldDelegate, UITextViewDelegate {

step5

viewWillAppearに下記のように書く titleTextFieldは自分のIBOutletです

override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        
        bodyTextView.text = "Placeholder"
        bodyTextView.textColor = UIColor.lightGray
        
        titleTextField.text = "Placeholder"
        titleTextField.textColor = UIColor.lightGray
        
        
    }

step6

textFieldDidBeginEditingとtextViewDidEndEditingを実装


      /*
     
     テキストフィールドもプレイスフォルダー実装
     
     */
    
    func textFieldDidBeginEditing(_ textField: UITextField) {
        if titleTextField.textColor == UIColor.lightGray {
            titleTextField.text = nil
            titleTextField.textColor = UIColor.black
        }
    }
    
    
    func textFieldDidEndEditing(_ textField: UITextField) {
        if (titleTextField.text?.isEmpty)! {
            titleTextField.text = "Placeholder"
            titleTextField.textColor = UIColor.lightGray
        }
    }

参考

Swift3 – UITextViewにPlaceholderをLabelを使わずに実装する方法