模拟本机占位符

一个常见的抱怨是iOS没有为文本视图提供原生占位符功能。下面的UITextView扩展试图通过提供人们期望从本机特性获得的便利来解决这个问题，只需要一行代码就可以向textview实例添加占位符。

这个解决方案的缺点是，因为它菊花链委托调用，它很容易(不太可能)在iOS更新中更改UITextViewDelegate协议。具体来说，如果iOS添加了新的协议方法，并且你在带有占位符的文本视图的委托中实现了它们中的任何一个，这些方法将不会被调用，除非你也更新了扩展来转发这些调用。

或者，内联占位符的答案是一个坚如磐石的和简单的可以。

使用例子:

•如果获得占位符的文本视图没有使用UITextViewDelegate:

    /* Swift 3 */

    class NoteViewController : UIViewController {
        @IBOutlet weak var noteView: UITextView!
        override func viewDidLoad() {
            noteView.addPlaceholder("Enter some text...",  color: UIColor.lightGray)
        }
    }

                                            -- 或者——

•如果获得占位符的文本视图使用了UITextViewDelegate:

    /* Swift 3 */

    class NoteViewController : UIViewController, UITextViewDelegate {
        @IBOutlet weak var noteView: UITextView!
        override func viewDidLoad() {
            noteView.addPlaceholder("Phone #", color: UIColor.lightGray, delegate: self)
        }
    }

实现(UITextView扩展):

/* Swift 3 */

extension UITextView: UITextViewDelegate
{

    func addPlaceholder(_ placeholderText : String, 
                      color : UIColor? = UIColor.lightGray,
                      delegate : UITextViewDelegate? = nil) {

        self.delegate = self             // Make receiving textview instance a delegate
        let placeholder = UITextView()   // Need delegate storage ULabel doesn't provide
        placeholder.isUserInteractionEnabled = false  //... so we *simulate* UILabel
        self.addSubview(placeholder)     // Add to text view instance's view tree               
        placeholder.sizeToFit()          // Constrain to fit inside parent text view
        placeholder.tintColor = UIColor.clear // Unused in textviews. Can host our 'tag'
        placeholder.frame.origin = CGPoint(x: 5, y: 0) // Don't cover I-beam cursor
        placeholder.delegate = delegate  // Use as cache for caller's delegate 
        placeholder.font = UIFont.italicSystemFont(ofSize: (self.font?.pointSize)!)
        placeholder.text = placeholderText
        placeholder.textColor = color
    }

      
    func findPlaceholder() -> UITextView? { // find placeholder by its tag 
        for subview in self.subviews {
            if let textview = subview as? UITextView {
                if textview.tintColor == UIColor.clear { // sneaky tagging scheme
                    return textview
                }
            }
        }
        return nil
    }
     
    /* 
     * Safely daisychain to caller delegate methods as appropriate...
     */

    public func textViewDidChange(_ textView: UITextView) { // ←  need this delegate method
        if let placeholder = findPlaceholder() {
            placeholder.isHidden = !self.text.isEmpty // ← ... to do this
            placeholder.delegate?.textViewDidChange?(textView)
        } 
    }

    /* 
     * Since we're becoming a delegate on behalf of this placeholder-enabled
     * text view instance, we must forward *all* that protocol's activity expected
     * by the instance, not just the particular optional protocol method we need to
     * intercept, above.
     */

    public func textViewDidEndEditing(_ textView: UITextView) {
        if let placeholder = findPlaceholder() {
            placeholder.delegate?.textViewDidEndEditing?(textView)
        } 
    }

    public func textViewDidBeginEditing(_ textView: UITextView) {
        if let placeholder = findPlaceholder() {
            placeholder.delegate?.textViewDidBeginEditing?(textView)
        } 
    }

    public  func textViewDidChangeSelection(_ textView: UITextView) {
        if let placeholder = findPlaceholder() {
            placeholder.delegate?.textViewDidChangeSelection?(textView)
        } 
    }

    public func textViewShouldEndEditing(_ textView: UITextView) -> Bool {
        if let placeholder = findPlaceholder() {
            guard let retval = placeholder.delegate?.textViewShouldEndEditing?(textView) else {
                return true
            }
            return retval
        }
        return true
    }

    public func textViewShouldBeginEditing(_ textView: UITextView) -> Bool {
        if let placeholder = findPlaceholder() {
            guard let retval = placeholder.delegate?.textViewShouldBeginEditing?(textView) else {
                return true
            }
            return retval
        } 
        return true
    }

    public func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
        if let placeholder = findPlaceholder() {
            guard let retval = placeholder.delegate?.textView?(textView, shouldChangeTextIn: range, replacementText: text) else {
                return true
            }
            return retval
        } 
        return true
    }

    public func textView(_ textView: UITextView, shouldInteractWith URL: URL, in characterRange: NSRange, interaction: UITextItemInteraction) -> Bool {
        if let placeholder = findPlaceholder() {
                guard let retval = placeholder.delegate?.textView?(textView, shouldInteractWith: URL, in: characterRange, interaction:
                    interaction) else {
                        return true
            }
            return retval
        }
        return true
    }

    public func textView(_ textView: UITextView, shouldInteractWith textAttachment: NSTextAttachment, in characterRange: NSRange, interaction: UITextItemInteraction) -> Bool {
        if let placeholder = findPlaceholder() {
            guard let retval = placeholder.delegate?.textView?(textView, shouldInteractWith: textAttachment, in: characterRange, interaction: interaction) else {
                return true
            }
            return retval
        }
        return true
    }
}

1. 作为一个像UITextView这样的基本iOS类的扩展，重要的是要知道这段代码没有与任何不激活占位符的textview交互，例如textview实例没有被调用addPlaceholder()初始化

2. 支持占位符的文本视图透明地变成一个UITextViewDelegate来跟踪字符计数，以控制占位符可见性。如果一个委托被传递给addPlaceholder()，这段代码会将委托回调给该委托。

3.作者正在研究如何检查UITextViewDelegate协议并自动代理它，而不必硬编码每个方法。这将使代码免受方法签名更改和添加到协议中的新方法的影响。

好吧，我的阴沟有点不同 我创建了一个小班来为你做这件事。

TextViewShader。m文件

#import "TextViewShader.h"

@implementation TextViewShader
-(id)initWithShadedTextView:(NSString *)text textViewToShade:(UITextView *)textview {
    self = [super initWithFrame:textview.frame];
    if (self) {
        if (shadeLabel==nil)
        {
            shadeLabel= [[UILabel alloc]initWithFrame:CGRectMake(10, 0, textview.frame.size.width, 30)];


    }
    shadeLabel.text =text;// @"Enter Your Support Request";
    shadeLabel.textColor = [UIColor lightGrayColor];
    [textview setDelegate: self];
    [textview addSubview:shadeLabel];
}
return self;
}

-(void)textViewDidChange:(UITextView *)textView{
        if (textView.text.length==0)
        {
            shadeLabel.hidden=false; 
        }
        else
        {
            shadeLabel.hidden=true;
        }

}

@end

TextViewShader.h文件

#import <UIKit/UIKit.h>

@interface TextViewShader : UIView<UITextViewDelegate>{
    UILabel *shadeLabel;

}
-(id)initWithShadedTextView:(NSString *)text textViewToShade:(UITextView *)textview ;
@end

这是简单的一行代码使用(不要忘记添加#import "TextViewShader.h")

 TextViewShader* shader = [[TextViewShader alloc]initWithShadedTextView:@"Enter Your Support Request" textViewToShade: youruitextviewToshade];

玩得开心!

下面是swift 3.1的代码

原始代码由杰森乔治在第一个答案。

不要忘记设置你的自定义类的TextView在接口生成器UIPlaceHolderTextView，然后设置占位符和占位符属性。

import UIKit

@IBDesignable
class UIPlaceHolderTextView: UITextView {

@IBInspectable var placeholder: String = ""
@IBInspectable var placeholderColor: UIColor = UIColor.lightGray

private let uiPlaceholderTextChangedAnimationDuration: Double = 0.05
private let defaultTagValue = 999

private var placeHolderLabel: UILabel?

override func awakeFromNib() {
    super.awakeFromNib()
    NotificationCenter.default.addObserver(
        self,
        selector: #selector(textChanged),
        name: NSNotification.Name.UITextViewTextDidChange,
        object: nil
    )
}

override init(frame: CGRect, textContainer: NSTextContainer?) {
    super.init(frame: frame, textContainer: textContainer)
    NotificationCenter.default.addObserver(
        self,
        selector: #selector(textChanged),
        name: NSNotification.Name.UITextViewTextDidChange,
        object: nil
    )
}

required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    NotificationCenter.default.addObserver(
        self,
        selector: #selector(textChanged),
        name: NSNotification.Name.UITextViewTextDidChange,
        object: nil
    )
}

deinit {
    NotificationCenter.default.removeObserver(
        self,
        name: NSNotification.Name.UITextViewTextDidChange,
        object: nil
    )
}

@objc private func textChanged() {
    guard !placeholder.isEmpty else {
        return
    }
    UIView.animate(withDuration: uiPlaceholderTextChangedAnimationDuration) {
        if self.text.isEmpty {
            self.viewWithTag(self.defaultTagValue)?.alpha = CGFloat(1.0)
        }
        else {
            self.viewWithTag(self.defaultTagValue)?.alpha = CGFloat(0.0)
        }
    }
}

override var text: String! {
    didSet{
        super.text = text
        textChanged()
    }
}

override func draw(_ rect: CGRect) {
    if !placeholder.isEmpty {
        if placeHolderLabel == nil {
            placeHolderLabel = UILabel.init(frame: CGRect(x: 0, y: 8, width: bounds.size.width - 16, height: 0))
            placeHolderLabel!.lineBreakMode = .byWordWrapping
            placeHolderLabel!.numberOfLines = 0
            placeHolderLabel!.font = font
            placeHolderLabel!.backgroundColor = UIColor.clear
            placeHolderLabel!.textColor = placeholderColor
            placeHolderLabel!.alpha = 0
            placeHolderLabel!.tag = defaultTagValue
            self.addSubview(placeHolderLabel!)
        }

        placeHolderLabel!.text = placeholder
        placeHolderLabel!.sizeToFit()
        self.sendSubview(toBack: placeHolderLabel!)

        if text.isEmpty && !placeholder.isEmpty {
            viewWithTag(defaultTagValue)?.alpha = 1.0
        }
    }

    super.draw(rect)
}
}

你能做的是在文本属性中设置文本视图的初始值，并将textColor更改为[UIColor grayColor]或类似的东西。然后，每当文本视图变为可编辑时，清除文本并显示游标，如果文本字段再次为空，则将占位符文本放回。根据需要将颜色更改为[UIColor blackColor]。

它与UITextField中的占位符功能不完全相同，但很接近。

    - (void)textViewDidChange:(UITextView *)textView
{
    placeholderLabel.hidden = YES;
}

在textview上放一个标签。

在h课上

@interface RateCommentViewController : UIViewController<UITextViewDelegate>{IBoutlet UITextview *commentTxtView;}

在。m类

- (void)viewDidLoad{      
    commentTxtView.text = @"Comment";
    commentTxtView.textColor = [UIColor lightGrayColor];
    commentTxtView.delegate = self;
}

- (BOOL) textViewShouldBeginEditing:(UITextView *)textView
{
    commentTxtView.text = @"";
    commentTxtView.textColor = [UIColor blackColor];
    return YES;
}

-(void) textViewDidChange:(UITextView *)textView
{
    if(commentTxtView.text.length == 0){
        commentTxtView.textColor = [UIColor lightGrayColor];
        commentTxtView.text = @"Comment";
        [commentTxtView resignFirstResponder];
    }
}