我想补充一下@sickp给出的答案。

在他的Swift代码中有一个问题，任何多字节文本(例如表情符号)都会出现。NSRange和String在Swift中是不兼容的，所以委托类将它们结合在一起是令人沮丧的。技巧是简单地转换String对象到NSString正确的解决方案，基于什么@sickp写，实际上是这样的:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    let currentText = (textField.text as NSString?) ?? NSString()
    let currentCharacterCount = currentText.length
    if range.length + range.location > currentCharacterCount {
        return false
    }
    let newLength = currentText.replacingCharacters(in: range, with: string).characters.count
    return newLength <= 25
}

有通用的解决方案设置最大长度在Swift。 通过IBInspectable，你可以在Xcode属性检查器中添加新的属性。

import UIKit
private var maxLengths = [UITextField: Int]()
extension UITextField {

    @IBInspectable var maxLength: Int {
        get {
            guard let length = maxLengths[self]
            else {
                return Int.max
            }
            return length
        }
        set {
            maxLengths[self] = newValue
            addTarget(
                self,
                action: Selector("limitLength:"),
                forControlEvents: UIControlEvents.EditingChanged
            )
        }
    }

    func limitLength(textField: UITextField) {
        guard let prospectiveText = textField.text
            where prospectiveText.characters.count > maxLength else {
                return
        }
        let selection = selectedTextRange
        text = prospectiveText.substringWithRange(
            Range<String.Index>(prospectiveText.startIndex ..< prospectiveText.startIndex.advancedBy(maxLength))
        )
        selectedTextRange = selection
    }

}

使用接口构建器，您可以在任何函数中链接并获得“编辑更改”的事件。 现在你可以检查长度

- (IBAction)onValueChange:(id)sender 
{
    NSString *text = nil;
    int MAX_LENGTH = 20;
    switch ([sender tag] ) 
    {
        case 1: 
        {
            text = myEditField.text;
            if (MAX_LENGTH < [text length]) {
                myEditField.text = [text substringToIndex:MAX_LENGTH];
            }
        }
            break;
        default:
            break;
    }

}

Swift 4 + 2。

通过实现UITextFieldDelegate方法

ViewController: class MyViewController: UIViewController { let MAX_LENGTH = 256 @IBOutlet weak var myTextField: UITextField! override viewDidLoad() { self.myTextField.delegate = self } } Delegate: extension MyViewController: UITextFieldDelegate { func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool { let userText = textView.text ?? "" var newText = "" if range.length > 0 { let txt = NSString(string: userText) if txt.length > 0 { newText = txt.replacingCharacters(in: range, with: text) } } else { newText = userText + text } return newText.count <= MAX_LENGTH } }

使用下面的扩展来设置UITextField和UITextView的最大字符长度。

斯威夫特4.0

    private var kAssociationKeyMaxLength: Int = 0
    private var kAssociationKeyMaxLengthTextView: Int = 0
    extension UITextField {


        @IBInspectable var maxLength: Int {
            get {
                if let length = objc_getAssociatedObject(self, &kAssociationKeyMaxLength) as? Int {
                    return length
                } else {
                    return Int.max
                }
            }
            set {
                objc_setAssociatedObject(self, &kAssociationKeyMaxLength, newValue, .OBJC_ASSOCIATION_RETAIN)
                addTarget(self, action: #selector(checkMaxLength), for: .editingChanged)
            }
        }

        @objc func checkMaxLength(textField: UITextField) {
            guard let prospectiveText = self.text,
                prospectiveText.count > maxLength
                else {
                    return
            }

            let selection = selectedTextRange

            let indexEndOfText = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
            let substring = prospectiveText[..<indexEndOfText]
            text = String(substring)

            selectedTextRange = selection
        }
    }

UITextView

extension UITextView:UITextViewDelegate {


        @IBInspectable var maxLength: Int {
            get {
                if let length = objc_getAssociatedObject(self, &kAssociationKeyMaxLengthTextView) as? Int {
                    return length
                } else {
                    return Int.max
                }
            }
            set {
                self.delegate = self

                objc_setAssociatedObject(self, &kAssociationKeyMaxLengthTextView, newValue, .OBJC_ASSOCIATION_RETAIN)
            }
        }

        public func textViewDidChange(_ textView: UITextView) {
            checkMaxLength(textField: self)
        }
        @objc func checkMaxLength(textField: UITextView) {
            guard let prospectiveText = self.text,
                prospectiveText.count > maxLength
                else {
                    return
            }

            let selection = selectedTextRange

            let indexEndOfText = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
            let substring = prospectiveText[..<indexEndOfText]
            text = String(substring)

            selectedTextRange = selection
        }
    }

你可以在下面设置限制。

我已经实现了一个UITextField扩展，以添加一个maxLength属性。

它基于Xcode 6 IBInspectables，所以你可以在接口构建器上设置maxLength限制。

实现如下:

UITextField + MaxLength。h

#import <UIKit/UIKit.h>

@interface UITextField_MaxLength : UITextField<UITextFieldDelegate>

@property (nonatomic)IBInspectable int textMaxLength;
@end

UITextField + MaxLength。m

#import "UITextField+MaxLength.h"

@interface UITextField_MaxLength()

@property (nonatomic, assign) id <UITextFieldDelegate> superDelegate;

@end

@implementation UITextField_MaxLength

- (BOOL)textField:(UITextField *) textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {

    //validate the length, only if it's set to a non zero value
    if (self.textMaxLength>0) {
        if(range.length + range.location > textField.text.length)
            return NO;

        if (textField.text.length+string.length - range.length>self.textMaxLength) {
            return NO;
        }
    }

    //if length validation was passed, query the super class to see if the delegate method is implemented there
    if (self.superDelegate && [self.superDelegate respondsToSelector:@selector(textField:shouldChangeCharactersInRange:replacementString:)]) {
        return [self.superDelegate textField:textField shouldChangeCharactersInRange:range replacementString:string];
    }
    else{
        //if the super class does not implement the delegate method, simply return YES as the length validation was passed
        return YES;
    }
}

- (void)setDelegate:(id<UITextFieldDelegate>)delegate {
    if (delegate == self)
        return;
    self.superDelegate = delegate;
    [super setDelegate:self];
}

//forward all non overriden delegate methods
- (id)forwardingTargetForSelector:(SEL)aSelector {
    if ([self.superDelegate  respondsToSelector:aSelector])
        return self.superDelegate;

    return [super forwardingTargetForSelector:aSelector];
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([self.superDelegate respondsToSelector:aSelector])
        return YES;

    return [super respondsToSelector:aSelector];
}
@end