为了完成八月的回答，建议函数的一个可能实现(参见UITextField的委托)。

我没有测试domness代码，但我的代码不会卡住，如果用户达到了限制，它与一个新的字符串兼容，取代一个更小或等于一个。

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    //limit the size :
    int limit = 20;
    return !([textField.text length]>limit && [string length] > range.length);
}

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
    }

}

斯威夫特4

import UIKit

private var kAssociationKeyMaxLength: 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
    }
}

编辑:内存泄漏问题修复。

我根据@Frouo给出了补充答案。我认为他的回答是最美丽的方式。因为这是一个我们可以重用的通用控件。

private var kAssociationKeyMaxLength: 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)
            self.addTarget(self, action: #selector(checkMaxLength), for: .editingChanged)
        }
    }

    func checkMaxLength(textField: UITextField) {

        guard !self.isInputMethod(), let prospectiveText = self.text,
            prospectiveText.count > maxLength
            else {
                return
        }

        let selection = selectedTextRange
        let maxCharIndex = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
        text = prospectiveText.substring(to: maxCharIndex)
        selectedTextRange = selection
    }

    //The method is used to cancel the check when use Chinese Pinyin input method.
    //Becuase the alphabet also appears in the textfield when inputting, we should cancel the check.
    func isInputMethod() -> Bool {
        if let positionRange = self.markedTextRange {
            if let _ = self.position(from: positionRange.start, offset: 0) {
                return true
            }
        }
        return false
    }

}

这个简单的方法怎么样?它对我来说很好。

extension UITextField {

    func  charactersLimit(to:Int) {

        if (self.text!.count > to) {
            self.deleteBackward()
        }
    }
}

然后:

someTextField.charactersLimit(to:16)

你不能直接这样做- UITextField没有maxLength属性，但你可以设置UITextField的委托，然后使用:

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