1、定義一個(gè)事件：

- (void)limitLength:(UITextField *)sender {
    bool isChinese;//判斷當(dāng)前輸入法是否是中文
    if ([[[UITextInputMode currentInputMode] primaryLanguage] isEqualToString: @"en-US"]) {
        isChinese = false;
    } else {
        isChinese = true;
    }
    
    if (sender == self.textField) {
        // 5位
        NSString *str = [[self.textField text] stringByReplacingOccurrencesOfString:@"?" withString:@""];
        if (isChinese) { //中文輸入法下
            UITextRange *selectedRange = [self.textField markedTextRange];
            //獲取高亮部分
            UITextPosition *position = [self.textField positionFromPosition:selectedRange.start offset:0];
            // 沒有高亮選擇的字，則對(duì)已輸入的文字進(jìn)行字?jǐn)?shù)統(tǒng)計(jì)和限制
            if (!position) {
                NSLog(@"漢字");
                if ( str.length>=6) {
                    NSString *strNew = [NSString stringWithString:str];
                    [self.textField setText:[strNew substringToIndex:5]];
                }
            } else {
                NSLog(@"輸入的英文還沒有轉(zhuǎn)化為漢字的狀態(tài)");
            }
        } else {
            NSLog(@"str=%@; 本次長(zhǎng)度=%ld",str,[str length]);
            if ([str length] >= 6) {
                NSString *strNew = [NSString stringWithString:str];
                [self.textField setText:[strNew substringToIndex:5]];
            }
        }
    }
}

2、對(duì)UITextField控件添加監(jiān)聽事件：

[self.textField addTarget:self action:@selector(limitLength:) forControlEvents:UIControlEventEditingChanged];