UITextField限制汉字数量最正确的姿势,解决iOS7下substringToIndex方法导致的崩溃

今天在写一个限制UITextField汉字数量的需求,原以为非常简单的一个需求,在实际开发中遇到了很多问题.

首先,汉字的输入时的联想词在输入到TextFiled时,并不会走

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

这导致了在这里进行长度限制的代码都会失效.此路不通.

经过查找资料,得知可以用注册UITextFieldTextDidChangeNotification通知来监测TextField的文字的变化

[[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(textFiledEditChanged:)
          name:@"UITextFieldTextDidChangeNotification" object:myTextField];

在textFiledEditChanged:方法中

- (void)textDidChanged:(NSNotification *)notification{

    NSInteger maxLength = 6;
    UITextField *textField = (UITextField *)notification.object;

    if (textField.text.length > maxLength)
    {
        textField.text = [textField.text substringToIndex:maxLength];
    }
}

　　这种方法看起来没什么问题,然而实测在iOS7下输入汉字时,一旦长度到达最大长度,再进行输入(此时输入状态还是拼音)就会立即崩溃.

*** Terminating app due to uncaught exception ‘NSRangeException‘, reason: ‘NSMutableRLEArray replaceObjectsInRange:withObject:length:: Out of bounds‘

该问题在iOS8以上的系统中不会出现,可能是苹果修复了该BUG.

再次查找资料,找到这篇博客http://www.jianshu.com/p/2d1c06f2dfa4

这篇博客解决了UItextFiled在截取字符串时,如果对emoji进行截取会变乱码的问题(原因是emoji实际上占了3个长度).

UITextField *textField = (UITextField *)obj.object;
    NSString *toBeString = textField.text;

    //获取高亮部分
    UITextRange *selectedRange = [textField markedTextRange];
    UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];

    // 没有高亮选择的字，则对已输入的文字进行字数统计和限制
    if (!position)
    {
        if (toBeString.length > MAX_STARWORDS_LENGTH)
        {
            NSRange rangeIndex = [toBeString rangeOfComposedCharacterSequenceAtIndex:MAX_STARWORDS_LENGTH];
            if (rangeIndex.length == 1)
            {
                textField.text = [toBeString substringToIndex:MAX_STARWORDS_LENGTH];
            }
            else
            {
                NSRange rangeRange = [toBeString rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_STARWORDS_LENGTH)];
                textField.text = [toBeString substringWithRange:rangeRange];
            }
        }
    }

测试后发现,这个方法在iOS8以上的系统运行良好,但是在iOS7下居然完全失效了.

原因是在iOS7下,position对象总是不为nil,导致走不进处理长度的代码.

研究后发现在需要增加一个判断条件,

改进后的代码:

UITextField *textField = (UITextField *)obj.object;
    NSString *toBeString = textField.text;

    //获取高亮部分
    UITextRange *selectedRange = [textField markedTextRange];
    UITextPosition *position = [textField positionFromPosition:selectedRange.start offset:0];

    // 没有高亮选择的字，则对已输入的文字进行字数统计和限制
    if (!position || !selectedRange)
    {
        if (toBeString.length > MAX_STARWORDS_LENGTH)
        {
            NSRange rangeIndex = [toBeString rangeOfComposedCharacterSequenceAtIndex:MAX_STARWORDS_LENGTH];
            if (rangeIndex.length == 1)
            {
                textField.text = [toBeString substringToIndex:MAX_STARWORDS_LENGTH];
            }
            else
            {
                NSRange rangeRange = [toBeString rangeOfComposedCharacterSequencesForRange:NSMakeRange(0, MAX_STARWORDS_LENGTH)];
                textField.text = [toBeString substringWithRange:rangeRange];
            }
        }
    }

再次测试后没有发现崩溃问题了.