UITextView字数最大值控制

-(BOOL)textView:(UITextView *)textView shouldChangeTextInRange:(NSRange)range replacementText:(NSString *)text{
    UITextRange *selectedRange = [textView markedTextRange];
    //获取高亮部分
    UITextPosition *pos = [textView positionFromPosition:selectedRange.start offset:0];
    //获取高亮部分内容
    //NSString * selectedtext = [textView textInRange:selectedRange];
    
    //如果有高亮且当前字数开始位置小于最大限制时允许输入
    if (selectedRange && pos) {
        NSInteger startOffset = [textView offsetFromPosition:textView.beginningOfDocument toPosition:selectedRange.start];
        NSInteger endOffset = [textView offsetFromPosition:textView.beginningOfDocument toPosition:selectedRange.end];
        NSRange offsetRange = NSMakeRange(startOffset, endOffset - startOffset);
        
        if (offsetRange.location < MAX_LIMIT_NUMS) {
            return YES;
        }
        else
        {
            return NO;
        }
    }
    
    
    NSString *comcatstr = [textView.text stringByReplacingCharactersInRange:range withString:text];
    
    NSInteger caninputlen = MAX_LIMIT_NUMS - comcatstr.length;
    
    if (caninputlen >= 0)
    {
        return YES;
    }
    else
    {
        NSInteger len = text.length + caninputlen;
        //防止当text.length + caninputlen < 0时，使得rg.length为一个非法最大正数出错
        NSRange rg = {0,MAX(len,0)};
        
        if (rg.length > 0)
        {
            NSString *s = @"";
            //判断是否只普通的字符或asc码(对于中文和表情返回NO)
            BOOL asc = [text canBeConvertedToEncoding:NSASCIIStringEncoding];
            if (asc) {
                s = [text substringWithRange:rg];//因为是ascii码直接取就可以了不会错
            }
            else
            {
                __block NSInteger idx = 0;
                __block NSString  *trimString = @"";//截取出的字串
                //使用字符串遍历，这个方法能准确知道每个emoji是占一个unicode还是两个
                [text enumerateSubstringsInRange:NSMakeRange(0, [text length])
                                         options:NSStringEnumerationByComposedCharacterSequences
                                      usingBlock: ^(NSString* substring, NSRange substringRange, NSRange enclosingRange, BOOL* stop) {
                                          
                                           NSInteger steplen = substring.length;
                                          
                                          if (idx >= rg.length) {
                                              *stop = YES; //取出所需要就break，提高效率
                                              return ;
                                          }
                                          
                                          trimString = [trimString stringByAppendingString:substring];
                                          
                                          idx = idx + steplen;
                                      }];
                
                s = trimString;
            }
            //rang是指从当前光标处进行替换处理(注意如果执行此句后面返回的是YES会触发didchange事件)
            [textView setText:[textView.text stringByReplacingCharactersInRange:range withString:s]];
            //既然是超出部分截取了，哪一定是最大限制了。
//            self.lbNums.text = [NSString stringWithFormat:@"%d/%ld",0,(long)MAX_LIMIT_NUMS];
        }
        return NO;
    }
}

-(void)textViewDidChange:(UITextView *)textView{
    UITextRange *selectedRange = [textView markedTextRange];
    //获取高亮部分
    UITextPosition *pos = [textView positionFromPosition:selectedRange.start offset:0];
    
    //如果在变化中是高亮部分在变，就不要计算字符了
    if (selectedRange && pos) {
        return;
    }
    
    NSString  *nsTextContent = textView.text;
    NSInteger existTextNum = nsTextContent.length;
    
    if (existTextNum > MAX_LIMIT_NUMS)
    {
        //截取到最大位置的字符(由于超出截部分在should时被处理了所在这里这了提高效率不再判断)
        NSString *s = [nsTextContent substringToIndex:MAX_LIMIT_NUMS];
        
        [textView setText:s];
    }
}