需求：用UITextView作为输入框，没有输入或输入一行以内的时候，textView的高度是一行的高度，随着输入内容的增加，textview的高度也跟着增加，但高度最大是能显示3行的高度，然后textview内部滚动显示。另外，textview可控制文字的行间距。

1、textView的设置
重点是textContainerInset和textContainer.lineFragmentPadding的设置，查看sdk说明文档，textContainerInset的默认值是(8, 0, 8, 0)，textContainer.lineFragmentPadding的默认值是5，这里为了方便计算，统一设为0，去掉textView里文字区域四周的空隙

- (UITextView *)textView {
    if (!_textView) {
        _textView = [[UITextView alloc] init];
        _textView.font = [UIFont systemFontOfSize:14];
        _textView.textColor = kAppearanceColor([UIColor colorWithHex:0x4D4D4D], [UIColor colorWithHex:0xE6E6E6]);
        _textView.tintColor = [UIColor colorWithHex:0x7147FF];
        _textView.returnKeyType = UIReturnKeySend;
        _textView.backgroundColor = [UIColor clearColor];
        _textView.delegate = self;
        _textView.textContainerInset = UIEdgeInsetsMake(0, 0, 0, 0);
        _textView.textContainer.lineFragmentPadding = 0;
        
        self.placeHolderLabel = [[UILabel alloc] init];
        self.placeHolderLabel.numberOfLines = 1;
        self.placeHolderLabel.font = [UIFont systemFontOfSize:14];
        self.placeHolderLabel.textColor= [UIColor colorWithHex:0x999999];
       [_textView addSubview:self.placeHolderLabel];
       [_textView setValue:self.placeHolderLabel forKey:@"_placeholderLabel"];
    }
    return _textView;
}

2、textView的布局

    [self.contentView addSubview:self.textView];
    [self.textView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.mas_equalTo(weakSelf.textBgView.mas_left).mas_offset(14);
        make.right.mas_equalTo(weakSelf.textBgView.mas_right).mas_offset(-14);
        make.top.mas_equalTo(weakSelf.textBgView.mas_top).mas_offset(9);
        make.bottom.mas_equalTo(weakSelf.textBgView.mas_bottom).mas_offset(-9);
    }];

3、输入时动态计算和调整高度

- (void)textViewDidChange:(UITextView *)textView {
    textView.attributedText = [[NSAttributedString alloc] initWithString:textView.text attributes:[self attributes]];
    
    CGFloat oldHeight = CGRectGetHeight(self.textBgView.frame);
    CGFloat height = [self calculateTextBgHeight:textView.text];
    if (fabs(height - oldHeight)>0.01) {
        [self.contentView mas_updateConstraints:^(MASConstraintMaker *make) {
            make.height.mas_equalTo(height + 10*2);
        }];
    } 
}

- (CGFloat)calculateTextBgHeight:(NSString *)content {
    NSAttributedString *attributedText = [[NSAttributedString alloc] initWithString:content attributes:[self attributes]];
    CGFloat maxWidth = kDeviceWidth - 87 - 76 - self.textView.textContainer.lineFragmentPadding *2;
    CGRect rect = [attributedText boundingRectWithSize:CGSizeMake(maxWidth, CGFLOAT_MAX) options:NSStringDrawingUsesLineFragmentOrigin|NSStringDrawingUsesFontLeading context:nil];
    CGFloat textHeight = ceilf(rect.size.height);
    CGFloat height = textHeight + 9*2;
    return MIN(MAX(height, 38), 78);
}

- (NSDictionary *)attributes {
    NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];
    paragraphStyle.lineHeightMultiple = 1.14;
    return @{NSFontAttributeName:[UIFont systemFontOfSize:14],
             NSParagraphStyleAttributeName: paragraphStyle
           };
}

参考资料

https://blog.csdn.net/weixin_33946605/article/details/91941424
https://www.jianshu.com/p/1d7d8cf9b1fe
https://www.jianshu.com/p/2d9fde661004
https://www.cnblogs.com/wgb1234/p/12449905.html