这是效果，输入框根据内容多少自动伸缩大小。

输入框.gif

1、继承UITextView

UITextField只能输入一行，要输入多行文本，那么只能用UITextView，如下所示，KTAutoHeightTextView继承自UITextView，然后使用自动布局添加三个高度上的约束，并连线：

Paste_Image.png

这里heightConstraint是用来控制实际textView的高度的，maxHeightConstraint和minHeightConstraint则分别用来控制最大和最小的高度。

2、IB_DESIGNABLE和IBInspectable的实时预览

除此之外，还定义了2个属性showsRoundCorner和placeholder，注意到开头有个IB_DESIGNABLE，这两个属性前面的还多出来了IBInspectable，IB_DESIGNABLE配合IBInspectable的作用就是在storyboard或者xib中实时更新属性效果的。有了这两个东西，会发现KTAutoHeightTextView的属性检查器中会多出两个属性：

Paste_Image.png

仅仅用IB_DESIGNABLE和IBInspectable还不足以达到实时在storyboard或者xib中预览圆角和placeholder的效果，还要在相应的属性setter中加上更新的代码，比如showsRoundCorner属性的setter：

- (void)setShowsRoundCorner:(BOOL)showsRoundCorner
{
    _showsRoundCorner = showsRoundCorner;
    if (showsRoundCorner)
    {
        self.layer.masksToBounds = YES;
        self.layer.cornerRadius = 5.0;
        self.layer.borderColor = [[UIColor lightGrayColor] CGColor];
        self.layer.borderWidth = 0.5;
    }
    else
    {
        self.layer.masksToBounds = NO;
        self.layer.cornerRadius = 0.0;
        self.layer.borderColor = [[UIColor clearColor] CGColor];
        self.layer.borderWidth = 0.0;
    }
}

在storyboard中设置“Shows RoundCorner”为On，那么就会看到预览的圆角效果了：

Paste_Image.png

3、placeholder实现

同样实现placeholder的setter：

- (void)setPlaceholder:(NSString *)placeholder
{
    _placeholder = [placeholder copy];
    [self setNeedsDisplay];
}

#pragma mark - Drawing

- (void)drawRect:(CGRect)rect
{
    [super drawRect:rect];
    
    if ([self.text length] == 0 && self.placeholder) {
        [[UIColor lightGrayColor] set];
        [self.placeholder drawInRect:CGRectInset(rect, 6.0f, 8.0f) withAttributes:[self kt_placeholderTextAttributes]];
    }
}

#pragma mark - Utilities

- (NSDictionary *)kt_placeholderTextAttributes
{
    NSMutableParagraphStyle *paragraphStyle = [[NSMutableParagraphStyle alloc] init];
    paragraphStyle.lineBreakMode = NSLineBreakByTruncatingTail;
    paragraphStyle.alignment = self.textAlignment;
    
    return @{ NSFontAttributeName : self.font,
              NSForegroundColorAttributeName : [UIColor lightGrayColor],
              NSParagraphStyleAttributeName : paragraphStyle };
}

这里在setter里面调用setNeedsDisplay方法，该方法调用之后，drawRect方法会被调用，然后在这里面加上绘制placeholder文本的逻辑。

4、自动高度变化的实现

这里的实现思路是监听文本编辑，文本变化的通知，然后调用setNeedsDisplay，注意setNeedsDisplay不光会导致drawRect方法被触发，还会导致layoutSubviews方法被触发来进行重布局，因此在layoutSubviews方法里面写上更新高度的逻辑，代码很简单：

- (void)awakeFromNib
{
    [super awakeFromNib];
    
    [self addTextViewNotificationObservers];
}

- (void)dealloc
{
    [self removeTextViewNotificationObservers];
}

- (void)layoutSubviews
{
    [super layoutSubviews];
    
    // calculate size needed for the text to be visible without scrolling
    CGSize sizeThatFits = [self sizeThatFits:self.frame.size];
    float newHeight = sizeThatFits.height;
    // if there is any minimal height constraint set, make sure we consider that
    if (self.maxHeightConstraint) {
        newHeight = MIN(newHeight, self.maxHeightConstraint.constant);
    }
    // if there is any maximal height constraint set, make sure we consider that
    if (self.minHeightConstraint) {
        newHeight = MAX(newHeight, self.minHeightConstraint.constant);
    }
    // update the height constraint
    self.heightConstraint.constant = newHeight;
}

#pragma mark -- notifications

- (void)addTextViewNotificationObservers
{
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(kt_didReceiveTextViewNotification:)
                                                 name:UITextViewTextDidChangeNotification
                                               object:self];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(kt_didReceiveTextViewNotification:)
                                                 name:UITextViewTextDidBeginEditingNotification
                                               object:self];
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(kt_didReceiveTextViewNotification:)
                                                 name:UITextViewTextDidEndEditingNotification
                                               object:self];
}

- (void)removeTextViewNotificationObservers
{
    [[NSNotificationCenter defaultCenter] removeObserver:self
                                                    name:UITextViewTextDidChangeNotification
                                                  object:self];
    [[NSNotificationCenter defaultCenter] removeObserver:self
                                                    name:UITextViewTextDidBeginEditingNotification
                                                  object:self];
    [[NSNotificationCenter defaultCenter] removeObserver:self
                                                    name:UITextViewTextDidEndEditingNotification
                                                  object:self];
}

- (void)kt_didReceiveTextViewNotification:(NSNotification *)notification
{
    [self setNeedsDisplay];
}

代码是在利用sizeThatFits在contentSize变化的时候去计算合适的大小fitHeight，同时利用一定的规则在合适的时候寻找高度约束heightConstraint。当存在heightConstraint的时候修改约束值，不存在的时候直接修改bounds，从而实现在自动布局或者非自动布局情况下都能自动匹配高度。

5、重写属性的setter

重写属性的setter，保证实时更新。注意setBounds的setter，是为了避免出现错误contentOffset的情况。

#pragma mark -- 重写系统的setter --

- (void)setBounds:(CGRect)bounds
{
    [super setBounds:bounds];
    
    if (self.contentSize.height <= self.bounds.size.height + 1){
        self.contentOffset = CGPointZero; // Fix wrong contentOffset
    }
}

- (void)setText:(NSString *)text
{
    [super setText:text];
    [self setNeedsDisplay];
}

- (void)setAttributedText:(NSAttributedString *)attributedText
{
    [super setAttributedText:attributedText];
    [self setNeedsDisplay];
}

- (void)setFont:(UIFont *)font
{
    [super setFont:font];
    [self setNeedsDisplay];
}

- (void)setTextAlignment:(NSTextAlignment)textAlignment
{
    [super setTextAlignment:textAlignment];
    [self setNeedsDisplay];
}

完整的项目https://github.com/tujinqiu/KTAutoHeightTextView