这是效果，输入框根据内容多少自动伸缩大小。
关于输入框遮挡键盘的处理在另外一篇 文章描述。
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：
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：
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：
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