为UITextView增加占位文字的封装

1.新建继承UITextView的类

定义可以对外访问用于修改的属性
<pre>
/** 占位文字 */
@property (nonatomic, copy) NSString *placeholder;
/** 占位文字的颜色 */
@property (nonatomic, strong) UIColor *placeholderColor;
</pre>

在.m文件定义一个label

<pre>
/** 占位文字label */
@property (nonatomic, weak) UILabel *placeholderLabel;
</pre>

懒加载把label添加到textView中

<pre>

• (UILabel *)placeholderLabel
  {
  if (!_placeholderLabel) {
  // 添加一个用来显示占位文字的label
  UILabel *placeholderLabel = [[UILabel alloc] init];
  placeholderLabel.numberOfLines = 0;
  placeholderLabel.x = 4;
  placeholderLabel.y = 7;
  [self addSubview:placeholderLabel];
  _placeholderLabel = placeholderLabel;
  }
  return _placeholderLabel;
  }
  参考文章：UIView扩展-直接用点语法改变UIView的尺寸http://www.jianshu.com/p/4e00189dea2b
  </pre>

初始化一些设置

<pre>

• (instancetype)initWithFrame:(CGRect)frame
  {
  if (self = [super initWithFrame:frame]) {
  // 垂直方向上永远有弹簧效果
  self.alwaysBounceVertical = YES;

    // 默认字体
    self.font = [UIFont systemFontOfSize:15];
    
    // 默认的占位文字颜色
    self.placeholderColor = [UIColor grayColor];
    
    // 监听文字改变
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange) name:UITextViewTextDidChangeNotification object:nil];
  

  }
  return self;
  }

• (void)textDidChange
  {
  // 只要有文字, 就隐藏占位文字label
  self.placeholderLabel.hidden = self.hasText;
  }

</pre>

移除通知

<pre>

• (void)dealloc
  {
  [[NSNotificationCenter defaultCenter] removeObserver:self];
  }
  </pre>

更新占位文字的尺寸

<pre>

• (void)layoutSubviews
  {
  [super layoutSubviews];

  self.placeholderLabel.width = self.width - 2 * self.placeholderLabel.x;
  [self.placeholderLabel sizeToFit];
  }
  参考文章：UIView扩展-直接用点语法改变UIView的尺寸http://www.jianshu.com/p/4e00189dea2b
  </pre>

重写相关的setting方法

<pre>

• (void)setPlaceholderColor:(UIColor *)placeholderColor
  {
  _placeholderColor = placeholderColor;

  self.placeholderLabel.textColor = placeholderColor;
  }

• (void)setPlaceholder:(NSString *)placeholder
  {
  _placeholder = [placeholder copy];

  self.placeholderLabel.text = placeholder;

  [self setNeedsLayout];
  }

• (void)setFont:(UIFont *)font
  {
  [super setFont:font];

  self.placeholderLabel.font = font;

  [self setNeedsLayout];
  }

• (void)setText:(NSString *)text
  {
  [super setText:text];

  [self textDidChange];
  }

• (void)setAttributedText:(NSAttributedString*)attributedText
  {
  [super setAttributedText:attributedText];

  [self textDidChange];
  }
  </pre>

封装的类的下载地址https://git.oschina.net/qjz.com/TextView_placeholderLabel/tree/master