注冊(cè)UITextViewTextDidChangeNotification 通知，每次文字改變的時(shí)候會(huì)重新調(diào)用drawRect方法，重新繪制placeholder。

@interface MyTextView : UITextView
/** 占位文字 */
@property (nonatomic, copy) NSString *placeholder;
/** 占位文字的顏色 */
@property (nonatomic, strong) UIColor *placeholderColor;
@end

@implementation MyTextView
-(instancetype)initWithFrame:(CGRect)frame{
if (self = [super initWithFrame:frame]) {
    
    // 當(dāng)UITextView的文字發(fā)生改變時(shí)，UITextView自己會(huì)發(fā)出一個(gè)UITextViewTextDidChangeNotification通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange) name:UITextViewTextDidChangeNotification object:self];
 }
return self;
}
- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}
/**
 * 監(jiān)聽(tīng)文字改變
 */
- (void)textDidChange
{
    // 重繪（重新調(diào)用）
[self setNeedsDisplay];
}

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

[self setNeedsDisplay];
}

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

[self setNeedsDisplay];
}

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

// setNeedsDisplay會(huì)在下一個(gè)消息循環(huán)時(shí)刻，調(diào)用drawRect:
[self setNeedsDisplay];
}

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

[self setNeedsDisplay];
}
-(void)drawRect:(CGRect)rect
{
//    如果有文字，就直接返回，不畫(huà)占位文字
  if (self.hasText) return;

// 文字屬性
NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
attrs[NSFontAttributeName] = self.font;
attrs[NSForegroundColorAttributeName] = self.placeholderColor?self.placeholderColor:[UIColor grayColor];
// 畫(huà)文字
CGFloat x = 5;
CGFloat w = rect.size.width - 2 * x;
CGFloat y = 8;
CGFloat h = rect.size.height - 2 * y;
CGRect placeholderRect = CGRectMake(x, y, w, h);
[self.placeholder drawInRect:placeholderRect withAttributes:attrs];
}