带placeholder的textView

注册UITextViewTextDidChangeNotification 通知，每次文字改变的时候会重新调用drawRect方法，重新绘制placeholder。

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

@implementation MyTextView
-(instancetype)initWithFrame:(CGRect)frame{
if (self = [super initWithFrame:frame]) {
    
    // 当UITextView的文字发生改变时，UITextView自己会发出一个UITextViewTextDidChangeNotification通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange) name:UITextViewTextDidChangeNotification object:self];
 }
return self;
}
- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}
/**
 * 监听文字改变
 */
- (void)textDidChange
{
    // 重绘（重新调用）
[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会在下一个消息循环时刻，调用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，不画占位文字
  if (self.hasText) return;

// 文字属性
NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
attrs[NSFontAttributeName] = self.font;
attrs[NSForegroundColorAttributeName] = self.placeholderColor?self.placeholderColor:[UIColor grayColor];
// 画文字
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];
}