iOS中用UILabel实现UITextView的占位文字

@interface BSPublishTextView : UITextView
/** 对外属性占位字符 placeholder */
@property (nonatomic, copy) NSString *placeholder;
 
/** 对外属性占位符颜色 */
@property (nonatomic, strong) UIColor *placeholderColor;
@end

@interface BSPublishTextView ()
/** 占位字符 label */
@property (nonatomic, weak) UILabel *placeholderLabel;
@end
 
@implementation BSPublishTextView
- (UILabel *)placeholderLabel{
    if (!_placeholderLabel) {
        UILabel *placeholderLabel = [[UILabel alloc]init];
        placeholderLabel.numberOfLines = 0;
　　　　 CGRect frame = placeholderLabel.frame;
        frame.origin.x = 4;
        frame.origin.y = 7;
        placeholderLabel.frame = frame;
        [self addSubview:placeholderLabel];
        _placeholderLabel = placeholderLabel;
    }
    return _placeholderLabel;
}
 
- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // 永久垂直弹簧效果
        self.alwaysBounceVertical = YES;
        self.font = [UIFont systemFontOfSize:15];
        self.placeholderColor = [UIColor grayColor];
     
        // 注册textView文字发生改变通知  UITextViewTextDidChangeNotification
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textChange) name:UITextViewTextDidChangeNotification object:nil];    
    }
    return self;
}
 
//通知调用方法
- (void)textChange{
    self.placeholderLabel.hidden = self.hasText;
}
 
- (void)dealloc{
    [[NSNotificationCenter defaultCenter]removeObserver:self];
}
 
- (void)layoutSubviews{
    [super layoutSubviews];
    //在 layoutSubviews 拿到 UITextView的宽度一定是正确的 ，这里如果外界即使更改了UITextView的宽度这里也是能算正确的
    //先设置placeholderLabel占位字符的宽度
    CGRect frame = self.placeholderLabel.frame;
    frame.size.width = self.frame.size.width - 2 * self.placeholderLabel.frame.origin.x;
    self.placeholderLabel.frame = frame;
 
    // placeholderLabel占位字符的大小size随里面的内容大小而变化
    [self.placeholderLabel sizeToFit];
}
 
// setNeedsDisplay方法 : 会在恰当的时刻自动调用drawRect:方法
// setNeedsLayout方法 : 会在恰当的时刻调用layoutSubviews方法
 
- (void)setPlaceholderColor:(UIColor *)placeholderColor{
    _placeholderColor = placeholderColor;
    self.placeholderLabel.textColor = placeholderColor;
}
 
- (void)setPlaceholder:(NSString *)placeholder{
    _placeholder = [placeholder copy];
    self.placeholderLabel.text = placeholder;
    [self setNeedsLayout]; //会在恰当的时刻调用layoutSubviews方法 这句代码可不写
}
 
- (void)setFont:(UIFont *)font{
    [super setFont:font];
    self.placeholderLabel.font = font;
    [self setNeedsLayout];  //会在恰当的时刻调用layoutSubviews方法 这句代码可不写
}
 
- (void)setText:(NSString *)text{
    [super setText:text];
    [self textChange];
}
 
- (void)setAttributedText:(NSAttributedString *)attributedText{
    [super setAttributedText:attributedText];
    [self textChange];
}
@end