UITextView添加占位文字

UITextView添加占位文字简单好用的方法。主要运用drawRect:方法对占位文字进行绘制。
创建CustomPlaceholderTextView文件继承于UITextView，在CustomPlaceholderTextView.h 文件中有如下属性：

//占位文字
@property (nonatomic, copy) NSString *placeholder;
//占位文字颜色
@property (nonatomic, strong) UIColor *placeholderColor;
//占位文字大小
@property (nonatomic, assign) CGFloat placeholderFontSize;
//文字大小
@property (nonatomic, assign) CGFloat fontSize;
//提示文字距左距离
@property (nonatomic, assign) CGFloat text_x;
//提示文字距上距离
@property (nonatomic, assign) CGFloat text_y;
//圆角弧度
@property (nonatomic, assign) CGFloat cornerRadius;
//描边宽
@property (nonatomic, assign) CGFloat borderWidth;
//描边颜色
@property (nonatomic, strong) UIColor *borderColor;

在CustomPlaceholderTextView.m文件中实现占位文字的绘制。
初始化 textView 并进行一些默认设置：

- (instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame])
    {
        //设置占位文字默认大小
        self.placeholderFontSize = 15.0;
        //设置占位文字默认颜色
        self.placeholderColor = [UIColor grayColor];
        //设置显示文字默认大小
        self.font = [UIFont systemFontOfSize:17.0];
        // 使用通知监听文字改变
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange:) name:UITextViewTextDidChangeNotification object:self];
    }
    return self;
}

当文字发生改变的时候调用 textDidChange: 方法

- (void)textDidChange:(NSNotification *)note
{
    // 会重新调用drawRect:方法
    [self setNeedsDisplay];
}

调用drawRect:

/**
 * 每次调用drawRect:方法，都会将以前画的东西清除掉
 */
- (void)drawRect:(CGRect)rect
{
    // 如果有文字，就直接返回，不需要画占位文字
    if (self.hasText) return;
    
    // 属性
    NSMutableDictionary *attrs = [NSMutableDictionary dictionary];

    attrs[NSFontAttributeName] = [UIFont systemFontOfSize:self.placeholderFontSize];
    attrs[NSForegroundColorAttributeName] = self.placeholderColor;
    
    // 画文字
    rect.origin.x = self.text_x;
    rect.origin.y = self.text_y;
    rect.size.width -= 2 * rect.origin.x;
    [self.placeholder drawInRect:rect withAttributes:attrs];
}

一些属性的 setter 方法：

//占位文字
- (void)setPlaceholder:(NSString *)placeholder
{
    _placeholder = [placeholder copy];
    [self setNeedsDisplay];
}

//占位文字颜色
- (void)setPlaceholderColor:(UIColor *)placeholderColor
{
    _placeholderColor = placeholderColor;
    [self setNeedsDisplay];
}

//占位文字大小
- (void)setPlaceholderFontSize:(CGFloat)placeholderFontSize
{
    _placeholderFontSize = placeholderFontSize;
    [self setNeedsDisplay];
}

//textView 文字大小
- (void)setFontSize:(CGFloat)fontSize
{
    _fontSize = fontSize;
    self.font = [UIFont systemFontOfSize:fontSize];
}

//边框圆角弧度
- (void)setCornerRadius:(CGFloat)cornerRadius
{
    _cornerRadius = cornerRadius;
    self.layer.cornerRadius = cornerRadius;
}

//边框宽度
- (void)setBorderWidth:(CGFloat)borderWidth
{
    _borderWidth = borderWidth;
    self.layer.borderWidth = borderWidth;
}

//边框颜色
- (void)setBorderColor:(UIColor *)borderColor
{
    _borderColor = borderColor;
    self.layer.borderColor = borderColor.CGColor;
}

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

- (void)setAttributedText:(NSAttributedString *)attributedText
{
    [super setAttributedText:attributedText];
    [self setNeedsDisplay];
}

调用 layoutSubviews 方法：

- (void)layoutSubviews
{
    [super layoutSubviews];
    [self setNeedsDisplay];
}

调用 dealloc 方法，移除监听：

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}