.h文件

#import <UIKit/UIKit.h>

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

.m文件

#import "XMGPlaceholderTextView.h"

@interface XMGPlaceholderTextView()
/** 占位文字label */
@property (nonatomic, weak) UILabel *placeholderLabel;
@end

@implementation XMGPlaceholderTextView

- (UILabel *)placeholderLabel
{
    if (!_placeholderLabel) {
        // 添加一個用來顯示占位文字的label
        UILabel *placeholderLabel = [[UILabel alloc] init];
        placeholderLabel.numberOfLines = 0;
        CGRect rect = placeholderLabel.frame;
        rect.origin.x = 4;
        rect.origin.y = 7;
        placeholderLabel.frame = rect;
        [self addSubview:placeholderLabel];
        _placeholderLabel = placeholderLabel;
    }
    return _placeholderLabel;
}

- (instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        // 垂直方向上永遠有彈簧效果
        self.alwaysBounceVertical = YES;
        
        // 默認字體
        self.font = [UIFont systemFontOfSize:15];
        
        // 默認的占位文字顏色
        self.placeholderColor = [UIColor grayColor];
        
        // 監聽文字改變
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange) name:UITextViewTextDidChangeNotification object:nil];
    }
    return self;
}

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

/**
 * 監聽文字改變
 */
- (void)textDidChange
{
    // 只要有文字, 就隱藏占位文字label
    self.placeholderLabel.hidden = self.hasText;
}

/**
 * 更新占位文字的尺寸
 */
- (void)layoutSubviews
{
    [super layoutSubviews];
    CGRect rect = self.placeholderLabel.frame;
    rect.size.width = self.frame.size.width - 2 * self.placeholderLabel.frame.origin.x;
    self.placeholderLabel.frame = rect;
    [self.placeholderLabel sizeToFit];
}

#pragma mark - 重寫setter
- (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];
}

/**
 * setNeedsDisplay方法 : 會在恰當的時刻自動調用drawRect:方法
 * setNeedsLayout方法 : 會在恰當的時刻調用layoutSubviews方法
 */

@end

說明：出自MJ