在UITextView中添加placeholder

#import <UIKit/UIKit.h>

@interface UIPlaceHolderTextView : UITextView {
    NSString *placeholder;
    UIColor *placeholderColor;
    
@private
    UILabel *placeHolderLabel;
}
@property (nonatomic, retain) UILabel *placeHolderLabel;
@property (nonatomic, copy) NSString *placeholder;
@property (nonatomic, retain) UIColor *placeholderColor;
-(void)textChanged:(NSNotification*)notification;
@end

#import "UIPlaceHolderTextView.h"

@implementation UIPlaceHolderTextView
@synthesize placeHolderLabel;
@synthesize placeholder;
@synthesize placeholderColor;

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    [placeHolderLabel release]; self.placeHolderLabel = nil;
    [placeholderColor release]; self.placeholderColor = nil;
     self.placeholder = nil;
    [super dealloc];
}

- (void)awakeFromNib
{
    [super awakeFromNib];
    [self setPlaceholder:@""];
    [self setPlaceholderColor:[UIColor lightGrayColor]];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textChanged:) name:UITextViewTextDidChangeNotification object:nil];
}

- (id)initWithFrame:(CGRect)frame
{
    if( (self = [super initWithFrame:frame]) )
    {
        [self setPlaceholder:@""];
        [self setPlaceholderColor:[UIColor lightGrayColor]];
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textChanged:) name:UITextViewTextDidChangeNotification object:nil];
    }
    return self;
}

- (void)textChanged:(NSNotification *)notification
{
    if([[self placeholder] length] == 0)
    {
        return;
    }
    
    if([[self text] length] == 0)
    {
        [[self viewWithTag:999] setAlpha:1];
    }
    else
    {
        [[self viewWithTag:999] setAlpha:0];
    }
}

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

- (void)drawRect:(CGRect)rect
{
    if( [[self placeholder] length] > 0 )
    {
        if ( placeHolderLabel == nil )
        {
            placeHolderLabel = [[UILabel alloc] initWithFrame:CGRectMake(8,8,self.bounds.size.width - 16,0)];
            placeHolderLabel.lineBreakMode = UILineBreakModeWordWrap;
            placeHolderLabel.numberOfLines = 0;
            placeHolderLabel.font = self.font;
            placeHolderLabel.backgroundColor = [UIColor clearColor];
            placeHolderLabel.textColor = self.placeholderColor;
            placeHolderLabel.alpha = 0;
            placeHolderLabel.tag = 999;
            [self addSubview:placeHolderLabel];
        }
        
        placeHolderLabel.text = self.placeholder;
        [placeHolderLabel sizeToFit];
        [self sendSubviewToBack:placeHolderLabel];
    }
    
    if( [[self text] length] == 0 && [[self placeholder] length] > 0 )
    {
        [[self viewWithTag:999] setAlpha:1];
    }
    
    [super drawRect:rect];
}
@end

from:http://stackoverflow.com/questions/1328638/placeholder-in-uitextview

备注：以上方法placeHolderLabel子视图会内存泄漏，下面的SSToolkit中的SSTextView方法会更好些！

//
//  SSTextView.h
//  SSToolkit
//
//  Created by Sam Soffes on 8/18/10.
//  Copyright 2010-2011 Sam Soffes. All rights reserved.
//

/**
 UITextView subclass that adds placeholder support like UITextField has.
 */
@interface SSTextView : UITextView

/**
 The string that is displayed when there is no other text in the text view.
 
 The default value is `nil`.
 */
@property (nonatomic, strong) NSString *placeholder;

/**
 The color of the placeholder.
 
 The default is `[UIColor lightGrayColor]`.
 */
@property (nonatomic, strong) UIColor *placeholderTextColor;

@end

//
//  SSTextView.m
//  SSToolkit
//
//  Created by Sam Soffes on 8/18/10.
//  Copyright 2010-2011 Sam Soffes. All rights reserved.
//

#import "SSTextView.h"

@interface SSTextView ()
- (void)_initialize;
- (void)_updateShouldDrawPlaceholder;
- (void)_textChanged:(NSNotification *)notification;
@end


@implementation SSTextView {
    BOOL _shouldDrawPlaceholder;
}


#pragma mark - Accessors

@synthesize placeholder = _placeholder;
@synthesize placeholderTextColor = _placeholderTextColor;

- (void)setText:(NSString *)string {
    [super setText:string];
    [self _updateShouldDrawPlaceholder];
}


- (void)setPlaceholder:(NSString *)string {
    if ([string isEqual:_placeholder]) {
        return;
    }
    
    _placeholder = string;
    [self _updateShouldDrawPlaceholder];
}


#pragma mark - NSObject

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


#pragma mark - UIView

- (id)initWithCoder:(NSCoder *)aDecoder {
    if ((self = [super initWithCoder:aDecoder])) {
        [self _initialize];
    }
    return self;
}


- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        [self _initialize];
    }
    return self;
}


- (void)drawRect:(CGRect)rect {
    [super drawRect:rect];
    
    if (_shouldDrawPlaceholder) {
        [_placeholderTextColor set];
        [_placeholder drawInRect:CGRectMake(8.0f, 8.0f, self.frame.size.width - 16.0f, self.frame.size.height - 16.0f) withFont:self.font];
    }
}


#pragma mark - Private

- (void)_initialize {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_textChanged:) name:UITextViewTextDidChangeNotification object:self];
    
    self.placeholderTextColor = [UIColor colorWithWhite:0.702f alpha:1.0f];
    _shouldDrawPlaceholder = NO;
}


- (void)_updateShouldDrawPlaceholder {
    BOOL prev = _shouldDrawPlaceholder;
    _shouldDrawPlaceholder = self.placeholder && self.placeholderTextColor && self.text.length == 0;
    
    if (prev != _shouldDrawPlaceholder) {
        [self setNeedsDisplay];
    }
}


- (void)_textChanged:(NSNotification *)notificaiton {
    [self _updateShouldDrawPlaceholder];    
}

@end