UITextFieldでは以下のようにすることでプレースホルダーを使用することができる。

UITextField *field = [[UITextField alloc] init];
[field setFrame:CGRectMake(10, 120, 190, 35)];
[field setPlaceholder:@"入力してください"];

複数行のテキスト入力欄が必要な場合はUITextViewをEditableにして対応するが、上述のような手法でプレースホルダーを用いることはできない。

■実装

UIPlaceHolderTextView.h

#import "UIPlaceHolderTextView.h"

@implementation UIPlaceHolderTextView
@synthesize placeholder;
@synthesize placeholderColor;
@synthesize placeholderLabel;

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

// 表示テキストに変更があった時
- (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];
}

// drawRect時にplaceholderのUILabelを準備してViewに追加する
- (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

クライアントコード

以下のようにすることで使用することができる。

UIPlaceHolderTextView *view = [[UIPlaceHolderTextView alloc] init];
[view setPlaceholder:@"(^o^)"];

■参考

• Placeholder in UITextView