Runtime实战记录给UITextView添加placeholder

@interface UITextView (NTES)
@property (nonatomic, strong) NSString* placeholder;
@property (nonatomic, strong) UILabel * placeholderLabel;
@property (nonatomic, strong) NSString* textValue;
@end

-(void)checkIfNeedToDisplayPlaceholder{
    // If our UITextView is empty, display our Placeholder label (if we have one)
    if (self.placeholderLabel == nil)
        return;
    
    self.placeholderLabel.hidden = (![self.text isEqualToString:@""]);
}

-(void)onTap{
    // When the user taps in our UITextView, we'll see if we need to remove the placeholder text.
    [self checkIfNeedToDisplayPlaceholder];
    
    // Make the onscreen keyboard appear.
    [self becomeFirstResponder];
}

-(void)keyPressed:(NSNotification*)notification{
    //  The user has just typed a character in our UITextView (or pressed the delete key).
    //  Do we need to display our Placeholder label ?
    [self checkIfNeedToDisplayPlaceholder];
}

-(void)setTextValue:(NSString *)textValue{
    //  Change the text of our UITextView, and check whether we need to display the placeholder.
    self.text = textValue;
    [self checkIfNeedToDisplayPlaceholder];
}
-(NSString*)textValue{
    return self.text;
}

NSString const *kKeyPlaceHolder = @"kKeyPlaceHolder";
-(void)setPlaceholder:(NSString *)_placeholder{
    //  Sets our "placeholder" text string, creates a new UILabel to contain it, and modifies our UITextView to cope with
    //  showing/hiding the UILabel when needed.
    objc_setAssociatedObject(self, &kKeyPlaceHolder, (id)_placeholder, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    self.placeholderLabel = [[UILabel alloc] initWithFrame:CGRectMake(8, 8, 0, 0)];
    self.placeholderLabel.numberOfLines = 1;
    self.placeholderLabel.text = _placeholder;
    self.placeholderLabel.textColor = UI_PLACEHOLDER_TEXT_COLOR;
    self.placeholderLabel.backgroundColor = [UIColor clearColor];
    self.placeholderLabel.userInteractionEnabled = true;
    self.placeholderLabel.font = self.font;
    [self addSubview:self.placeholderLabel];
    
    [self.placeholderLabel sizeToFit];
    
    //  Whenever the user taps within the UITextView, we'll give the textview the focus, and hide the placeholder if necessary.
    [self addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onTap)]];
    
    //  Whenever the user types something in the UITextView, we'll see if we need to hide/show the placeholder label.
    [[NSNotificationCenter defaultCenter] addObserver:self selector: @selector(keyPressed:) name:UITextViewTextDidChangeNotification object:nil];
    
    [self checkIfNeedToDisplayPlaceholder];
}
-(NSString*)placeholder{
    //  Returns our "placeholder" text string
    return objc_getAssociatedObject(self, &kKeyPlaceHolder);
}

NSString const *kKeyLabel = @"kKeyLabel";
-(void)setPlaceholderLabel:(UILabel *)placeholderLabel{
    //  Stores our new UILabel (which contains our placeholder string)
    objc_setAssociatedObject(self, &kKeyLabel, (id)placeholderLabel, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector: @selector(keyPressed:) name:UITextViewTextDidChangeNotification object:nil];
    
    [self checkIfNeedToDisplayPlaceholder];
}
-(UILabel*)placeholderLabel{
    //  Returns our new UILabel
    return objc_getAssociatedObject(self, &kKeyLabel);
}

// 告诉编译器、xxx属性的setter、getter方法由开发者自己来生成
//@dynamic placeholder;
//@dynamic placeholderLabel;
//@dynamic textValue;