gistfile1.m

- (void) viewDidLoad {
    //...
    comments.text = @"Comments";
    comments.textColor = [UIColor lightGrayColor];
    isEmpty = YES;
    //...
}

- (BOOL)textViewShouldBeginEditing:(UITextView*)textView {
    if (isEmpty) {
        comments.text = @"";
        comments.textColor = [UIColor blackColor];
        isEmpty = NO;
    }
    return YES;
}

- (void) textViewDidEndEditing:(UITextView*)textView {
    if(comments.text.length == 0){
        comments.textColor = [UIColor lightGrayColor];
        comments.text = @"Comments";
        isEmpty = YES;
    }
}

Android中的hint属性，iOS中的placeholder属性！！！