让UITextView文字垂直居中

让UITextView和UITextField同样拥有垂直居中的属性，建议单独一个类继承自UITextView

只需要初始化UITextView之后用KVO监听 "contentSize" 属性即可

_textView = [[XHMessageTextView alloc] init];

    _textView.delegate = self;

    _textView.placeHolder = @"我来评论...";

//    _textVie.contentVerticalAlignment = UIControlContentVerticalAlignmentCenter;

    //byliuchao

    _textView.frame = CGRectMake(10, 2, KWIDTH - 70, 38);

    _textView.font = [UIFont systemFontOfSize:14];

    _textView.textColor = FONTCOLOR;

    _textView.placeHolderTextColor = [UIColor colorWithRed:189/255.0 green:193/255.0 blue:199/255.0 alpha:0.5];

    _textView.layer.borderColor = [UIColor colorWithRed:189/255.0 green:193/255.0 blue:199/255.0 alpha:0.5].CGColor;

    _textView.layer.cornerRadius = 5.0;

    //end

    [_textView addObserver:self forKeyPath:@"contentSize"options:NSKeyValueObservingOptionNew context:nil];

    _textView.layer.borderWidth = 0.5;

    _textView.layer.masksToBounds = YES;

    _textView.returnKeyType = UIReturnKeyDone;

    [dockview addSubview:_textView];

#pragma mark - KVO

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context

{

    CGFloat topCorrect = ([_textView bounds].size.height - [_textView contentSize].height);

    topCorrect = (topCorrect <0.0 ? 0.0 : topCorrect);

    _textView.contentOffset = (CGPoint){.x = 0, .y = -topCorrect / 2};

}