oc textview垂直居中

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

{

if ([keyPath isEqualToString:@"contentSize"])

    {

UITextView *tv = object;

CGFloat deadSpace = ([tv bounds].size.height - [tv contentSize].height);

CGFloat inset = MAX(0, deadSpace/2.0);

        tv.contentInset = UIEdgeInsetsMake(inset, tv.contentInset.left, inset, tv.contentInset.right);

    }

}