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：

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[textView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:NULL];
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- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    UITextView *tv = object;
    // Center vertical alignment
    CGFloat topCorrect = ([tv bounds].size.height - [tv contentSize].height * [tv zoomScale])/2.0;
    topCorrect = ( topCorrect < 0.0 ? 0.0 : topCorrect );
    tv.contentOffset = (CGPoint){.x = 0, .y = -topCorrect};

//    // Bottom vertical alignment
//    CGFloat topCorrect = ([tv bounds].size.height - [tv contentSize].height);
//    topCorrect = (topCorrect <0.0 ? 0.0 : topCorrect);
//    tv.contentOffset = (CGPoint){.x = 0, .y = -topCorrect};
}