先为UITextView的contentSize键值添加一个观察者，当视图加载时:

First add an observer for the contentSize key value of the UITextView when the view is loaded:

- (void) viewDidLoad {
     [textField addObserver:self forKeyPath:@"contentSize" options:(NSKeyValueObservingOptionNew) context:NULL];
     [super viewDidLoad];
}

然后在每次contentSize值改变时添加这个方法来调整contentOffset:

Then add this method to adjust the contentOffset every time the contentSize value changes:

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
     UITextView *tv = object;
     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};
}