addObserverでresize event
self.contentView.addObserver(self, forKeyPath: "frame", options: NSKeyValueObservingOptions(rawValue: 0), context: nil)

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    //処理
}