iOS - Swift UISearchController的取消按钮

// 记录是否已经找到取消按钮
lazy var hasFindCancelBtn: Bool = {
    return false
}()
// 定时器(用来定时查找取消按钮)
lazy var link: CADisplayLink = {
    CADisplayLink(target: self, selector: #selector(findCancel))
}()

func findCancel() {
    let btn = searchBar.value(forKey: "_cancelButton") as AnyObject
    if btn.isKind(of: NSClassFromString("UINavigationButton")!) {
        LXFLog("就是它")
        link.invalidate()
        link.remove(from: RunLoop.current, forMode: .commonModes)
        hasFindCancelBtn = true
        let cancel = btn as! UIButton
        cancel.setTitleColor(UIColor.red, for: .normal)
        cancel.setTitleColor(UIColor.orange, for: .highlighted)
    }
}

searchBar.delegate = self

func searchBarTextDidBeginEditing(_ searchBar: UISearchBar) {
    if !hasFindCancelBtn {
        link.add(to: RunLoop.current, forMode: .commonModes)
    }
}