swift2.3，用 pushViewController 跳转没反应是什么原因？

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.backgroundColor = UIColor.blackColor()
        let btn = UIButton(frame:CGRectMake(0, 0, self.view.frame.size.width/2, 50))
        btn.setTitle("点击测试", forState: UIControlState.Normal)
        btn.center = self.view.center
        btn.addTarget(self, action:#selector(self.testNavTab), forControlEvents: UIControlEvents.TouchDown)
        view.addSubview(btn)
    }

    func testNavTab(){
        let homevi = homeview()
        //self.navigationController?.pushViewController(homevi, animated: true)
        self.presentViewController(homevi, animated: true, completion: nil)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}

以上是全部代码，如果用 presentViewController 就可以正常跳转（ homeview 是一个空白页面），但是 pushViewController 就没有任何反应...