Swift 为什么从 Parse 检索数据时我必须按两次按钮才能在 tableview 中加载数据

Question

下面是我的代码，它从 Parse 服务器类中检索图像并将其加载到 TableView 中。当我按 GetAlbum UIbutton 显示数据时，我必须按两次才能成功在表格 View 中显示数据，我不知道是什么原因。请帮忙(PS:我是swift的新手)

var Saveddata: [UIImage] = []

@IBAction func getAlbum(_ sender: AnyObject) {
    if (self.textEntercode.text == "")
    {
        let alert = UIAlertController(title: "No Entered value", message:   "Please enter value", preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.cancel, handler: nil));
        //event handler with closure

        present(alert, animated: true, completion: nil);
    }
    else {
        //-------------Get Data from Parse Server-------------------
        var photo = textEntercode.text
        let query = PFQuery(className: "Photostable10")

        query.whereKey("photo", equalTo: photo)
        //query.whereKey("Score", greaterThan: 1000)

        query.findObjectsInBackground(block: { (objects : [PFObject]?, error: Error?) -> Void in
            if error == nil {
                for object in objects! {
                    if let userPicture = object.value(forKey: "photo") {
                        let userPicture = object.value(forKey: "photo")! as! PFFile
                        userPicture.getDataInBackground({ (imageData: Data?, error: Error?) -> Void in
                            let image = UIImage(data: imageData!)
                            if image != nil {
                                self.Saveddata.append(image!)
                            }
                        })
                    }
                }
            }
        })
        self.tableview.reloadData()
    } //else close
}

func numberOfSections(in tableView: UITableView) -> Int {
    return 1
}

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "PostCell")!

    cell.textLabel!.text = "Post"
    cell.imageView?.image = Saveddata[indexPath.row]
    return cell

}

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return self.Saveddata.count
}

Answer 1

您对 self.tableView.reloadData() 的调用在错误的位置，并且是从错误的队列中调用的。您需要在主队列上调用它，并且需要在更新数据模型后在查询完成处理程序中调用它。

query.findObjectsInBackground(block: { (objects : [PFObject]?, error: Error?) -> Void in
    if error == nil {
        for object in objects! {
            if let userPicture = object.value(forKey: "photo") {
                let userPicture = object.value(forKey: "photo")! as! PFFile
                userPicture.getDataInBackground({ (imageData: Data?, error: Error?) -> Void in
                    let image = UIImage(data: imageData!)
                    if image != nil {
                        self.Saveddata.append(image!)
                    }
                })
            }
        }
        DispatchQueue.main.async {
            self.tableview.reloadData()
        }
    }
})