swift - 清空 Collection View Swift

Question

我遵循了 1 个教程，我能够用一些数据( ImageView 和文本)填充 collectionView:

let appleProducts = ["A", "B", "C", "D"]
let imageArray = [UIImage(named: "pug1"), UIImage(named: "pug2"), UIImage(named: "pug3"), UIImage(named: "pug4")]
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
{

    return appleProducts.count
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
{

    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath as IndexPath) as! CollectionViewCell

    cell.imageView?.image = self.imageArray[indexPath.row]

    cell.title?.text = self.appleProducts[indexPath.row]
    return cell

}

现在从演示项目传递到我的，我想用我从 FirebaseDatabse 获得的数据(图片和文本)填充这个 CollectionView，所以我创建了这个方法:

struct item {
let pictureId: String!
let picture: String!
}
var items = [item]()
   func getLatestAddedItems(){
    self.items.removeAll()
    let databaseRef = FIRDatabase.database().reference()
    databaseRef.child("Items").observe(.value, with: {
        snapshot in

        //self.items.insert(item(picture: picture), at: 0)
        for childSnap in snapshot.children.allObjects {

            let snap = childSnap as! FIRDataSnapshot
            //print(snap.key)

            let picture = (snap.value as? NSDictionary)?["bookImage"] as? String ?? ""
            //print(picture)
            self.items.append(item(pictureId:snap.key, picture:picture))

        }
        print(self.items.count)


    })
}

然后我创建这个按钮来调用 GetLatestAddedItems 方法:

@IBAction func selectAction(_ sender: AnyObject) {
    getLatestAddedItems()
        }

这个检查结果:

@IBAction func gettableAction(_ sender: AnyObject) {

    print(self.items[0].picture)
    print(self.items[1].picture)
    print(self.items[2].picture)
    print(self.items.count)    }

输出结果:

picture 1 link
picture 2 link
picture 3 link
3

现在在 ContentView 方法中进行必要的更改后，一切看起来都很好且正确:

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
{

    return items.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
{

    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath as IndexPath) as! CollectionViewCell

    //cell.imageView?.image = self.imageArray[indexPath.row]
     let url = NSURL(string: items[indexPath.row].picture)
     let data = NSData(contentsOf: url! as URL)
     cell.imageView?.image = UIImage(data: data! as Data)

    cell.title?.text = self.items[indexPath.row].pictureId
    return cell

}

现在我得到一个空的 ContentView，第一次使用按钮时它起作用了，因为我调用了 getLatestAddedItems() 它将获取数据并将数据添加到 Items 表，我尝试在 ViewWillAppear 或 Viewdidload 中调用它但没有任何变化.

这就是我认为返回的 items.count 返回 0，所以没有任何建议会出现？

Answer 1

将您的 collectionView 的协议(protocol)委托(delegate)初始化移动到 ViewController 生命周期范围之一，例如 viewDidLoad() 或viewWillAppear(_animated : Bool) 如果您使用自定义 viewController(即在 viewController 中嵌入一个 collectionView)

并在每次用户从其数据库接收到值时重新加载您的 collectionView。

override func viewDidLoad(){
  super.viewDidLoad()

  self.collectionView.dataSource = self 
  self.collectionView.delegate = self 

 }

func getLatestAddedItems(){
 self.items.removeAll()
 let databaseRef = FIRDatabase.database().reference()
 databaseRef.child("Items").observe(.childAdded, with: {
    snapshot in

    for childSnap in snapshot.children.allObjects {

        let snap = childSnap as! FIRDataSnapshot
        let picture = (snap.value as? NSDictionary)?["bookImage"] as? String ?? ""
         self.items.append(item(pictureId:snap.key, picture:picture))
         print(self.items.count)
         self.collectionView.reloadData() 
     }
   })
}

PS:- 所有对你的 firebase 服务器的调用都是异步的，这需要一些时间从你的 FBDB 中检索数据，所以把 print(self .items.count) 应该在 firebase 观察调用的 completionBlock 内，否则如果它被放在外面，它甚至会在您的数据从 FBDB 检索之前被调用。