swift - onAppear 中的 SwiftUI 异步数据获取

Question

我有一个类getDataFromDatabase，它具有readData()从 Firebase 读取数据的功能。

class getDataFromDatabase : ObservableObject {

    var arrayWithQuantity = [Int]()
    var arrayWithTime = [Double]()
    
    func readData(completion: @escaping(_ getArray: Array<Int>?,_ getArray: Array<Double>?) -> Void) {
       
        let db = Firestore.firestore()
        
        db.collection("amounts").getDocuments { (querySnapshot, err) in
            
            if let e = err{
                print("There's any errors: \(e)")
            }
            
            if err != nil{
                print((err?.localizedDescription)!)
                return
            }
            
            for i in querySnapshot!.documents{
                
                let quantityFromDb = i.get("amount") as! Int
                let timeFromDb = i.get("averageTimeRecognition") as! Double
                
                
                self.arrayWithQuantity.append(quantityFromDb)
                self.arrayWithTime.append(timeFromDb)
            }
            completion(self.arrayWithQuantity, self.arrayWithTime)
        }
    }
}

我在 funcreadData()中使用onAppear：

struct CheckDatabaseView: View {
    
    @State private var quantityFromDatabase: Array<Int> = []
    @State private var timeFromDatabase: Array<Double> = []
    @State private var flowersName: Array<String> = ["Carnation", "Daisy", "Hyacinth", "Iris", "Magnolia", "Orchid", "Poppy", "Rose", "Sunflower", "Tulip"]
    @State private var isReady: Bool = false
    
    
    var body: some View {
        
        ScrollView(.vertical, showsIndicators: false){
            ZStack(alignment: .top){
                VStack(spacing: 40){
                    Text("Hello, world!")
//                    BarView(value: CGFloat(timeFromDatabase[0]), name: flowersName[0])
                    
                }
            }
            .frame(minWidth: 0, maxWidth: .infinity, minHeight: 0, maxHeight: .infinity, alignment: .top)
            
        }
        .navigationBarTitle(Text("Your datas in database").foregroundColor(.blue), displayMode: .inline)
        .onAppear{
            
            let gd = getDataFromDatabase()
            gd.readData { (quantity, time) in
                self.quantityFromDatabase = quantity!
                self.timeFromDatabase = time!       
            }
        }
    }
}

我不能使用值self.quantityFromDatabase，self.timeFromDatabase因为是空的。我知道问题在于数据的异步检索。我试过了DispatchQueue.main.async，但我仍然没有得到这些值。另一种获取方法是什么？我需要这个值，因为我想在VStack（那里的注释行）中绘制图表。

编辑

正如@Rexhin Hoxha 在下面写的那样，我修改了代码，但我不确定方法是否正确。我更改var arrayWithQuantity = [Int]()并var arrayWithTime = [Double]()添加@Published了类 getDataFromDatabase（现在是 GetDataFromDatabaseViewModel）：

class GetDataFromDatabaseViewModel : ObservableObject {

    @Published var arrayWithQuantity = [Int]()
    @Published var arrayWithTime = [Double]()
    
    func readData() {
       
        let db = Firestore.firestore()
        
        db.collection("amounts").getDocuments { (querySnapshot, err) in
            
            if let e = err{
                print("There's any errors: \(e)")
            }
            
            if err != nil{
                print((err?.localizedDescription)!)
                return
            }
            
            for i in querySnapshot!.documents{
                
                let quantityFromDb = i.get("amount") as! Int
                let timeFromDb = i.get("averageTimeRecognition") as! Double
                
                
                self.arrayWithQuantity.append(quantityFromDb)
                self.arrayWithTime.append(timeFromDb)
            }
            print("Array with quantity: \(self.arrayWithQuantity.count)")
        }
    }
}

同样在我初始化的结构中@ObservedObject var gd = GetDataFromDatabaseViewModel()，onAppear现在看起来像这样：

.onAppear{
            
            self.gd.readData()
            print("Quantity after reading: \(self.gd.arrayWithQuantity.count)")
        }

但在 onAppear 中打印仍然打印一个空数组。我在哪里做错了？

标签： swiftfirebaseasynchronousgoogle-cloud-firestoreswiftui

Answer 1

所以问题出在你的完成处理程序中。它在您检索数据之前返回。

解决方案是使您的数组@Published 并从视图中实时读取数据。您必须删除完成处理程序。

调用“onAppear()”上的函数并使用@ObservedObject 绑定到您的ViewModel (getDataFromDatabase)。这就是它在 SwiftUI 中的完成方式。

请大写第一个字母并使用更通用的名称，例如“YouViewName”ViewModel。

你的名字适用于方法/函数，但不适用于类