在 SwiftUI 中更新 CoreData 时更新状态变量

Posted 2023-03-05

【中文标题】在 SwiftUI 中更新 CoreData 时更新状态变量

【英文标题】：Update State-variable Whenever CoreData is Updated in SwiftUI

【发布时间】：2020-01-28 17:43:30

【问题描述】：

从CoreData更新一些数据后，我还想更新一个State-变量为返回结果的数量。

当 CoreData 发生变化时，Stepper 应该始终设置为返回结果的数量。但是，当我使用Stepper 时，onAppear 也会触发。如何检查 onAppear 是否更改了 CoreData 或使用了 Stepper？这可能吗？

import SwiftUI

struct ContentView: View 
@State var numberOfResults = 0

@FetchRequest(entity: YourModel.entity(), sortDescriptors: [], predicate:NSPredicate(format: "isSelected == %@", NSNumber(booleanLiteral: true))) var objects: FetchedResults<YourModel>

var body: some View
    return VStack

        Stepper("text", value: $numberOfResults, in: 0...objects.wrappedValue.count, step:5)
            .onReceive(objects.publisher, perform: _ in
                self.numberOfResults = self.objects.count
                print("onReceive")
            )
        
    

【参考方案1】：

如果你使用@FetchRequest 和 onReceive numberOfResults 会在发布者发送消息时更新

import SwiftUI

struct DidSetCoreData: View 
    @State var numberOfResults = 0
    @State var initSetup1: Bool = true
    @State var initSetup2: Bool = true
    @State var adjustedCount = 0
    @FetchRequest(entity: YourModel.entity(), sortDescriptors: [], predicate:NSPredicate(format: "isSelected == %@", NSNumber(booleanLiteral: true))) var objects: FetchedResults<YourModel>

    var body: some View
    return VStack
            Text("Total Count= \(objects.count)")
            Text("Adjusted Total = \($adjustedCount.wrappedValue)")
            //You need the separate adjusted count variable to save the changes

            //Option 1 Stepper - Keeps the max step flexible, eliminates the need for the numberOfResults var
            Stepper("AdjTotal - FlexibleMax", value: $adjustedCount, in: 0...objects.count, step:5)
            .onReceive(objects.publisher.count(), perform: count in
                //onReceive will be called everytime there is a change to objects.count or body refresh
                print("onReceive - Option 1")
                if self.initSetup1
                    //The variable will only be setup once
                    self.adjustedCount = count
                    self.initSetup1 = false
                    print("initSetupComplete - Option 1")
                
            )

            //Option 2 Stepper
            Stepper("AdjTotal - initMax", value: $adjustedCount, in: 0...$numberOfResults.wrappedValue, step:5)

            .onReceive(objects.publisher.count(), perform: count in
                //onReceive will be called everytime there is a change to objects.count or body refresh
                print("onReceive - Option 2")
                if self.initSetup2
                    //The variables will only be setup once
                    self.numberOfResults = count //Limits the max step to only the original count
                    self.adjustedCount = self.numberOfResults
                    self.initSetup2 = false
                    print("initSetupComplete - Option 2")
                
            )
            //Option 3 Stepper - Limits the StepperMax to the Stepper value
            Stepper("AdjTotal - ValueMax", value: $numberOfResults, in: 0...$numberOfResults.wrappedValue, step:5)
            .onReceive(objects.publisher.count(), perform: count in
                //onReceive will be called everytime there is a change to objects.count or body refresh
                print("onReceive - Option 3")
                if self.initSetup3
                    //The variable will only be setup once
                    self.numberOfResults = count
                    self.initSetup3 = false
                    print("initSetupComplete - Option 3")
                
           )
        
    

【讨论】：

onAppear 也会在每次更改状态变量时触发。我在 Stepper 中使用状态变量。因此，每当我使用Stepper 时，它都会设置为numberOfResults。我更改了代码以使其更清晰。您还知道如何使步进器工作吗？

您可以尝试两种方法。但这取决于您是否需要此视图上的 FetchRequest 变量来做其他事情。如果您不需要 FetchRequest 进行其他任何操作，可以使用 init() 期间的一次性 FetchRequest 来设置 numberOfResults 变量。 developer.apple.com/library/archive/documentation/Cocoa/… 如果您确实需要 FetchRequest，我将上面的代码更改为包含一个 initSetup Bool。

【参考方案2】：

您不需要@State var numberOfResults。您可以在 Text() 中仅使用 objects.count。包装器@FetchRequest 为您完成所有工作。每当您将YourModel 实体对象添加到ManagedObjectContext 中时，它将触发FetchRequest 刷新并为您提供实际结果。 @FetchRequest 已经在做 @State 之类的事情了

【讨论】：

我需要它作为状态变量，因为我在 Stepper 中使用了该变量。它最初应该设置为最大值。