CoreData 删除后的列表刷新答案

【问题标题】：List refresh after CoreData deletionCoreData 删除后的列表刷新
【发布时间】：2021-08-29 17:39:58
【问题描述】：

我有一个包含 CoreData 实体记录列表的视图，并且在视图中声明了以下 FetchRequest

@FetchRequest(entity: Locations.entity(), sortDescriptors: [NSSortDescriptor(key: "locTimestamp", ascending: false)])
var locations: FetchedResults<Locations>

我有两个删除功能，一个是只删除给定 ID 的一条记录

func deleteLocation(forID id: UUID) {
    let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Locations")
    request.predicate = NSPredicate(format: "locID == %@", id as CVarArg)
    do {
        let location = try myContext.fetch(request) as! [NSManagedObject]
        myContext.delete(location.first!)
        try myContext.save()
    } catch let error {
        errTitle = "Error"
        errMessage = error.localizedDescription
        isError = true
    }
}

效果很好，因此在视图中包含记录的列表被刷新，第二个用于删除所有记录

func deleteAllLocations() {
    let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Locations")
    let deleteRequest = NSBatchDeleteRequest(fetchRequest: request)
    do {
        try myContext.execute(deleteRequest)
        try myContext.save()
    } catch let error {
        errTitle = "Error"
        errMessage = error.localizedDescription
        isError = true
    }
}

执行（从 CoreData 实体中删除记录）但视图中的列表行未刷新。这可能是什么问题？

【问题讨论】：

标签： core-data swiftui nsbatchdeleterequest

【解决方案1】：

一些谷歌搜索显示批量删除需要更多步骤才能工作（来自here 的信息）

...

使用NSBatchDeleteRequest 时的一些重要注意事项是：

NSBatchDeleteRequest 直接修改了NSPersistentStore，不加载任何数据到内存中。
NSBatchDeleteRequest 不会修改 NSManagedObjectContext。如果需要通知NSManagedObjectContext 批量删除，请使用mergeChanges(fromRemoteContextSave:, into:)（如示例中所示）。

// Specify a batch to delete with a fetch request
let fetchRequest: NSFetchRequest<NSFetchRequestResult>
fetchRequest = NSFetchRequest(entityName: "Business")

// Create a batch delete request for the
// fetch request
let deleteRequest = NSBatchDeleteRequest(
    fetchRequest: fetchRequest
)

// Specify the result of the NSBatchDeleteRequest
// should be the NSManagedObject IDs for the
// deleted objects
deleteRequest.resultType = .resultTypeObjectIDs

// Get a reference to a managed object context
let context = persistentContainer.viewContext

// Perform the batch delete
let batchDelete = try context.execute(deleteRequest)
    as? NSBatchDeleteResult

guard let deleteResult = batchDelete?.result
    as? [NSManagedObjectID]
    else { return }

let deletedObjects: [AnyHashable: Any] = [
    NSDeletedObjectsKey: deleteResult
]

// Merge the delete changes into the managed
// object context
NSManagedObjectContext.mergeChanges(
    fromRemoteContextSave: deletedObjects,
    into: [context]
)

【讨论】：