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我有一个带有属性 recipeID 的实体 Recipe。 如何在 Swift 中将 max(recipeID) 作为 Int 值获取？
我是 swift 的新手，请帮助我。 提前致谢。
func fetchMaxID() {
let context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
let fetchRequest = NSFetchRequest(entityName: "Recipe")
fetchRequest.fetchLimit = 1
let sortDescriptor = NSSortDescriptor(key: "recipeID", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
do {
let maxID = try [managedObjectContext?.executeFetchRequest(fetchRequest)].first
print(maxID)
} catch _ {
}
}
最佳答案
Apple 推荐并且最快的方式是使用 NSExpressions。 moc 是一个 NSManagedObjectContext。
private func getLastContactSyncTimestamp() -> Int64? {
let request: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest()
request.entity = NSEntityDescription.entity(forEntityName: "Contact", in: self.moc)
request.resultType = NSFetchRequestResultType.dictionaryResultType
let keypathExpression = NSExpression(forKeyPath: "timestamp")
let maxExpression = NSExpression(forFunction: "max:", arguments: [keypathExpression])
let key = "maxTimestamp"
let expressionDescription = NSExpressionDescription()
expressionDescription.name = key
expressionDescription.expression = maxExpression
expressionDescription.expressionResultType = .integer64AttributeType
request.propertiesToFetch = [expressionDescription]
var maxTimestamp: Int64? = nil
do {
if let result = try self.moc.fetch(request) as? [[String: Int64]], let dict = result.first {
maxTimestamp = dict[key]
}
} catch {
assertionFailure("Failed to fetch max timestamp with error = \(error)")
return nil
}
return maxTimestamp
}
关于swift - 核心数据 - 如何从实体属性中获取最大值(Swift)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39102900/