[英]How to accumulate length of managed object String property in CoreData?
這是CoreData中的一個Item實體,Item包含一個屬性title(類型是String?):
let item = Item(context: moc)
item.title = "hello world"
我想計算所有 Item.title.count 的總和:
let item_0 = Item(context: moc)
item_0.title = "a" // title.count is 1
let item_1 = Item(context: moc)
item_1.title = "ab" // title.count is 2
let item_2 = Item(context: moc)
item_2.title = "abc" // title.count is 3
// sum Item.title.count is 1 + 2 + 3 = 6
我不想循環遍歷所有 Item 對象,我想使用如下內容:
let req = NSFetchRequest<NSFetchRequestResult>(entityName: "Item")
req.resultType = .dictionaryResultType
let countDesc = NSExpressionDescription()
countDesc.name = "titlesCount"
countDesc.expressionResultType = .integer64AttributeType
countDesc.expression = NSExpression(forFunction: "sum:", arguments: [NSExpression(forKeyPath: "title.count")])
req.propertiesToFetch = [countDesc]
let results = try moc.fetch(req) as! [NSDictionary] // Crash!!!
let dict = results.first!
let count = dict["titlesCount"] as! Int64
不幸的是,上面的代碼崩潰如下:
*** 由於未捕獲的異常“NSInvalidArgumentException”而終止應用程序,原因:“無效的密鑰路徑(在屬性名稱后繼續):標題”
有沒有辦法使用 NSExpression 對 Item.title.count 求和? 我不想遍歷所有項目。
非常感謝! ;)
如何獲得每個項目的長度然后總結它們?
let req = NSFetchRequest<NSFetchRequestResult>(entityName: "Item")
req.resultType = .dictionaryResultType
let countDesc = NSExpressionDescription()
countDesc.name = "titlesCount"
countDesc.expressionResultType = .integer64AttributeType
countDesc.expression = NSExpression(forFunction: "length:", arguments: [NSExpression(forKeyPath: "name")])
req.propertiesToFetch = [countDesc]
do {
let results = try moc.fetch(req) as! [NSDictionary]
return results.reduce(0) { $0 + ($1["titlesCount"] as! Int) } // this sums up all the individual item lengths
} catch {
print("Error fetching data from context \(error)")
}
我希望它有幫助! :)
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