如何在Coredata＆＃xff1f中积累托管对象字符串属性的长度;

Question

这是Coredata中的项目实体，项目包含一个属性标题（类型为字符串？）：

let item = Item(context: moc)
item.title = "hello world"

我想计算所有项目的总和。Title.Count.Count：

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

我不想遍历所有项目对象，我想要要使用以下内容：

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，以下代码崩溃如下：

***终止由于未知的例外“ nsinvalidargumentException”，原因是：'无效键盘（在属性名称之后继续）：title'

是否有办法使用nsexpression来sum.title.title.count.count.count？我不想迭代所有项目。

多谢！ ）

Answer 1

如何获得每个项目的长度然后将它们求和？

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)")
}

我希望它有帮助！ :)