iOS Swift總結CoreData屬性值

Question

Setup: Xcode version: 7.1 Beta, 
Firmware: iOS 9, 

我試圖從核心數據訪問一個值來獲得購買產品的總計。我已經檢查在同一個問題，但它是在的OBJ C. CoreData細節：

實體：車

屬性：FNAME（字符串），fCost（整數16），fQuantity（整數16）

檢索代碼如下：

let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate 
let managedContext : NSManagedObjectContext = appDelegate.managedObjectContext! 
let fetchRequest = NSFetchRequest(entityName: "Cart") 
fetchRequest.returnsObjectsAsFaults = false 
do { 
    let results = try managedContext.executeFetchRequest(fetchRequest) 
    print("===\(results)") 

} catch let error as NSError { 
    print(error) 
} 

打印（結果）輸出是如下：

[<Tif.Cart: 0x7f8d19e60d00> (entity: Cart; id: 0xd000000000240000 <x-coredata://5D617B2C-35BC-43B4-BEC1-40AABA96C558/Cart/p9> ; data: { 
    fCost = 30; 
    fName = "Milk"; 
    fQuantity = 2; 
}), <Tif.Cart: 0x7f8d19e7cd80> (entity: Cart; id: 0xd000000000280000 <x-coredata://5D617B2C-35BC-43B4-BEC1-40AABA96C558/Cart/p10> ; data: { 
    fCost = 60; 
    fName = "Butter"; 
    fQuantity = 1; 
})] 

我正在使用tableView單元顯示核心數據並執行總計。

var cart = [NSManagedObject]() 
let order = cart[indexPath.row] 
cell.fNLabel.text = order.valueForKey("fName")! as? String 
cell.fCLabel.text = "Rs \(order.valueForKey("fCost") as! NSNumber)" 
cell.fQLabel.text = "\(order.valueForKey("fQuantity") as! NSNumber)" 
let fQ = order.valueForKey("fQuantity") as! Int 
let fC = order.valueForKey("fCost") as! Int 
let total = fQ * fC 
totalArray.append(total) 
grandTotal = totalArray.sum() // sum() is an extension that calculates the sum of integer values 
print(grandTotal) 
grandTotalLabel.text = "\(grandTotal)" 
cell.fTLabel.text = "Rs \(total)" 

我得到grandTotal，但每次我拉表時，值都被添加到最後一個值。

我也讀過關於使用NSFetchRequestController選項，但我根本無法使用它。

Answer 1

看起來您正在計算tableView:cellForRowAtIndexPath:方法中的總計。這絕對不是正確的地方，因爲它只會計算可見單元格的總計。

let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate 
let managedContext : NSManagedObjectContext = appDelegate.managedObjectContext! 
let fetchRequest = NSFetchRequest(entityName: "Cart") 
fetchRequest.returnsObjectsAsFaults = false 
do { 
    let results = try managedContext.executeFetchRequest(fetchRequest) 
    print("===\(results)") 

    // Calculate the grand total... 
    var grandTotal = 0 
    for order in results { 
     let fQ = order.valueForKey("fQuantity") as! Int 
     let fC = order.valueForKey("fCost") as! Int 
     grandTotal += fC * fQ 
    } 
    print("Grand total = \(grandTotal)") 

} catch let error as NSError { 
    print(error) 
}