Ios prepareforsgue首先返回nil

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致命错误：当我从单元格中分离时，在展开可选值时意外发现nil

代码如下：

func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
    if indexPath.section == 1 {
        let listIngredients = recipeItem.ingredients[indexPath.row]
        selectedIngredient = listIngredients.ingredient
    }
    tableView.deselectRowAtIndexPath(indexPath, animated: false)
    performSegueWithIdentifier("showIngredientInRecipe", sender: self)


}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "showIngredientInRecipe" {
        let svc = segue.destinationViewController as! UINavigationController
        let destination = svc.topViewController as! IngredientDetailViewController

        destination.ingredientItem = selectedIngredient


        print("selectedIngredient \n \(selectedIngredient)")

    }

}

以下是我从调试器获得的信息：

        selectedIngredient
        nil

        selectedIngredient
        Optional(Ingredient {
            name = Rye Whiskey;
            inStock = 1;
            type = IngredientType {
                name = Spirit;
            };
        })

        fatal error: unexpectedly found nil while unwrapping an Optional value

如您所见，
selectedgredient
打印两次。第一次为nil，第二次为预期内容。如果我将
destination.ingredientem=selectedIngredient
替换为
destination.ingredientem=recipeItem.components[0]。component
序列运行正常，没有错误。
您正在检查indexPath的节是否为1，如果不是，它仍将执行序列。确保您的可选单元格位于第1节（或第0节？）中，并将您的
PerformsgueWithIdentifier（“showIngredientInRecipe”，发件人：self）
调用if语句以使其更安全。
修复：

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "showIngredientInRecipe" {
        if let selectedIndexPath = self.tableView.indexPathForSelectedRow {
            if selectedIndexPath.section == 1 {
                let listIngredients = recipeItem.ingredients[selectedIndexPath.row]
                selectedIngredient = listIngredients.ingredient
                let svc = segue.destinationViewController as! UINavigationController
                let destination = svc.topViewController as! IngredientDetailViewController
                destination.ingredientItem = selectedIngredient
                print("selectedIngredient \n \(selectedIngredient)")
            }
        }
    }
}

您的代码很容易出错，因为您可以执行segue而不考虑
部门
获取选定项的更好方法是在委托内部调用
indexPathForSelectedRow（）
方法

谢谢我将
performsguewithidentifier（“showIngredientInRecipe”，sender:self）
移动到if语句中，但我还是先得到了nil，然后才得到了预期的结果。
let svc=segue.destinationViewController
已经给了您
IngredientDetailViewController
。你能告诉我它在哪一行给你零吗
func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
    if indexPath.section == 1 {
         performSegueWithIdentifier("showIngredientInRecipe", sender: self)
    }
}

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
    if segue.identifier == "showIngredientInRecipe" {
        //get selected item
        let indexPath = self.tableView.indexPathForSelectedRow() {
            let selectedIngredient = recipeItem.ingredients[indexPath.row]
            print("selectedIngredient \n \(selectedIngredient)")
            //segue
            let svc = segue.destinationViewController as! UINavigationController
            let destination = svc.topViewController as! IngredientDetailViewController
            destination.ingredientItem = selectedIngredient
        }
    }
}