ios - 受管对象中的String属性数组始终为nil

Question

首先，我将文本字段中的内容提取到字典中:

var dict: [String: String] = [:]
for metricPair in metricStackView.arrangedSubviews {
    if metricPair.subviews[0] is UITextField {
        let unitTextField = metricPair.subviews[0] as! UITextField
        let valueTextField = metricPair.subviews[1] as! UITextField
        
        if let textContent = unitTextField.text, let valueTextContent = valueTextField.text {
            let trimmedKey = textContent.trimmingCharacters(in: .whitespacesAndNewlines)
            let trimmedValue = valueTextContent.trimmingCharacters(in: .whitespacesAndNewlines)
            dict.updateValue(trimmedValue, forKey: trimmedKey)
        }
    }
}

我将其保存到核心数据中:

let goal = Goal(context: self.context)
goal.date = Date()
 
for item in dict {
 goal.metrics?.append(item.key)
}
 
goal.progress.insert(progress)

我的托管对象如下所示:

extension Goal {
    @nonobjc public class func createFetchRequest() -> NSFetchRequest<Goal> {
        return NSFetchRequest<Goal>(entityName: "Goal")
    }

    @NSManaged public var date: Date
    @NSManaged public var metrics: [String]?
    @NSManaged public var progress: Set<Progress>
}

即使在保存上下文之前，我仍然不断获得 nil类型的 metrics属性的 [String]。当我登录 item.key时:

for item in dict {
 goal.metrics?.append(item.key)
}

内容正确显示。

Answer 1

据我从上面的代码可以看出，metrics数组未初始化。
尝试替换此行:

goal.metrics?.append(item.key)

这些行:

if case nil = goal.metrics?.append(item.key) {
    goal.metrics = [item.key]
}

或仅使用以下几行:

if goal.metrics == nil {
    goal.metrics = []
}
goal.metrics?.append(item.key)