Question

我有一个包含textField和button的VC。我有一个简单的任务是将textField文本数据传递给我的模型实体。但是，虽然我想将textField数据分配给实体数据，但我的数据仍为零。我无法理解是什么导致了这样的问题

class InputTextViewController: UIViewController {

    @IBOutlet weak var messageTextField: UITextField!

    //I receive user from parent VC by segue
    var userSendMessageTo: User!
    var tableView: UITableView?
    var message: Message { return Message(userSendMessageTo) } 
    // initialising object of my entity class

    override func viewDidLoad() {
        super.viewDidLoad()
        print(userSendMessageTo.name)
        //name is right
    }

    func createMessage() {
        message.messageText = messageTextField.text
        message.messageTime = Date()
        print(message.messageText) //nil
        print(message.messageTime) //nil

    }


    @IBAction func sendMessge(_ sender: Any) {

        createMessage()
        userSendMessageTo.mesaageHistory.append(message)

        print(userSendMessageTo.mesaageHistory[0].messageText) //nil
    }

我的模特

class Message {

    var messageText: String?
    var messageTime: Date?
    var messageInage: UIImage?
    var user: User

    init(_ user: User) {
        self.user = user
    }

}

class User {

    let name: String

    var mesaageHistory = [Message]()

    init(name: String) {
        self.name = name
    }


}

Answer 1

构造var message: Message { return Message(userSendMessageTo) }每次调用时都会返回一个新的Message对象。

因此，您正在创建一个消息对象，为其分配文本，然后将其丢弃。然后创建第二个消息对象，为其指定日期并将其丢弃。然后创建第三个消息对象并检查其文本（为零），然后创建第四个消息对象并检查其日期，也是零。

请改为尝试：

class InputTextViewController: UIViewController {

    @IBOutlet weak var messageTextField: UITextField!
    var userSendMessageTo: User!
    var tableView: UITableView?
    var message: Message?

    func createMessage() {
        message = Message()
        message!.messageText = messageTextField.text
        message!.messageTime = Date()
        print(message!.messageText) //won't be nil
        print(message!.messageTime) //won't be nil
    }
}

Answer 2

您已将message声明为计算变量。这意味着每次引用message时，您实际上都在执行以下代码块：

Message(userSendMessageTo)

换句话说，对message 的每次引用都会创建Message 的新实例。

message.messageText = messageTextField.text - 创建Message
message.messageTime = Date() - 创建Message
print(message.messageTime) - 创建一个新的Message实例，其messageTime属性为nil

此处无需使用房产;您的createMessage函数应该返回新的Message：

func createMessage(withText text: text) -> Message {
    let message = Message(userSendMessageTo)
    message.messageText = text
    message.messageTime = Date()
    return message
}

@IBAction func sendMessge(_ sender: Any) {

    let message = self.createMessage(withText: messageTextField.text)
    userSendMessageTo.mesaageHistory.append(message)

    print(userSendMessageTo.mesaageHistory[0].messageText)
}

说实话，如果你为Message创建一个合适的初始化程序，你可以完全摆脱createMessage函数：

class Message {

    var messageText: String
    var messageTime: Date
    var messageInage: UIImage?
    var user: User

class Message {

    var messageText: String
    var messageTime: Date
    var messageInage: UIImage?
    var user: User

    init(_ user: User, text: String = "", date: Date = Date()) {
        self.user = user
        self.messageTime = date
        self.messageText = text
    } 

}

然后你的行动方法就变成了：

@IBAction func sendMessge(_ sender: Any) {
    let message = Message(userSendMessageTo, text: messageTextField.text, date:Date())
    userSendMessageTo.mesaageHistory.append(message)

    print(userSendMessageTo.mesaageHistory[0].messageText)
}

变量赋值后的零值

2 个答案: