swift - 在应用程序之间切换时未调用 CellForRowAt(内部 gif)

Question

我有一个 ChatViewController 和聊天消息。我使用 Storyboard 约束和标识符初始化消息气泡的布局，以通过代码更改它们:

func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = chatUser.dequeueReusableCell(withIdentifier: "chatUserCell", for: indexPath) as! ChatUserTableViewCell
    print("cellforrowcalled")
    cell.transform = CGAffineTransform(rotationAngle: CGFloat(Double.pi))

    // I check here the message sender
    cell.isFromCurrentUser = messages[indexPath.row].fromUid == UserApi.shared.CURRENT_USER_UID!

    cell.labelChatMessage.text = messages[indexPath.row].chatText

    cell.labelChatMessageDate.text = messages[indexPath.row].chatDate?.dateValue().timeAgo(numericDates: false)

    return cell
}

在我的 ChatCellViewController 中，我根据“isFromCurrentUser” bool 值更改约束:

var isFromCurrentUser: Bool! {
    didSet {
        labelChatMessage.backgroundColor = isFromCurrentUser ? .systemBlue : .darkGray

        if isFromCurrentUser {
            constraintOtherUserTimestamp.isActive = false
            constraintOtherUserLeading.isActive = false
            constraintOtherUserTrailing.isActive = false
            constraintCurrentUserTimestamp.isActive = true
            constraintCurrentUserLeading.isActive = true
            constraintCurrentUserTrailing.isActive = true
        } else {
            constraintCurrentUserTimestamp.isActive = false
            constraintCurrentUserLeading.isActive = false
            constraintCurrentUserTrailing.isActive = false
            constraintOtherUserTimestamp.isActive = true
            constraintOtherUserLeading.isActive = true
            constraintOtherUserTrailing.isActive = true
        }
    }
}

它的工作原理与您在 .gif 中看到的一样，但是当我更改应用程序并重新打开我的应用程序时，未调用 cellForRowAt，我失去​​了约束并且 View 正在使用初始 Storyboard约束:

Answer 1

您应该收听应用程序进入前台的通知，即 UIApplication.willEnterForegroundNotification

在您的 viewDidLoad() 中，添加观察者:

NotificationCenter.default.addObserver(self, selector: #selector(resumeState), name: UIApplication.willEnterForegroundNotification, object: nil)

然后声明resumeState()函数，当应用程序进入前台状态时将调用此函数。

@objc func resumeState() {
    yourTableView.reloadData()
}

也不要忘记在 deinit 中删除观察者

deinit {
    print("\(self.description) deinitialized")
    NotificationCenter.default.removeObserver(self)
}