Swift4: 'MessageKind' has no member 'url'

问题

Below is example code of MessageKit library. Currently, its behavior is that all image messages must be loaded completely before displaying to end user so it takes a lot of time to wait. What i want is to load image after all messages have been loaded.

Look at 1/ you will see that the "kind" variable which will be a mediaItem Look at 2/ you will see that the mediaItem has "url" variable Look at 3/ you will see that the message is created with above mediaItem

The question is how to get the url of the message at 3/? I tried below but swift does not understand

let imkind = message.kind
imkind.photo.url // error Enum element 'photo' cannot be referenced as an instance member

Below are 1/ 2/ 3/

1/ Message type protocol

public protocol MessageType {

    /// The sender of the message.
    var sender: Sender { get }

    /// The unique identifier for the message.
    var messageId: String { get }

    /// The date the message was sent.
    var sentDate: Date { get }

    /// The kind of message and its underlying kind.
    var kind: MessageKind { get }

}

2/ Create an image to put into the message Kind as below:

private struct ImageMediaItem: MediaItem {

    var url: URL?
    var image: UIImage?
    var placeholderImage: UIImage
    var size: CGSize

    init(image: UIImage) {
        self.image = image
        self.size = CGSize(width: 240, height: 240)
        self.placeholderImage = UIImage()
    }

}

3/ Create a message based on the MessageType above:

internal struct MockMessage: MessageType {

    var messageId: String
    var sender: Sender
    var sentDate: Date
    var kind: MessageKind

    private init(kind: MessageKind, sender: Sender, messageId: String, date: Date) {
        self.kind = kind
        self.sender = sender
        self.messageId = messageId
        self.sentDate = date
    }

    init(image: UIImage, sender: Sender, messageId: String, date: Date) {
        let mediaItem = ImageMediaItem(image: image)
        self.init(kind: .photo(mediaItem), sender: sender, messageId: messageId, date: date)
    }

}

4/ messageKind in the pod library

/// An enum representing the kind of message and its underlying kind.
public enum MessageKind {

    /// A standard text message.
    ///
    /// - Note: The font used for this message will be the value of the
    /// `messageLabelFont` property in the `MessagesCollectionViewFlowLayout` object.
    ///
    /// Using `MessageKind.attributedText(NSAttributedString)` doesn't require you
    /// to set this property and results in higher performance.
    case text(String)

    /// A message with attributed text.
    case attributedText(NSAttributedString)

    /// A photo message.
    case photo(MediaItem)

    /// A video message.
    case video(MediaItem)

    /// A location message.
    case location(LocationItem)

    /// An emoji message.
    case emoji(String)

    /// A custom message.
    /// - Note: Using this case requires that you implement the following methods and handle this case:
    ///   - MessagesDataSource: customCell(for message: MessageType, at indexPath: IndexPath, in messagesCollectionView: MessagesCollectionView) -> UICollectionViewCell
    ///   - MessagesLayoutDelegate: customCellSizeCalculator(for message: MessageType, at indexPath: IndexPath, in messagesCollectionView: MessagesCollectionView) -> CellSizeCalculator
    case custom(Any?)

}

回答1:

guard case .photo(let mediaItem) = imkind else {
        return
    }

    print(mediaItem.url)

You have to unwrap enum case using this code. The problem is that you tries to retrieve enum as mediaItem. But you have to retrieve the internal data.

回答2:

you can use a switch case as MessageKind is an enum

switch message.kind {
        case .photo(let photoItem):
            if let urlsel = photoItem.url{
                print(urlsel)    // here is your imageUrl
                
        }
        default:
            break
        }

来源：https://stackoverflow.com/questions/53787852/swift4-messagekind-has-no-member-url