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Set typing indicator hidden inside performBatchUpdates to prevent flow layout from requesting message type from DataSource for typing indicator section.

What does this implement/fix? Explain your changes.

Showing the typing indicator works fine, but when hiding it, a crash occurs in the following function of MessagesDataSource with message index out of bounds:

    func messageForItem(at indexPath: IndexPath, in messagesCollectionView: MessagesCollectionView) -> MessageType {
        return messages[indexPath.section]
    }

The stacktrace indicates this is coming from the following function in MessagesCollectionViewFlowLayout:

  open func cellSizeCalculatorForItem(at indexPath: IndexPath) -> CellSizeCalculator {
    if isSectionReservedForTypingIndicator(indexPath.section) {
      return typingIndicatorSizeCalculator
    }
    let message = messagesDataSource.messageForItem(at: indexPath, in: messagesCollectionView) // Here
    switch message.kind {
// ...

This is because messagesCollectionView.performBatchUpdates() causes the MessagesCollectionViewFlowLayout to update. But because we have already set the typing indicator to isHidden = true before calling performBatchUpdates(), the call to isSectionReservedForTypingIndicator in flow layout will return false. This causes a call to messageForItem for a section dedicated to the typing indicator, which causes the index out of bounds crash since we are not expecting the library to request anything for this section.

When I made the change in this PR to my local codebase, it fixed the issue and the typing indicator hiding worked properly. This was tested within the app I am using MessageKit.

I've seen the example app crash a couple of times aswell and I think this might be because of the same issue. Specifically the advanced example.

Does this close any currently open issues?

Found this crash myself while developing an app that is using MessageKit.

Possible fix for #1788

Any relevant logs, error output, etc?

Any other comments?

…

Where has this been tested?

Devices/Simulators:

• iPhone 11 Pro
• iPhone 12 Pro

iOS Version:

• iOS 15.4
• iOS 16.0

Swift Version: Latest

MessageKit Version: 4.1.1