Add dragAndDropManager to the MosaicProps

[cbeer]

Changes proposed in this pull request:

This should allow containing applications to provide an external react-dnd context. It seems like this may have been supported by previous react-dnd (#74), but we're seeing this crop up again in ProjectMirador/mirador#3094 when we use react-dnd in our application.

HTML5Backend.js?56ae:380 Uncaught Error: Cannot have two HTML5 backends at the same time.
    at HTML5Backend.setup (HTML5Backend.js?56ae:380)
    at _default.setup (MultiBackend.js?7d82:75)
    at DragDropManagerImpl.handleRefCountChange (DragDropManagerImpl.js?e1b0:40)
    at Object.dispatch (redux.js?0a4a:222)
    at HandlerRegistryImpl.addTarget (HandlerRegistryImpl.js?0db8:99)
    at registerTarget (registration.js?c129:3)
    at registerHandler (drop.js?169c:50)

Passing the dragDropManager through as a prop allows Mosaic to use the same context:

      <DndContext.Consumer>
				{(ctx) => (
          <Mosaic
            manager={ctx.dragDropManager}
            ...
          />
        )}

[nomcopter]

What's the benefit of this over using MosaicWithoutDragDropContext directly?

[cbeer]

I wasn't able to get MosaicWithoutDragDropContext to work for me -- react-dnd kept complaining that there was no dragDropContext:

Could not find the drag and drop manager in the context of RootDropTargetsClass. Make sure to render a DndProvider component in your top-level component. Read more: http://react-dnd.github.io/react-dnd/docs/troubleshooting#could-not-find-the-drag-and-drop-manager-in-the-context

I've replicated the failure in a simple test case app: https://github.com/cbeer/mosaic-test-case/blob/master/src/App.js

I think it's because react-mosaic has react-dnd as a direct dependency instead of a peer dependency, so the context isn't getting propagated into react-mosaic-component?

[nomcopter]

I wasn't able to get MosaicWithoutDragDropContext to work for me -- react-dnd kept complaining that there was no dragDropContext:

Could not find the drag and drop manager in the context of RootDropTargetsClass. Make sure to render a DndProvider component in your top-level component. Read more: http://react-dnd.github.io/react-dnd/docs/troubleshooting#could-not-find-the-drag-and-drop-manager-in-the-context

I've replicated the failure in a simple test case app: https://github.com/cbeer/mosaic-test-case/blob/master/src/App.js

I think it's because react-mosaic has react-dnd as a direct dependency instead of a peer dependency, so the context isn't getting propagated into react-mosaic-component?

Ah interesting - I think you are on the right track. Can you try pinning your version of react-dnd to the identical version that react-mosaic uses in that test case app? If they both use the same instance of react-dnd it should work.

[cbeer]

Hm. I'm not seeing any difference pinning react-dnd to ^10.0.2 or 10.0.2.

[mejackreed]

Just curious about seeing if this could move forward. @nomcopter would you mind taking a look again? Thanks!

[nomcopter]

Sorry about the crazy delay - hope this is still useful!