Automatically update preview while changing the markdown source

[kieferrm]

From microsoft/vscode#6106:

we need to change the extension to update the preview based on document edits so that the preview updates while the user is typing

[kieferrm]

Here is our initial thinking for how to update the side-by-side preview:

1. The extension is listening to the document change events emitted by the workspace
2. If we receive a document change event for a markdown document we translate that into a document change event for the virtual markdown preview document and let the registered document content provider fire that document change event
3. Ensure the html preview editor reloads when it receives a document changed event from the document content provider

[bpasero]

I think that is the idea, also @jrieken fyi who provides the API for text document provider.

[jrieken]

Yes - your document provider can offer an event that signals change. This is an sample: https://github.com/Microsoft/vscode/blob/master/extensions/vscode-api-tests/src/workspace.test.ts#L299

[kieferrm]

@jrieken thanks. That's makes for a slick solution.