dcc location callback nav

[AnnMarieW]

Improve page navigation when dcc.Location is updated in a callback.

Currently when dcc.Location is updated in a callback, the browser url is updated, but it does not navigate to the new page unless the browser is refreshed. This PR makes it unnecessary to refresh the browser -- giving the same fast navigation as when user clicks on a dcc.Link.

See a demo app in the usage-dcc-location-callback-nav folder.

As requested on the Dash Community Forum

Limitations

The following issues occur when there are multiple dcc.Location components in a multi-page app. These are all existing issues and are not new based on changes in this PR. It they aren't fixed as part of the PR, then it would be helpful to document the workarounds.

• There are potential conflicts when callbacks update multiple dcc.Location location causing the browser to crash.

• When using in a multi-page app with multiple dcc.Location components, they must all be on the same page - typically the app.py file. (Or the index.py if using the multi-page structure where app is defined in it's own file)

• When navigating to a page with the callback to update the dcc.Location the callback is fired even when prevent_initial_call=True This has been reported in issue 1346 and issue 1985

• Browser forward/back doesn't work when there are multiple dcc.Location components

Contributor Checklist

• [ ] I have broken down my PR scope into the following TODO tasks
  • [ ] Investigate whether it's necessary to add the "callback-nav" option. Should this just be the default, or are there other use-cases for refresh=True and refresh=False
  • [ ] Investigate why the browser forward/back does not work when using more than one dcc.Location
• [x] I have run the tests locally and they passed. (refer to testing section in contributing)
• [x] I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• [ ] I have added entry in the CHANGELOG.md
• [ ] If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • [ ] this GitHub #PR number updates the dash docs
  • [ ] here is the show and tell thread in Plotly Dash community