Allow customising icons for snippets via SnippetViewSet.icon

[laymonage]

Rebase of #9804, fixes #4095, incorporates #10216 as part of #10206.

Unfortunately this does not fix #4040, as it requires overwriting the SnippetChooserBlock.Meta.icon attribute at runtime.

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Please check the following:

• Do the tests still pass?¹
• Does the code comply with the style guide?
  • Run make lint from the Wagtail root.
• For Python changes: Have you added tests to cover the new/fixed behaviour?
• For front-end changes: Did you test on all of Wagtail’s supported environments?²
  • Please list the exact browser and operating system versions you tested:
  • Please list which assistive technologies ³ you tested:
• For new features: Has the documentation been updated accordingly?

Please describe additional details for testing this change.

Define a subclass of SnippetViewSet, e.g. with the Person model in bakerydemo in wagtail_hooks.py:

from wagtail.snippets.models import register_snippet
from wagtail.snippets.views.snippets import SnippetViewSet


class PersonViewSet(SnippetViewSet):
    icon = "user"


register_snippet(Person, viewset=PersonViewSet)

Note: when merging, add Daniel Kirkham as co-author of this PR.

Footnotes

1. Development Testing ↩

2. Browser and device support ↩

3. Accessibility Target ↩

[squash-labs]

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Test this branch here: https://laymonagecustom-snippets-icons-mdw1t.squash.io

[laymonage]

Changing this to reduce boilerplate

[laymonage]

Previously this page doesn't have any header icon, but there's an icon beside the object's name:

Screenshot

If we add an icon, I feel like it would look a bit weird to have the same icon for both, so I used list-ul as the header_icon and came up with object_icon for the object's icon specifically for this view:

Screenshot

I chose the list-ul icon because that's what we currently use for the Workflow information dialog:

Screenshot

The SnippetViewSet.icon will be passed as object_icon instead of header_icon.

[laymonage]

A bit hesitant to add this, but the problem is that the bulk actions view instance is shared across all the snippet models, and it has its own registration mechanism. Thus, the icon needs to be resolved at request time rather than during the view class's instantiation. Adding this allows us to do self.model.snippet_viewset.icon in get_context_data() of the view.

I imagine this will be necessary as we add more config options to the ViewSet anyway. For example, if the base URL path is customisable through the viewset, then we will need a way to access the ViewSet for a given model. Another way is to create a registry that maps the model to the viewset, but I'm not sure it's worth it...

Or something simpler, we could change the SNIPPET_MODELS global list into a dictionary that maps each model to the viewset instance? People who rely on get_snippet_models() will still be able to iterate through it like a list of models, but we will be able to do something like get_snippet_models()[MyModel] and get the viewset. Still, it's not as handy compared to MyModel.snippet_viewset, though.

Happy to change the naming, e.g. admin_viewset or just viewset. (Though developers might want to have other viewsets that are not SnippetViewSet for the model as well?)

We can also do this in SnippetViewSet.on_register(), but we currently patch some other attributes to the model in this function, so I figured I put this here.

[gasman]

Looks good to me and works well in my tests!

[gasman]

Merged in 5f154a5