Add development guidelines about frontend widget architecture

doc/development/fe_guide/architecture.md

@@ -17,6 +17,13 @@ when appropriate.
 
 You can find the Frontend Architecture experts on the [team page](https://about.gitlab.com/company/team/).
 
+## Widget Architecture
+
+The [Plan stage](https://about.gitlab.com/handbook/engineering/development/dev/fe-plan/)
+is refactoring the right sidebar to consist of **widgets**. They have a specific architecture to be
+reusable and to expose an interface that can be used by external Vue applications on the page.
+Learn more about the [widget architecture](widgets.md).
+
 ## Examples
 
 You can find [documentation about the desired architecture](vue.md) for a new

doc/development/fe_guide/widgets.md

+---
+stage: none
+group: unassigned
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Widgets
+
+Frontend widgets are standalone Vue applications or Vue component trees that can be added on a page
+to handle a part of the functionality.
+
+Good examples of widgets are [sidebar assignees](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/sidebar/components/assignees/sidebar_assignees_widget.vue) and [sidebar confidentiality](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/sidebar/components/confidential/sidebar_confidentiality_widget.vue).
+
+When building a widget, we should follow a few principles described below.
+
+## Vue Apollo is required
+
+All widgets should use the same stack (Vue + Apollo Client).
+To make it happen, we must add Vue Apollo to the application root (if we use a widget
+as a component) or provide it directly to a widget. For sidebar widgets, use the
+[sidebar Apollo Client and Apollo Provider](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/sidebar/graphql.js):
+
+```javascript
+import SidebarConfidentialityWidget from '~/sidebar/components/confidential/sidebar_confidentiality_widget.vue';
+import { apolloProvider } from '~/sidebar/graphql';
+
+function mountConfidentialComponent() {
+  new Vue({
+    apolloProvider,
+    components: {
+      SidebarConfidentialityWidget,
+    },
+    /* ... */
+  });
+}
+```
+
+## Required injections
+
+All editable sidebar widgets should use [`SidebarEditableItem`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/sidebar/components/sidebar_editable_item.vue) to handle collapsed/expanded state. This component requires the `canUpdate` property provided in the application root.
+
+## No global state mappings
+
+We aim to make widgets as reusable as possible. That's why we should avoid adding any external state
+bindings to widgets or to their child components. This includes Vuex mappings and mediator stores.
+
+## Widget's responsibility
+
+A widget is responsible for fetching and updating an entity it's designed for (assignees, iterations, and so on).
+This means a widget should **always** fetch data (if it's not in Apollo cache already).
+Even if we provide an initial value to the widget, it should perform a GraphQL query in the background
+to be stored in Apollo cache.
+
+Eventually, when we have an Apollo Client cache as a global application state, we won't need to pass
+initial data to the sidebar widget. Then it will be capable of retrieving the data from the cache.
+
+## Using GraphQL queries and mutations
+
+We need widgets to be flexible to work with different entities (epics, issues, merge requests, and so on).
+Because we need different GraphQL queries and mutations for different sidebars, we create
+[_mappings_](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/sidebar/constants.js#L9):
+
+```javascript
+export const assigneesQueries = {
+  [IssuableType.Issue]: {
+    query: getIssueParticipants,
+    mutation: updateAssigneesMutation,
+  },
+  [IssuableType.MergeRequest]: {
+    query: getMergeRequestParticipants,
+    mutation: updateMergeRequestParticipantsMutation,
+  },
+};
+```
+
+To handle the same logic for query updates, we **alias** query fields. For example:
+
+- `group` or `project` become `workspace`
+- `issue`, `epic`, or `mergeRequest` become `issuable`
+
+Unfortunately, Apollo assigns aliased fields a typename of `undefined`, so we need to fetch `__typename` explicitly:
+
+```plaintext
+query issueConfidential($fullPath: ID!, $iid: String) {
+  workspace: project(fullPath: $fullPath) {
+    __typename
+    issuable: issue(iid: $iid) {
+      __typename
+      id
+      confidential
+    }
+  }
+}
+```
+
+## Communication with other Vue applications
+
+If we need to communicate the changes of the widget state (for example, after successful mutation)
+to the parent application, we should emit an event:
+
+```javascript
+updateAssignees(assigneeUsernames) {
+  return this.$apollo
+    .mutate({
+      mutation: this.$options.assigneesQueries[this.issuableType].mutation,
+      variables: {...},
+    })
+    .then(({ data }) => {
+      const assignees = data.issueSetAssignees?.issue?.assignees?.nodes || [];
+      this.$emit('assignees-updated', assignees);
+    })
+}
+```
+
+Sometimes, we want to listen to the changes on the different Vue application like `NotesApp`.
+In this case, we can use a renderless component that imports a client and listens to a certain query:
+
+```javascript
+import { fetchPolicies } from '~/lib/graphql';
+import { confidentialityQueries } from '~/sidebar/constants';
+import { defaultClient as gqlClient } from '~/sidebar/graphql';
+
+created() {
+  if (this.issuableType !== IssuableType.Issue) {
+    return;
+  }
+
+  gqlClient
+    .watchQuery({
+      query: confidentialityQueries[this.issuableType].query,
+      variables: {...},
+      fetchPolicy: fetchPolicies.CACHE_ONLY,
+    })
+    .subscribe((res) => {
+      this.setConfidentiality(issuable.confidential);
+    });
+},
+methods: {
+  ...mapActions(['setConfidentiality']),
+},
+```
+
+[View an example of such a component.](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/notes/components/sidebar_subscription.vue)