[Session] Code cleanup (#3854)

* Split utils into files

* Move reducer to another file

* Write types explicitly

* Remove unnnecessary check

* Move things around a bit

* Move more stuff into agent factories

* Move more stuff into agent

* Fix gates await

* Clarify comments

* Enforce more via types

* Nit

* initSession -> resumeSession

* Protect against races

* Make agent opaque to reducer

* Check using plain condition

src/state/session/reducer.ts

@@ -0,0 +1,188 @@
+import {AtpSessionEvent} from '@atproto/api'
+
+import {createPublicAgent} from './agent'
+import {SessionAccount} from './types'
+
+// A hack so that the reducer can't read anything from the agent.
+// From the reducer's point of view, it should be a completely opaque object.
+type OpaqueBskyAgent = {
+  readonly api: unknown
+  readonly app: unknown
+  readonly com: unknown
+}
+
+type AgentState = {
+  readonly agent: OpaqueBskyAgent
+  readonly did: string | undefined
+}
+
+export type State = {
+  readonly accounts: SessionAccount[]
+  readonly currentAgentState: AgentState
+  needsPersist: boolean // Mutated in an effect.
+}
+
+export type Action =
+  | {
+      type: 'received-agent-event'
+      agent: OpaqueBskyAgent
+      accountDid: string
+      refreshedAccount: SessionAccount | undefined
+      sessionEvent: AtpSessionEvent
+    }
+  | {
+      type: 'switched-to-account'
+      newAgent: OpaqueBskyAgent
+      newAccount: SessionAccount
+    }
+  | {
+      type: 'updated-current-account'
+      updatedFields: Partial<
+        Pick<
+          SessionAccount,
+          'handle' | 'email' | 'emailConfirmed' | 'emailAuthFactor'
+        >
+      >
+    }
+  | {
+      type: 'removed-account'
+      accountDid: string
+    }
+  | {
+      type: 'logged-out'
+    }
+  | {
+      type: 'synced-accounts'
+      syncedAccounts: SessionAccount[]
+      syncedCurrentDid: string | undefined
+    }
+
+function createPublicAgentState(): AgentState {
+  return {
+    agent: createPublicAgent(),
+    did: undefined,
+  }
+}
+
+export function getInitialState(persistedAccounts: SessionAccount[]): State {
+  return {
+    accounts: persistedAccounts,
+    currentAgentState: createPublicAgentState(),
+    needsPersist: false,
+  }
+}
+
+export function reducer(state: State, action: Action): State {
+  switch (action.type) {
+    case 'received-agent-event': {
+      const {agent, accountDid, refreshedAccount, sessionEvent} = action
+      if (agent !== state.currentAgentState.agent) {
+        // Only consider events from the active agent.
+        return state
+      }
+      if (sessionEvent === 'network-error') {
+        // Don't change stored accounts but kick to the choose account screen.
+        return {
+          accounts: state.accounts,
+          currentAgentState: createPublicAgentState(),
+          needsPersist: true,
+        }
+      }
+      const existingAccount = state.accounts.find(a => a.did === accountDid)
+      if (
+        !existingAccount ||
+        JSON.stringify(existingAccount) === JSON.stringify(refreshedAccount)
+      ) {
+        // Fast path without a state update.
+        return state
+      }
+      return {
+        accounts: state.accounts.map(a => {
+          if (a.did === accountDid) {
+            if (refreshedAccount) {
+              return refreshedAccount
+            } else {
+              return {
+                ...a,
+                // If we didn't receive a refreshed account, clear out the tokens.
+                accessJwt: undefined,
+                refreshJwt: undefined,
+              }
+            }
+          } else {
+            return a
+          }
+        }),
+        currentAgentState: refreshedAccount
+          ? state.currentAgentState
+          : createPublicAgentState(), // Log out if expired.
+        needsPersist: true,
+      }
+    }
+    case 'switched-to-account': {
+      const {newAccount, newAgent} = action
+      return {
+        accounts: [
+          newAccount,
+          ...state.accounts.filter(a => a.did !== newAccount.did),
+        ],
+        currentAgentState: {
+          did: newAccount.did,
+          agent: newAgent,
+        },
+        needsPersist: true,
+      }
+    }
+    case 'updated-current-account': {
+      const {updatedFields} = action
+      return {
+        accounts: state.accounts.map(a => {
+          if (a.did === state.currentAgentState.did) {
+            return {
+              ...a,
+              ...updatedFields,
+            }
+          } else {
+            return a
+          }
+        }),
+        currentAgentState: state.currentAgentState,
+        needsPersist: true,
+      }
+    }
+    case 'removed-account': {
+      const {accountDid} = action
+      return {
+        accounts: state.accounts.filter(a => a.did !== accountDid),
+        currentAgentState:
+          state.currentAgentState.did === accountDid
+            ? createPublicAgentState() // Log out if removing the current one.
+            : state.currentAgentState,
+        needsPersist: true,
+      }
+    }
+    case 'logged-out': {
+      return {
+        accounts: state.accounts.map(a => ({
+          ...a,
+          // Clear tokens for *every* account (this is a hard logout).
+          refreshJwt: undefined,
+          accessJwt: undefined,
+        })),
+        currentAgentState: createPublicAgentState(),
+        needsPersist: true,
+      }
+    }
+    case 'synced-accounts': {
+      const {syncedAccounts, syncedCurrentDid} = action
+      return {
+        accounts: syncedAccounts,
+        currentAgentState:
+          syncedCurrentDid === state.currentAgentState.did
+            ? state.currentAgentState
+            : createPublicAgentState(), // Log out if different user.
+        needsPersist: false, // Synced from another tab. Don't persist to avoid cycles.
+      }
+    }
+  }
+}