Add a new home feed-api wrapper and give a header indicating the fallback behavior (#2534)

* Add a new home feed-api wrapper and give a header indicating the fallback behavior

* Sneak in a quick fix: use the correct text color in the delete modal

* Use imported constant

src/lib/api/feed/home.ts

@@ -0,0 +1,88 @@
+import {AppBskyFeedDefs} from '@atproto/api'
+import {FeedAPI, FeedAPIResponse} from './types'
+import {FollowingFeedAPI} from './following'
+import {CustomFeedAPI} from './custom'
+import {PROD_DEFAULT_FEED} from '#/lib/constants'
+
+// HACK
+// the feed API does not include any facilities for passing down
+// non-post elements. adding that is a bit of a heavy lift, and we
+// have just one temporary usecase for it: flagging when the home feed
+// falls back to discover.
+// we use this fallback marker post to drive this instead. see Feed.tsx
+// for the usage.
+// -prf
+export const FALLBACK_MARKER_POST: AppBskyFeedDefs.FeedViewPost = {
+  post: {
+    uri: 'fallback-marker-post',
+    cid: 'fake',
+    record: {},
+    author: {
+      did: 'did:fake',
+      handle: 'fake.com',
+    },
+    indexedAt: new Date().toISOString(),
+  },
+}
+
+export class HomeFeedAPI implements FeedAPI {
+  following: FollowingFeedAPI
+  discover: CustomFeedAPI
+  usingDiscover = false
+  itemCursor = 0
+
+  constructor() {
+    this.following = new FollowingFeedAPI()
+    this.discover = new CustomFeedAPI({feed: PROD_DEFAULT_FEED('whats-hot')})
+  }
+
+  reset() {
+    this.following = new FollowingFeedAPI()
+    this.discover = new CustomFeedAPI({feed: PROD_DEFAULT_FEED('whats-hot')})
+    this.usingDiscover = false
+    this.itemCursor = 0
+  }
+
+  async peekLatest(): Promise<AppBskyFeedDefs.FeedViewPost> {
+    if (this.usingDiscover) {
+      return this.discover.peekLatest()
+    }
+    return this.following.peekLatest()
+  }
+
+  async fetch({
+    cursor,
+    limit,
+  }: {
+    cursor: string | undefined
+    limit: number
+  }): Promise<FeedAPIResponse> {
+    if (!cursor) {
+      this.reset()
+    }
+
+    let returnCursor
+    let posts: AppBskyFeedDefs.FeedViewPost[] = []
+
+    if (!this.usingDiscover) {
+      const res = await this.following.fetch({cursor, limit})
+      returnCursor = res.cursor
+      posts = posts.concat(res.feed)
+      if (res.feed.length === 0 || !cursor) {
+        posts.push(FALLBACK_MARKER_POST)
+        this.usingDiscover = true
+      }
+    }
+
+    if (this.usingDiscover) {
+      const res = await this.discover.fetch({cursor, limit})
+      returnCursor = res.cursor
+      posts = posts.concat(res.feed)
+    }
+
+    return {
+      cursor: returnCursor,
+      feed: posts,
+    }
+  }
+}

src/lib/api/feed/merge.ts

@@ -26,7 +26,7 @@ export class MergeFeedAPI implements FeedAPI {
 
   reset() {
     this.following = new MergeFeedSource_Following(this.feedTuners)
-    this.customFeeds = [] // just empty the array, they will be captured in _fetchNext()
+    this.customFeeds = []
     this.feedCursor = 0
     this.itemCursor = 0
     this.sampleCursor = 0