Introduce an image sizes cache to improve feed layouts (close #213) (#335)

* Introduce an image sizes cache to improve feed layouts (close #213)

* Clear out resolved promises from the image cache

src/state/models/cache/image-sizes.ts

@@ -0,0 +1,37 @@
+import {Image} from 'react-native'
+import {Dim} from 'lib/media/manip'
+
+export class ImageSizesCache {
+  sizes: Map<string, Dim> = new Map()
+  private activeRequests: Map<string, Promise<Dim>> = new Map()
+
+  constructor() {}
+
+  get(uri: string): Dim | undefined {
+    return this.sizes.get(uri)
+  }
+
+  async fetch(uri: string): Promise<Dim> {
+    const dim = this.sizes.get(uri)
+    if (dim) {
+      return dim
+    }
+    const prom =
+      this.activeRequests.get(uri) ||
+      new Promise<Dim>(resolve => {
+        Image.getSize(
+          uri,
+          (width: number, height: number) => resolve({width, height}),
+          (err: any) => {
+            console.error('Failed to fetch image dimensions for', uri, err)
+            resolve({width: 0, height: 0})
+          },
+        )
+      })
+    this.activeRequests.set(uri, prom)
+    const res = await prom
+    this.activeRequests.delete(uri)
+    this.sizes.set(uri, res)
+    return res
+  }
+}