RN: Cleanup ImageSource Flow Type

Summary:
Cleans up the documentation and types for `ImageSource`.

The only material changes here are:

-  `ImageSource`'s array variant will now be `$ReadOnlyArray` instead of `Array`.
- `ImageURISource.headers` is now an object of string properties.

Changelog:
[General][Changed] - Refined `ImageSource` Flow type for array-variant and headers.

Reviewed By: kacieb

Differential Revision: D23355812

fbshipit-source-id: c3407db037dfb1d3514a028d1a237eb76ee6fedd

Libraries/Image/ImageSource.js

@@ -4,17 +4,18 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @flow
+ * @flow strict
  * @format
  */
 
 'use strict';
 
-// This is to sync with ImageSourcePropTypes.js.
-// We explicitly don't want this to be strict so that we can pass in objects
-// that might have more keys. This also has to be inexact to support taking
-// instances of classes like FBIcon.
-// https://fburl.com/8lynhvtw
+/**
+ * Keep this in sync with `DeprecatedImageSourcePropType.js`.
+ *
+ * This type is intentinoally inexact in order to permit call sites that supply
+ * extra properties.
+ */
 export type ImageURISource = $ReadOnly<{
   /**
    * `uri` is a string representing the resource identifier for the image, which
@@ -23,27 +24,32 @@ export type ImageURISource = $ReadOnly<{
    * function).
    */
   uri?: ?string,
+
   /**
    * `bundle` is the iOS asset bundle which the image is included in. This
    * will default to [NSBundle mainBundle] if not set.
    * @platform ios
    */
   bundle?: ?string,
+
   /**
    * `method` is the HTTP Method to use. Defaults to GET if not specified.
    */
   method?: ?string,
+
   /**
    * `headers` is an object representing the HTTP headers to send along with the
    * request for a remote image.
    */
-  headers?: ?Object,
+  headers?: ?{[string]: string},
+
   /**
    * `body` is the HTTP body to send with the request. This must be a valid
    * UTF-8 string, and will be sent exactly as specified, with no
    * additional encoding (e.g. URL-escaping or base64) applied.
    */
   body?: ?string,
+
   /**
    * `cache` determines how the requests handles potentially cached
    * responses.
@@ -65,21 +71,24 @@ export type ImageURISource = $ReadOnly<{
    * @platform ios
    */
   cache?: ?('default' | 'reload' | 'force-cache' | 'only-if-cached'),
+
   /**
    * `width` and `height` can be specified if known at build time, in which case
    * these will be used to set the default `<Image/>` component dimensions.
    */
   width?: ?number,
   height?: ?number,
+
   /**
    * `scale` is used to indicate the scale factor of the image. Defaults to 1.0 if
    * unspecified, meaning that one image pixel equates to one display point / DIP.
    */
   scale?: ?number,
+
   ...
 }>;
 
-// We have to export any because of an issue in Flow with objects that come from Relay:
-// https://fburl.com/8ljo5tmr
-// https://fb.facebook.com/groups/flow/permalink/1824103160971624/
-export type ImageSource = ImageURISource | number | Array<ImageURISource>;
+export type ImageSource =
+  | number
+  | ImageURISource
+  | $ReadOnlyArray<ImageURISource>;