[RN] Prefetch remote avatar images 1/2

react/features/mobile/image-cache/functions.js

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+import { ImageCache } from 'react-native-img-cache';
+
+/**
+ * Notifies about the successful download of an <tt>Image</tt> source. The name
+ * is inspired by <tt>Image</tt>. The downloaded <tt>Image</tt> source is not
+ * available because (1) I do not know how to get it from {@link ImageCache} and
+ * (2) we do not need it bellow. The function was explicitly introduced to cut
+ * down on unnecessary <tt>ImageCache</tt> <tt>observer</tt> instances.
+ *
+ * @private
+ * @returns {void}
+ */
+function _onLoad() {
+    // ImageCache requires an observer; otherwise, we do not need it because we
+    // merely want to initiate the download and do not care what happens with it
+    // afterwards.
+}
+
+/**
+ * Initiates the retrieval of a specific <tt>Image</tt> source (if it has not
+ * been initiated already). Due to limitations of {@link ImageCache}, the source
+ * may have at most one <tt>uri</tt>. The name is inspired by <tt>Image</tt>.
+ *
+ * @param {Object} source - The <tt>Image</tt> source with preferably exactly
+ * one <tt>uri</tt>.
+ * @public
+ * @returns {void}
+ */
+export function prefetch(source) {
+    ImageCache.get().on(source, /* observer */ _onLoad, /* immutable */ true);
+}