Added usePlugin hooks

Summary:
`usePlugin(pluginFactory)` returns the current plugin instance's api that was exposed by the plugin directory.

Passing `pluginFactory` is technically strictly not needed, but having the user pass it, we can make sure it is strongly typed

Reviewed By: nikoant

Differential Revision: D22286293

fbshipit-source-id: 4268b6849b8cd3d524103de7eadbd6c0a65c7a61

desktop/flipper-plugin/src/plugin/PluginContext.tsx

@@ -7,9 +7,28 @@
  * @format
  */
 
-import {createContext} from 'react';
-import {SandyPluginInstance} from './Plugin';
+import {createContext, useContext} from 'react';
+import {SandyPluginInstance, FlipperPluginFactory} from './Plugin';
 
 export const SandyPluginContext = createContext<
   SandyPluginInstance | undefined
 >(undefined);
+
+export function usePlugin<PluginFactory extends FlipperPluginFactory<any, any>>(
+  plugin: PluginFactory,
+): ReturnType<PluginFactory> {
+  const pluginInstance = useContext(SandyPluginContext);
+  if (!pluginInstance) {
+    throw new Error('Plugin context not available');
+  }
+  // In principle we don't *need* the plugin, but having it passed it makes sure the
+  // return of this function is strongly typed, without the user needing to create it's own
+  // context.
+  // But since we pass it, let's make sure the user did request the proper context
+  if (pluginInstance.definition.module.plugin !== plugin) {
+    throw new Error(
+      `Plugin context (${pluginInstance.definition.module.plugin}) didn't match the type of the requested plugin (${plugin})`,
+    );
+  }
+  return pluginInstance.instanceApi;
+}