Add preview components

src/contexts/ClientContext.ts

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+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from "react";
+
+import { Client, discoverServer } from "cypher";
+import { prefixFetch } from "cypher/src/utils/fetch";
+
+type State = {
+    clientURL: string;
+    client: Client;
+}[];
+
+// Actions are a discriminated union.
+export enum ActionTypes {
+    AddClient = "ADD_CLIENT",
+    RemoveClient = "REMOVE_CLIENT",
+}
+
+export interface AddClient {
+    action: ActionTypes.AddClient;
+    clientURL: string;
+}
+
+export interface RemoveClient {
+    action: ActionTypes.RemoveClient;
+    clientURL: string;
+}
+
+export type Action = AddClient | RemoveClient;
+
+export const INITIAL_STATE: State = [];
+export const reducer = async (state: State, action: Action): Promise<State> => {
+    switch (action.action) {
+        case ActionTypes.AddClient:
+            return state.filter((x) => x.clientURL !== action.clientURL);
+
+        case ActionTypes.RemoveClient:
+            if (!state.filter((x) => x.clientURL === action.clientURL)) {
+                const resolvedURL = await discoverServer(action.clientURL);
+                state.push({
+                    clientURL: resolvedURL,
+                    client: prefixFetch(resolvedURL),
+                });
+            }
+    }
+    return state;
+};
+
+// The null is a hack to make the type checker happy
+// create context does not need an argument
+export default React.createContext<typeof reducer | null>(null);