Define client and presenter for follow

src/content/client/FollowMasterClient.ts

@@ -0,0 +1,47 @@
+import * as messages from '../../shared/messages';
+import { Key } from '../../shared/utils/keys';
+
+export default interface FollowMasterClient {
+  startFollow(newTab: boolean, background: boolean): void;
+
+  responseHintCount(count: number): void;
+
+  sendKey(key: Key): void;
+
+  // eslint-disable-next-line semi
+}
+
+export class FollowMasterClientImpl implements FollowMasterClient {
+  private window: Window;
+
+  constructor(window: Window) {
+    this.window = window;
+  }
+
+  startFollow(newTab: boolean, background: boolean): void {
+    this.postMessage({
+      type: messages.FOLLOW_START,
+      newTab,
+      background,
+    });
+  }
+
+  responseHintCount(count: number): void {
+    this.postMessage({
+      type: messages.FOLLOW_RESPONSE_COUNT_TARGETS,
+      count,
+    });
+  }
+
+  sendKey(key: Key): void {
+    this.postMessage({
+      type: messages.FOLLOW_KEY_PRESS,
+      key: key.key,
+      ctrlKey: key.ctrlKey || false,
+    });
+  }
+
+  private postMessage(msg: messages.Message): void {
+    this.window.postMessage(JSON.stringify(msg), '*');
+  }
+}