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diff --git a/ext/fg/js/frame-offset-forwarder.js b/ext/fg/js/frame-offset-forwarder.js
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+/*
+ * Copyright (C) 2020 Alex Yatskov <alex@foosoft.net>
+ * Author: Alex Yatskov <alex@foosoft.net>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* global
+ * apiForward
+ */
+
+class FrameOffsetForwarder {
+ constructor() {
+ this._started = false;
+
+ this._forwardFrameOffset = (
+ window !== window.parent ?
+ this._forwardFrameOffsetParent.bind(this) :
+ this._forwardFrameOffsetOrigin.bind(this)
+ );
+
+ this._windowMessageHandlers = new Map([
+ ['getFrameOffset', ({offset, uniqueId}, e) => this._onGetFrameOffset(offset, uniqueId, e)]
+ ]);
+ }
+
+ start() {
+ if (this._started) { return; }
+ window.addEventListener('message', this.onMessage.bind(this), false);
+ this._started = true;
+ }
+
+ async getOffset() {
+ const uniqueId = yomichan.generateId(16);
+
+ const frameOffsetPromise = yomichan.getTemporaryListenerResult(
+ chrome.runtime.onMessage,
+ ({action, params}, {resolve}) => {
+ if (action === 'frameOffset' && isObject(params) && params.uniqueId === uniqueId) {
+ resolve(params);
+ }
+ },
+ 5000
+ );
+
+ window.parent.postMessage({
+ action: 'getFrameOffset',
+ params: {
+ uniqueId,
+ offset: [0, 0]
+ }
+ }, '*');
+
+ const {offset} = await frameOffsetPromise;
+ return offset;
+ }
+
+ onMessage(e) {
+ const {action, params} = e.data;
+ const handler = this._windowMessageHandlers.get(action);
+ if (typeof handler !== 'function') { return; }
+ handler(params, e);
+ }
+
+ _onGetFrameOffset(offset, uniqueId, e) {
+ let sourceFrame = null;
+ for (const frame of document.querySelectorAll('frame, iframe:not(.yomichan-float)')) {
+ if (frame.contentWindow !== e.source) { continue; }
+ sourceFrame = frame;
+ break;
+ }
+ if (sourceFrame === null) {
+ this._forwardFrameOffsetOrigin(null, uniqueId);
+ return;
+ }
+
+ const [forwardedX, forwardedY] = offset;
+ const {x, y} = sourceFrame.getBoundingClientRect();
+ offset = [forwardedX + x, forwardedY + y];
+
+ this._forwardFrameOffset(offset, uniqueId);
+ }
+
+ _forwardFrameOffsetParent(offset, uniqueId) {
+ window.parent.postMessage({action: 'getFrameOffset', params: {offset, uniqueId}}, '*');
+ }
+
+ _forwardFrameOffsetOrigin(offset, uniqueId) {
+ apiForward('frameOffset', {offset, uniqueId});
+ }
+}