Injected script to use Promise-based async calls with WebKit user script message handlers

apicall.js

// SPDX-License-Identifier: MIT

(function (_console, _window, _Map, _Promise) {
	"use strict";

	let currentTxnId = 0;
	const txnMap = new _Map();
	const warning = _console.warn;

	function makeTxnId() {
		while (txnMap.has(currentTxnId))
			currentTxnId++;
		return currentTxnId++;
	}

	function finishAsyncApiCall(txnId, how, value) {
		let entry = txnMap.get(txnId);
		if (entry === undefined) {
			warning("attempted to " + how + " unknown promise " + txnId);
			return false;
		} else {
			entry[how](value);
			txnMap.delete(txnId);
			return true;
		}
	}

	function asyncApiCall(name, args) {
		const message = {
			txnId: makeTxnId(),
			args: args,
		};

		return new _Promise((resolve, reject) => {
			//
			// Store the resolve/reject callbacks indexed by the transaction identifier;
			// so later the UIProcess can call window.__finishAsyncApiCall() passing the
			// identifier back, which allows finding back which functions to call and
			// settle the promise.
			//
			txnMap.set(message.txnId, {
				resolve: resolve,
				reject: reject,
				message: message,
			});
			window.webkit.messageHandlers[name].postMessage(message);
		});
	}

	_window.__finishAsyncApiCall = finishAsyncApiCall;
	_window.asyncApiCall = asyncApiCall;
})(console, window, Map, Promise);