background.js

const {
    REQUEST_TYPE_SYNC_EMITFULL,
    REQUEST_TYPE_SYNC_FULL,
    REQUEST_TYPE_SYNC_TO_BG,
    REQUEST_TYPE_SYNC_TO_TAB,
    canSendAction,
    handleSendMessageResponse,
    markActionAsSynchronised
} = require("./shared.js");

function createSyncMiddleware() {
    return () => next => action => {
        if (canSendAction(action)) {
            // mark as having been sync'd
            markActionAsSynchronised(action);
            // send the state update
            sendStateUpdate(action);
        }
        // continue with the next middleware
        next(action);
    };
}

function sendStateUpdate(action) {
    const fetchTabs = new Promise(resolve => chrome.tabs.query({}, tabs => resolve(tabs)));
    return fetchTabs.then(tabs => {
        tabs.forEach(tab => {
            chrome.tabs.sendMessage(
                tab.id,
                {
                    type: REQUEST_TYPE_SYNC_TO_TAB,
                    action
                },
                undefined,
                handleSendMessageResponse
            );
        });
        // handle popup
        chrome.runtime.sendMessage(
            {
                type: REQUEST_TYPE_SYNC_TO_TAB,
                action
            },
            undefined,
            handleSendMessageResponse
        );
    });
}

function syncStore(store) {
    const { dispatch, getState } = store;
    const handler = (request, sender, sendResponse) => {
        if (!request) {
            return;
        }
        if (request.type === REQUEST_TYPE_SYNC_TO_BG) {
            dispatch(request.action);
        } else if (request.type === REQUEST_TYPE_SYNC_EMITFULL) {
            sendResponse({
                type: REQUEST_TYPE_SYNC_FULL,
                state: getState()
            });
        }
    };
    chrome.runtime.onMessage.addListener(handler);
    return store;
}

module.exports = {
    createSyncMiddleware,
    syncStore
};