chrome.runtime.onMessage.addListener(
    function (request, sender, sendResponse) {
        if (!request.action) return;
        // console.log("Recieved request:",request);
        switch (request.action) {
            case ACTION_EXTRACT:
                let data = extract(request.itemsSelector, request.fieldSelectors);
                if (sendResponse) sendResponse(data);
                break;
            case ACTION_GOTO_URL:
                window.location.replace(request.url);
                if (sendResponse) sendResponse(request.url);
                break;
            case ACTION_REPORT_IN:
                if (sendResponse) sendResponse(request.action);
                break;
            case ACTION_QUERY_URL:
                if (sendResponse) sendResponse(window.location.href);
                break;
            default:
                break;
        }
    }
);

function extract(itemsSelector, fieldSelectors) {
    // since some elements may be loaded asynchronously.
    // if one field is never found, we should return undefined,
    // so that senders can detect to retry until elements loaded.
    // If user writes wrong selectors, the task retries infinitely.
    let fieldFound = {};
    let items = Array.from(document.querySelectorAll(itemsSelector));
    // items may not loaded yet, tell the sender to retry.
    if (!items.length) return MSG_ELEMENT_NOT_FOUND;
    let results = items.map(
        item => {
            return fieldSelectors.map(
                selector => {
                    let [cls, attr] = selector.split('@').slice(0, 2);
                    let fieldVals = Array.from(item.querySelectorAll(cls));
                    if (!fieldVals.length) {
                        return;
                    }
                    fieldFound[selector] = true;
                    return fieldVals.map(find => attr ? find[attr] : find.textContent.trim()).join('\n')
                }
            )
        }
    );
    // if it exists a field, which is not found in any row, the sender should retry.
    let shouldWait = fieldSelectors.reduce((p, c) => p || !fieldFound[c], false);
    return shouldWait ? MSG_ELEMENT_NOT_FOUND : results
}