BrowserExtension webRequest.onBeforeRequest return promise

问题

I have the following in a Chrome and FireFox extension:

function webListener(requestDetails) {
    var asyncCancel = new Promise((resolve, reject) => {
        resolve({ cancel : true });
    });
    return asyncCancel;
}

chrome.webRequest.onBeforeRequest.addListener(
    webListener, {
        urls: ["<all_urls>"],
        types: ["script"]
    }, ["blocking", "requestBody"]
);

The problem is that it doesn't cancel the request. I read Chrome docs and Firefox docs and for Firefox it says it's based on Chrome's API and can return a promise to handle the request asynchronously..

However, unless I make it synchronous, it fails to cancel (IE: If I return just {cancel : true} instead of the promise, it works).

Am I doing something wrong or does Chrome and Firefox only support synchronous request handling here?

回答1:

The Chrome documentation reads to me as though you must use a synchronous, blocking response to cancel a request, and that has been my experience also.

Emphasis mine :

If the optional opt_extraInfoSpec array contains the string 'blocking' (only allowed for specific events), the callback function is handled synchronously. That means that the request is blocked until the callback function returns. In this case, the callback can return a webRequest.BlockingResponse that determines the further life cycle of the request. Depending on the context, this response allows cancelling or redirecting a request (onBeforeRequest), cancelling a request or modifying headers (onBeforeSendHeaders, onHeadersReceived), and cancelling a request or providing authentication credentials (onAuthRequired).

My reading is that you can only influence the request lifetime(like cancelling it) if your event handler is synchronous.

Furthermore, the docs also don't mention anything about supporting a Promise instead of a BlockingResponse directly, so it seems that that is Firefox-specific functionality.

来源：https://stackoverflow.com/questions/47910732/browserextension-webrequest-onbeforerequest-return-promise