Electron: webview frames and "did-frame-finish-load" event


I am working with a webview which load a website including several frames, only one get my interest.
I setup an event listener as:

webview.addEventListener(“did-frame-finish-load”, function (event) {
webview.send(“alert-something”,event) ;
And in my injected code I put the event processing function
ipcRenderer.on(“alert-something”, function (event, data) {

When I put a breakpoint on this function I can see the event but it only tell me mainFrame = false
So I’m really wondering how I can know which frame is the source of the event…
Any advice would be greatly received…