Service worker, postMessage to the client, ClassCastException, ServiceWorkerClient #180
Description
I was trying to post a message to the WindowClient from service worker.
The code is a GWT generated JavaScript accessed from service-worker.js (plain JavaScript) through JsInterop.
Main thread JavaScript module posts the message to the service worker, service worker processes the message and sends the message back to the calling client.
Below is the code that demonstrates approaches attempted.
...
private void postMessage(String payload, String clientId) {
ServiceWorkerGlobalScope swgs = (ServiceWorkerGlobalScope) DomGlobal.self;
ServiceWorkerClients clients = swgs.getClients();
// approach 1
// the exception is ClassCastException thrown in get(...) and the client is not received into then
clients.get(clientId).then(client -> {
if (client != null) {
client.postMessage(payload);
}
return null;
});
// approach 2
clients.matchAll().then(all -> {
for (int i = 0; i < all.length; i++) {
Object windowClient = all.at(i); // windowClient is actually an instance of WindowClient as seen in the debugger
if (windowClient instanceof ServiceWorkerClient) {
// it never gets here
}
// force cast gives ClassCastException
ServiceWorkerClient swc = Js.cast(windowClient);
swc.postMessage(payload);
}
return null;
});
...As mentioned in the code snippet above, what is actually received in the approach 2 is WindowClient .
I was able to workaround the issue by passing an array of windowClients ( all in the snippet above) through JsInterop and posting the message from JavaScript and it worked.
@JsType(isNative = true)
public class UtilsJs {
public static native void postMessage(Object windowClients, String clientId, String payload);
}The browser was Chrome.