Error when canceling a service subscription from the service side

Web App Development
1

I see in the webs-service reference that the message object has a cancel method to notify a subscribed call that no more responses will be sent (see link). When I call this method, I see the following error:

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So, here a few questions:

  • If I want to keep only one active subscription to a service, is it necessary to cancel the previous one?
  • Can the subscriptions be called from the service side instead of the Webapp side?
  • Is this the correct way to cancel subscriptions?

To simplify the understanding of the problem, I created a modified version of the HelloWorldService that consistently reproduces the issue.

  • Webapp code: The only change is the addition of the subscribe: true on the service request.
function displayReponse() {
  var value = document.getElementById("input").value;
  webOS.service.request("luna://com.sample.helloworld.service/", {
    method: "hello",
    subscribe: true,
    parameters: { name: value },
    onFailure: showFailure,
    onSuccess: showSuccess,
  });
}

function showSuccess(res) {
  document.getElementById("result1").innerHTML = "Service Responded!";
  document.getElementById("result2").innerHTML = res.data;
}

function showFailure(err) {
  document.getElementById("result1").innerHTML = "Failed!";
  document.getElementById("result2").innerHTML =
    "(" + err.errorCode + ") " + err.errorText;
  console.log(err);
}
  • Service code: I implemented a timer here that sends responses to the subscribed message every second. If a new call to the method comes in, it attempts to replace the subscription causing the error.
var pkgInfo = require("./package.json");
var Service = require("webos-service");
var service = new Service(pkgInfo.name);

var name = "World";
var subscription = null;
var interval = null;
var count = 0;

var hello = service.register("hello");

hello.on("request", function (message) {
  console.log("In hello callback");
  if (message.payload && message.payload.name) {
    name = message.payload.name;
  }

  // First, cancel the previous subscription and clear the interval
  if (subscription) {
    subscription.cancel();  // CRASH!!! This call causes the error!!
    if (interval) {
      clearInterval(interval);
    }
  }

  // Now, save the new subscription
  subscription = message;

  // Create a new interval to send the message every second
  interval = setInterval(function () {
    subscription.respond({
      returnValue: true,
      data: "Hello, " + name + "! " + count,
    });
    count++;
  }, 1000);
});

hello.on("cancel", function (message) {
  console.log("In cancel callback");
  subscription = null;
  if (interval) {
    clearInterval(interval);
  }
});

IMPORTANT NOTES:

  • Both, the app and the service seems to keep working despite of the error.
  • The error can be reproduced on simulator. On real TV I don’t see the error on the screen.

Thanks in advance.

2

I think that I found the problem. Although the documentation fo the cancel method on the message object defines the syntax as:

message.cancel();

Checking the example on the example in the nodejs-module-webos-service repo, I see that the cancel method receives an object. Note: Yes, I know WebOS OSE is not the same as WebOS TV, but after testing, te error has gone. Also, I see in the message.cancel implementation here that the function receives an argument expected to be an object.
Now, the question is if there is a way to replace a subscription of the same APP client. Based on tests (need to confirm checking the luna service code) I see that the correct way to do it is:

  1. Respond to the new subscription message to allow complete the subscription.
  2. Call message.cancel on the first subscription to notify that no more messages will be sent to that one.
  3. Save the new subscription and start sending messages to the new one.

Is this approach correct?