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Issue in documentation in Emp API for LWC(Lightning Web Component))
While implementing emp API in LWC to subscribe the platform event I find out that there is small issue in sample code snipit given in documentation.
Issue is in handleSubscribe method in js below is the code given in documentation:
Issue is in handleSubscribe method in js below is the code given in documentation:
handleSubscribe() {
// Callback invoked whenever a new event message is received
const messageCallback = function(response) {
console.log('New message received : ', JSON.stringify(response));
// Response contains the payload of the new message received
};
// Invoke subscribe method of empApi. Pass reference to messageCallback
subscribe(this.channelName, -1, messageCallback).then(response => {
// Response contains the subscription information on successful subscribe call
console.log('Successfully subscribed to : ', JSON.stringify(response.channel));
this.subscription = response;
this.toggleSubscribeButton(true);
});
}
Correction is required in messageCallback method which should be as below:
const messageCallback = (response) => {
console.log('New message received : ', JSON.stringify(response));
this.payload = JSON.stringify(response);
console.log('this.payload: ' + this.payload);
// Response contains the payload of the new message received
};
We have to use arrow function as it does not have its own scope.
const messageCallback = (response) => { console.log('New message received : ', JSON.stringify(response)); this.payload = JSON.stringify(response); console.log('this.payload: ' + this.payload); // Response contains the payload of the new message received };All Answers
Bellow is the code
handleSubscription() { console.log("LOG"); const messageCallback = response => { console.log("New message received : ", JSON.stringify(response)); }; subscribe( "/event/MYNAMESPACE__MYCUSTOMEVENTNAME__e", -1, messageCallback ) .then(response => { console.log( "Successfully subscribed to : ", JSON.stringify(response.channel) ); }) .catch(error => { console.log(error); }); }When I click in a button that has onclick={handleSubscription} I only receive the console output "LOG".
One of the components is a Aura component that subscribes to a Platform Event 1. The Second component is a LWC that subscibes to a Platform Event 2. I removed the Aura from the page, then the button that subscribes to the Platform Event 2 worked wthout any changes.
https://github.com/salesforce/lwc/issues/1618
For now Im just not leaving both components on the same lightning page. I also did a test where I have two Aura components subscribing to two different Platform Events and they work perfectly like you said @Prabhjot Singh 27. The issue happens only when I have one Aura and one LWC.
Also, there is another weird behaviors happening. When I load a page which has an Aura component with empApi and then move to another lightning page that has a LWC using empApi, this page throws an exception. If I reload this lightning page the error is gone.
Scenario 1:
Place one Aura in a Lightning Page that subscribe to an Event A
Place one LWC in the same Lightning Page that subscribes to an Event B.
Result: The LWC subscribe will not work and it will also not thrown any exception if you put a .catch(error => { console.log(error); }) in the promisse.
Scenario 2:
Place one Aura in a Lightning Page A that subscribe to a Platform Event 1
Place one LWC in a Lightning Page B in the same Lightning APP that subscribe to a Platform Event 2
Go to Lightning Page A, wait the Aura to subscribe to the Platform Event 1
Click on the Lightning Page B and the error thrown will be the following
Reload the page with Ctrl + R and the error is gone and the LWC will subscribe to the Platform Event 2
Prabhjot Singh - You are a genious.. you helped complete my IOT project.. thank you
const messageCallback = (response) => { console.log('New message received : ', JSON.stringify(response)); this.payload = JSON.stringify(response); console.log('this.payload: ' + this.payload); // Response contains the payload of the new message received };@Prabhjot - I was hoping your solution would solve the issue but I get the same error as @allan31
Salesforce is already aware of it and it is trying to understand.
To bad SF does not correct their documentation...