FR: Support for overriding Firebase Cloud Messaging Endpoint

[sahejsingh]

Describe your environment

• Firebase Product: firebase cloud messaging (fcm) service

GO Service trying to send message to FCM.

Step 3: Describe the problem

Unable to do local integration testing with FCM.

Steps to reproduce:

• Integrate with FCM Go Library
• use messaging.Send() to send the message

At this point I would like to verify if the message has been received properly and I am handling the response properly as well.

As there is no fcm emulator available yet, I wrote a simple HTTP server that can handle the Send API. Now to make this work in local environment, I want to override the messagingEndpoint (https://github.com/firebase/firebase-admin-go/blob/master/messaging/messaging.go#L947) by using the https://godoc.org/google.golang.org/api/option#WithEndpoint option.

Relevant Code:

Here is the proposed fix:

Introduced ep below in the NewClient method defined at https://github.com/firebase/firebase-admin-go/blob/master/messaging/messaging.go#L911

// NewClient creates a new instance of the Firebase Cloud Messaging Client.
//
// This function can only be invoked from within the SDK. Client applications should access the
// the messaging service through firebase.App.
func NewClient(ctx context.Context, c *internal.MessagingConfig) (*Client, error) {
	if c.ProjectID == "" {
		return nil, errors.New("project ID is required to access Firebase Cloud Messaging client")
	}

	hc, ep, err := transport.NewHTTPClient(ctx, c.Opts...)
	if err != nil {
		return nil, err
	}

	if ep == "" {
		ep = messagingEndpoint
	}

	return &Client{
		fcmClient: newFCMClient(hc, ep, c),
		iidClient: newIIDClient(hc),
	}, nil
}

Clients of this code then call it like so

var opts []option.ClientOption
        
if emulate {
   opts = append(opts, option.WithEndpoint("http://localhost:3000/v1"))
}

app, err := firebase.NewApp(ctx, conf, opts...)

Question

Is this approach ok?

If yes, I am happy to raise a PR to make this option work.

[sahejsingh]

similar to #298

[hiranya911]

Already implemented in #373

Although I'd generally recommend a more blackbox testing approach for SDK users, leveraging Go interfaces to swap out the SDK when needed.

type FCMSender interface {
  Send(ctx context.Context, msg *messaging.Message) (string, error)
}