Retry if registerMethods hasn't been called yet

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Issue

The internal registerMethods call for __getTabData changes the behavior of the native sender because it adds the onMessage listener. This is not preventable.

The same behavior change also affected the custom sender which, instead of ignoring a context that didn't register any methods yet, it would receive the error "No METHOD registered here".

Messaging a target

Native behavior

• Documentation: How sendMessage responds #66

webext-messenger extension of behavior

• if the messenger isn't loaded, the sender retries
• 🆕 if the messenger is loaded but registerMethods isn't called, the sender retries
• if the messenger is loaded and registerMethods is called, but the specific METHOD isn't registered, the sender throws "No METHOD found"

Related

• Closes Warn if a message was received when no handlers have been registered at all #74
• Closes Provide way to explicitly mark receiver as ready to start handling messages #75

Future improvements

• Clarify what "No METHOD registered in the receiving end" means, possibly replacing the message with "The receiving end did not handle the METHOD request" or something like that.

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This line will make the sender retry when it receives this new error.

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This essentially fixes a regression caused by

• New named targets solution + add some notes/docs #53

😅

The Messenger however continues to work under the assumption that if a target receives a message and the specific METHOD isn't defined, it's an error. Changing this assumption might not be straightforward and is not needed at this stage.

[fregante]

I can reimplement this better actually and completely drop the private registerMethods call from the content scripts, for example.