Add RPC support for composite components

[heruan]

Fixes #4198

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This change is 

[Legioth]

Review status: 1 unresolved discussion, 0 of 1 LGTMs obtained

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flow-server/src/main/java/com/vaadin/flow/server/communication/rpc/PublishedServerEventHandlerRpcHandler.java, line 139 at r1 (raw file):

        } else if (methods.size() == 1) {
            invokeMethod(instance, methods.get(0), args);
        } else if (Composite.class.equals(clazz)) {

This assumes there's only one level of composites. It's also possible (but not very common) that a composite is created out of another composite.

I'm also not sure if we should also support @ClientCallable methods on the composite class itself, and in that case also choose what to do if both the composite and the content defines a method method with the same name.

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Comments from Reviewable

[heruan]

Review status: 1 unresolved discussion, 0 of 1 LGTMs obtained

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flow-server/src/main/java/com/vaadin/flow/server/communication/rpc/PublishedServerEventHandlerRpcHandler.java, line 139 at r1 (raw file):

Previously, Legioth (Leif Åstrand) wrote…

This assumes there's only one level of composites. It's also possible (but not very common) that a composite is created out of another composite.

I'm also not sure if we should also support @ClientCallable methods on the composite class itself, and in that case also choose what to do if both the composite and the content defines a method method with the same name.

My intention here is to let the handler check if there's a method on the Composite first, so if the developer is importing its own client code there's still a chance to catch the call. I didn't consider multiple level of Composites though, I'll try to figure something out.

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Comments from Reviewable

[heruan]

Now this supports composites of composites, with this flow:

1. The instance class has the method: invoke that.
2. The instance is a Composite, try to invoke on the content.
3. If it fails, step up in the hierarchy.

[heruan]

@Legioth What do you think about adding a specific RpcMethodNotFoundException to be thrown/caught if the method is not found? Catching the IllegalStateException feels a bit risky to me there, as it might originate from other errors.

[Legioth]

Using any exception for control flow sounds wrong to me. What about splitting out a separate method that finds a method to invoke (potentially recursively)?

[heruan]

Now a new method looks for the target method in the class hierarchy; if not found and instance is a Composite it starts over with the composite content.

[heruan]

Do you feel this needs other improvements, or tests?

[Legioth]

Seems fine to me, but won't merge this right away to reduce the risk of further regressions during the release candidate period.

[denis-anisimov]

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Review status: all discussions resolved, 0 of 1 LGTMs obtained, and 1 stale

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Comments from Reviewable