AudioPluginInterfaceCallback object is not transmitted to service if client and service are different apps

[atsushieno]

In AudioPluginInterface.aidl there is setCallback() method that passes a callback object to the AudioPluginInterface service.

Its corresponding service setCallback() implementation receives in_callback correctly if the client and service are the same app (e.g. aapinstrumentsample on its own PluginManagerActivity), but if they are different apps then in_callback is nullptr.

[atsushieno]

It is blocking notify_parameters_changed() dogfooding implementation in aap-juce (https://github.com/atsushieno/aap-core/issues/130).

[atsushieno]

I wondered if cross-process callbacks are not supported, but it seems to work at least on Java level. Locally confirmed that this example works (it mentions an example git repo): https://stackoverflow.com/questions/13948671/how-can-i-use-aidl-remote-service-to-deal-with-defferent-clients-concurrent-req/34217138#34217138

[atsushieno]

Other possibilities:

• NdkBinder does not support it yet
• it is too early to send callback until onServiceConnected() call is done (it is currently done within the call)

[atsushieno]

After more investigation, it turned out that NdkBinder lacks android.os.Binder equivalent, and it leads to missing Stub class in the generated code.

When we bind our interface e.g. MyService in .aidl, there will be IMyService.Stub class and IMyService.Stub.Proxy class if it were Java, along with IMyServiceDefault class. aidl(.exe) with -lang=ndk generates only IMyServiceDefault which only provides interface methods stubs.

android.os.Binder seems to do a lot of work and it is not easily ported to NdkBinder land (at least the full implementation equivalent would be hard task). We need to get around it.