|as_promise| converts a FIDL asynchronous call in the new C++ bindings

into a promise. Example usage:

// Let's say an async FIDL call originally uses a callback.

fidl::Client

<MyProtocol

> client(...);

client->Foo({ ... }).Then([] (fidl::Result

<MyProtocol

::Foo>

&

result) {

assert(result.is_ok());

});

// It can be turned into a promise by wrapping the call with |as_promise|

// as opposed to attaching a callback via |Then|:

auto p1 = fidl_fpromise::as_promise(client->Foo({ ... }));

// And used like any other regular promise:

auto p2 = p1.then([] (auto

&

result) {

assert(result.is_ok());

});

some_executor.schedule_task(std::move(p2));

The signature of the resulting promise is akin to

fpromise::promise

<SuccessType

, ErrorType>

where |SuccessType| is the payload type for when the FIDL call succeeds, or

|void| if the FIDL call has an empty/zero-argument payload.

where |ErrorType| is |fidl::Error| if the FIDL call does not use application

errors, and |fidl::ErrorsIn

<MyProtocol

::FidlMethod>| otherwise. |MyProtocol|

and |FidlMethod| are all placeholders to be replaced by the actual protocol

and method names.