The following FIDL attributes are discussed:
An attribute preceeds a FIDL element, for example:
[Layout = "Simple"] struct MyStruct { ...
It's used to either modify the characteristics of the element, or provide documentation.
Note that the attribute applies only to the next element, not all subsequent ones. Elements after the current one revert to having no attributes.
Causes the service's name to be made available for lookup. A service with a [Discoverable] attribute can be found at run-time. That is to say, you can “request” this service, and zircon will locate it and provide access to it.
In FIDL, comments can start with two (“//”) or three slashes (“///”), or they can be embodied within a [Doc] attribute. The two-slash variant does not propagate the comments to the generated target, whereas both the three-slash and [Doc] variants do.
That is:
/// Foo struct MyFooStruct { ...
and
[Doc = "Foo"] struct MyFooStruct { ...
have the same effect — the text of the comment is emitted into the generated code, in a manner compatible with the syntax of the target language.
This marks internal libraries, such as library zx. It should be used only by Fuchsia developers.
This attribute currently has one valid value, Simple, and is meaningful only on protocols.
It's used to indicate that all arguments and returns must contain objects that are of a fixed size. The arguments and returns themselves, however, can be dynamically sized string or vector of primitives.
To clarify with an example, the following is valid:
[Layout = "Simple"] protocol MyProtocol { DynamicCountOfFixedArguments(bytes:1024 inputs); };
Here, the argument is a dynamically sized vector of unsigned 8-bit integers called inputs, with a maximum bound of 1024 elements.
The benefit of [Layout = "Simple"] is that the data can be directly mapped without having to be copied and assembled.
