Currently, Window Manager realizes one surface for one application.

Some application such like navigation wants to create 2 surface. One is map surface and one is User interface surface to receive user instruction.

So try to implement multi surface support.

The idea is that some services can provides service surfaces to applications and notify Window Manager of binding service surface to the application.

The image is below.

There are 2 applications in application side, one is application and one is service provider(the bottom left one).

Window Manager need to bind each surfaces created by the application and provider to control both surfaces when the state changes, so provider should notify Window Manager of attaching its surface to application. Then, Window Manager hold the information.

Application receives the surface alias from surface provider and request Window Manager to set render order.

This model realize that map service can provide other surface to other applications.