Interfaces, requests, and events

The Wayland protocol works by issuing requests and events that act on objects. Each object has an interface which defines what requests and events are possible, and the signature of each. Let's consider an example interface: wl_surface.

Requests

A surface is a box of pixels that can be displayed on-screen. It's one of the primitives we build things like application windows out of. One of its requests is "damage", which the client uses to indicate that some part of the surface has changed and needs to be redrawn. Here's an annotated example of a "damage" message on the wire (in hexadecimal):

0000000A    Object ID (10)
00180002    Message length (24) and request opcode (2)
00000000    X coordinate (int): 0
00000000    Y coordinate (int): 0
00000100    Width        (int): 256
00000100    Height       (int): 256

This is a snippet of a session - the surface was allocated earlier and assigned an ID of 10. When the server receives this message, it looks up the object with ID 10 and finds that it's a wl_surface instance. Knowing this, it looks up the signature for the request with opcode 2. It then knows to expect four integers as the arguments, and it can decode the message and dispatch it for processing internally.

Events

Requests are sent from the client to the server. The server can also send messages back - events. One event that the server can send regarding a wl_surface is "enter", which it sends when that surface is being displayed on a specific output (the client might respond to this, for example, by adjusting its scale factor for a HiDPI display). Here's an example of such a message:

0000000A    Object ID (10)
000B0000    Message length (12) and event opcode (0)
00000005    Output (object ID): 5

This message references another object, by its ID: the wl_output object which the surface is being shown on. The client receives this and dances to a similar tune as the server did. It looks up object 10, associates it with the wl_surface interface, and looks up the signature of the event corresponding to opcode 0. It decodes the rest of the message accordingly (looking up the wl_output with ID 5 as well), then dispatches it for processing internally.

Interfaces

The interfaces which define the list of requests and events, the opcodes associated with each, and the signatures with which you can decode the messages - are agreed upon in advance. I'm sure you're dying to know how - simply turn the page to end the suspense.