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-# agl-compositor
-
-## Design and overall architecture
-
-The compositor is built on top of libweston and on libweston-desktop. The
-latter, among other things, is required as it provides the server side
-implementation of the XDG shell protocol which underlying toolkits
-(like qtwayland/chromium) make use to deliver desktop like functionality.
-The former is used to provide back-ends and rendering support, besides
-implementing the wayland protocol.
-
-xdg-shell is currently de facto protocol for handling all window management
-related functionality. In order to avoid polluting the wayland protocol
-namespace, the XDG extension, together with some other useful protocols, like
-the linux-dmabuf one, are all developed under the wayland-protocols umbrella.
-
-The whole purpose of wayland-protocols is to enhance the Wayland protocol with
-new functionality and bring new extensions entirely. Compositors are free to
-implement, modify, enhance, and add new extensions to wayland-protocols but
-they need to do so in consensus.
-
-Besides the core wayland protocol and extended functionality from
-wayland-protocols, a compositor can provide additional protocol extension
-specifications (obviously specific to that compositor). agl-compositor uses
-private extensions to deliver specific functionality or improve the current
-functionality to align with AGL demands. See [protocol extension](#protocol-extensions)
-about them.
-
-Traditionally, clients were entirely separated from the window manager, the
-desktop environment and the server. In wayland all these are conceptually under
-the same entity though they are implemented as different UNIX processes, or a
-different namespace with front and back-end APIs, exposed by different
-libraries. As such, the compositor and the shell driving the UI should be seen
-as one and the same, and in practice, this happens on traditional desktop
-environments.
-
-agl-compositor has logic in place to spawn/run the shell, but in case of AGL,
-the shell can be represented under different forms, as well as the fact that
-the process management has another layer baked in to handle MAC (Mandatory
-Access Control) labels and use a custom application framework
-management. These are all tightly integrated and therefore, the agl-compositor
-will not automatically start the shell client, although there's code to handle
-that.
-
-## Customizing and tailoring the compositor
-
-The compositor only adds basic functionality, and depending on use-cases, one
-might want to further enhance or tailor the compositor to fit their need. The
-compositor only contains the bare minimum, necessary bits to bring up the
-screen and make use of private extensions to integrate better with the existing
-AGL infrastructure. It is encouraged and desirable to modify the compositor if
-more customization is required.
-
-## Protocol extensions
-
-Compositors can define and implement custom extensions to further control
-application behaviour. For AGL, respectively, for agl-compositor, we have two
-private extensions defined. One targeted at defining surface roles commonly
-found desktop environments (like panels, and backgrounds), which acts as the
-shell client, and one targeted at regular application(s) that might require
-additional functionality.
-
-This document describes the overall functionality and some implementation
-details and potential corner-cases. Refer to the protocol XML specification for
-further and up-to-date documentation.
-
-### Surface roles
-
-Internally, agl-compositor assigns to regular XDG applications, a default,
-desktop role. Further roles like a background or a panel, are available to
-further customize layer and orientation of surfaces.
-
-These have **no** particular meaning, except that it hints the compositor where
-they should be stacked or where to position them. The background one occupies
-the most lower layer, the desktop ones on top and the panel surfaces the upper
-most layer.
-
-Additional roles have been added, in a different extension, to add further
-functionality with the control/security functions being transferred over to a
-policy framework in case that is necessary. See [policy api][#policy] for
-further details.
-
-### agl-shell
-
-Clients can make use of this protocol to define different kind of roles for
-different kind of surfaces. This defines panels and a background surface. It
-includes to ability to activate other applications, assuming that those are
-already running. Activation happens by using using the app_id, respectively
-using set_app_id request as defined by the XDG shell protocol. Established
-client-side implementation of the XDG shell protocol will have a function used
-to set it up, or it should provide or expose an API to do so.
-
-Clients will be **required** to set their application ids accordingly in order
-for the client shell to activate them.
-
-### agl-shell-desktop
-
-This extension is targeted at keeping some of the functionally already
-established in AGL a) to allow applications display/activate other
-surfaces/application window, and b) to set further roles, specially dialog
-pop-ups and split-type of surfaces.
-
-Clients can make use of this protocol to set further roles, like independently
-positioned pop-up dialog windows, split type of surfaces or fullscreen ones.
-Additional roles can be added by extending the protocol. These roles serve as
-hints for the compositor and should be used before the actual surface creation
-takes place such that the compositor can take the necessary steps to ensure
-those requirements.
-
-#### Additional surface roles
-
-Like mentioned earlier, the compositor is already making use of some (internal)
-roles, and with this extension we add some further roles. These are: split
-(there's vertical and a horizontal one), fullscreen, and pop-up one. Internally
-these are encoded with different values such that there's a translation needed,
-between the protocol values and the internal values.
-
-Besides the roles, additional data can to be passed, but only relevant
-depending on the role.
-
-#### Receiving application state events from (other) applications
-
-agl-shell-desktop exposes two events which client can install handlers for, one
-that signals when regular XDG application have been created, and one that
-signals state changes (active/hidden) as well as destroyed/no longer present
-surfaces. These events can be useful to add additional functionality if
-needed.
-
-### Activating (other) applications
-
-Both agl-shell and agl-shell-desktop have requests to activate other
-application based on their XDG shell app_id. In case the application is
-present/running it it will attempt to make the surface backing that application
-the current activate one, with each output having independently active
-surfaces.
-
-### Explicit output
-
-The activation and setting surface roles requires passing an output
-(wl_output). The output is the wayland interface representation of an output
-and is **mandatory**. Clients can retrieve it (the output) if they wish to
-place the surface on other outputs by using the toolkits exposing wayland
-objects. A human-like representation is provided by binding directly, by using
-other extension implemented by the client (i.e., xdg_output is the one
-recommended) or by using abstraction implementation written on top.
-
-### Protocol design, future work and potential improvements
-
-Both protocols assume immediate, synchronous behaviour and to some extent lack
-some error handling functionality.
-
-Role assignment to surfaces could probably be improved with an additional
-interface with can add further data, if that roles assumes that to be true.
-
-There seems to be some overlapping functionality with respect to activating
-applications, so a potential improvement would be that the agl-shell protocol
-only contains the ability to set shell related surface roles, and
-agl-shell-desktop to be used when needing to activate applications.
-For the client shell this means that it will need to bind to both of the
-extensions, one that set-ups the background and panel roles, with the other
-needed to activate applications.
-
-## Policy
-
-The compositor contains an API useful for defining policy rules. It contains
-the bare minimum and installs, by default, an allow-all kind of engine.
-
-Users wanting to create their own policy engine should create a specialized
-version and use `struct ivi_policy_api` where they can install their own
-callbacks.
-
-The default policy found in src/policy-default.c should more than sufficient to
-get started. Users can either re-puporse the default policy or create a new one
-entirely different, based on their needs.
-
-### Hooks
-
-These are hooks for allowing the creation, committing and activation of surfaces
-(`ivi_policy_api::surface_create()`, `ivi_policy_api::surface_commited()`,
- `ivi_policy_api::surface_activate()`).
-
-Another hook, `ivi_policy_api::policy_rule_allow_to_add()` can be used to
-control if policy rules (the next type) can be added or not. Finally, we have
-`ivi_policy_api::policy_rule_try_event()` which is executed for each policy
-rules currently added, by using the policy API `ivi_policy_add()`.
-
-Users can customize the hooks by using some sort of database to retrieve
-the application name to compare against, or incorporate some kind of policy
-rule engine.
-
-### Policy rules
-
-Policy (injection) rules can be added using the policy API framework. The
-protocol allows to define policy rules that should be executed by using the
-`ivi_policy_api::policy_rule_try_event()` callback. These are particularly useful
-when handling state changes. The framework API allows adding new states and
-events and the default implementation has code for handling events like showing
-or hiding the application specified in the policy rule.
-
-#### Default events and states
-
-By default the when creating the policy framework it will add the 'show', and
-'hide' events and the 'start', 'stop' and 'reverse' states. An special type,
-assigned by default is 'invalid'.
-
-#### State changes
-
-A state change has to be propaged from to the compositor, by using
-`ivi_policy_state_change()`, to signal the compositor the (state) change itself,
-in order to apply the policy rules, and implicitly to call the event
-handler `ivi_policy_api::policy_rule_try_event()`.