How the Window Manager works


Typically, both the application that owns/creates a window and the window manager that controls that window need to access it jointly. The application needs to display its content into the main part of the window, and the window manager needs information about the location and depth ordering of all windows to render them.

To share a window between an application and the window manager, the application holds a strong reference (Arc) to the window, while the window manager holds a weak reference (Weak) to that same window. This allows the window manager to control an manage a window without conceptually owning it.

We use a Mutex to wrap each window to allow the application task and window manager task to safely access it jointly. However, Mutex introduces the possibility of deadlock: when an application wants to access its window, it must acquire the Mutex lock, operate on the window, and then release the lock. If the application doesn't release the lock on its window, the window manager will be forced to block until the lock is released, preventing it from performing typical operations like switching between windows, delivering events, or deleting windows.

To solve this problem, we define two structures: Window and WindowInner. WindowInner only contains the information required by the window manager. The window manager holds a list of references to WindowInner objects, while only the application owns the outer Window object (which itself does contain a reference to the underlying WM-owned WindowInner object. The Window struct also contains other application-relevant states that describe the window.

The WindowInner structure

The window_inner crate defines a WindowInner structure. It has states and methods of displaying the window on the screen.

A WindowInner has a framebuffer to which it can display the content of the window. The framebuffer takes a type parameter of pixels it consists of. When the window is rendered to the screen, a compositor may composite every pixel with different principles according to the type. Currently, we have implemented a normal RGB pixel and a pixel of an alpha channel.

Both an application's window and the window manager has a reference to the same WindowInner object. The application can configure and draw in the framebuffer and the manager can display and composite the window with others.

This structure also has an event producer. The window manager gets events from I/O devices such as keyboards and push them to the corresponding producer.


A Window object represents a window and is owned by an application. It contains its profile, a title, a consumer and a list of displayables. The consumer can get events pushed to the producer in its profile by the manager.

A Window provides methods to display the displayables in it and render itself to the screen. The window manager is responsible for compositing it with other windows through a framebuffer compositor.


The displayable crate defines a Displayable trait. A Displayable is an item which can display itself onto a framebuffer. It usually consists of basic graphs and acts as a component of a window such as a button or a text box. Currently, we have implemented a TextDisplay which is a block of text. In the future, we will implement other kinds of displayables.

An application can own multiple displayables and display any type of Displayable in its window.

The WindowManager

The window_manager crate defines a WindowManager structure. This structure consists of the profiles of an active window, a list of shown windows and a list of hidden windows. The hidden ones are totally overlapped by others. The structure implements basic methods to manipulate the list such as adding or deleting a window.

The WindowManager structure contains a bottom framebuffer which represents the background image and a final framebuffer of a floating window border and a mouse arrow. In refreshing an area, it renders the framebuffers in order background -> hidden list -> shown list -> active -> top. It provides several methods to update a rectangle area or several pixels for better performance.

The structure defines a loop for generic events, a loop for keyboard events and a loop for mouse events. Theseus will initialize them as tasks to handle inputs. The window manager structure provides methods to operate on the window list as reactions to these inputs. It can move a window when we drag it with mouse or pass other events to the active window. The owner application of the active window can handle these events.

The window_manager crate owns a WINDOW_MANAGER instance which contains all the existing windows. It invokes the methods of WindowManager to manage these windows.