Window Server Client-Side Library Overview
The Window Server keeps track of window sizes, positions, visibility and validity (keeping window content up to date). It also receives and distributes user input in the form of key presses and pointer events.
Note that much of the Window Server API is low-level and is encapsulated in the UI Control Framework API.
Architecture
The Window Server is used by all applications that have a user interface. The Window Server controls access by many client applications, to the machine’s screen, keyboard and pointer. The primary user input for these applications comes from the keyboard and pointer, and their primary user-visible output is to the screen. These input and output devices are shared between all applications on the system. Each client thread opens a session to the server, and issues requests for service.
The Window Server thread runs at a higher priority than any application; only the kernel runs at a higher priority. Therefore, all applications' requests for screen updates, and all handling of machine events such as pointers and keys, are handled at higher priority than any individual application.
Each client application runs in its own thread. The Window Server presents an interface to client applications such that they can run without direct interaction with the other applications on the machine. Drawing is clipped to the visible area of the application’s windows. Pointer events are only received if they are related to the application’s windows. Similarly, keyboard events are only given to an application whose window group has focus or to one that has captured them. A client application may ignore the majority of events relating to other applications. It won't even be told about most of them.
Each client application communicates with the Window Server using a Window Server session, or other object created from the session. The application waits to receive events by setting up one or more active objects. Events include user input and requests that windows be redrawn. Applications may create systems of windows and draw to them.
Window server client-side API
The following diagram shows the key classes in the Window Server client-side library. Below the diagram we provide a summary of the key concepts.
Figure: Window Server client-side API classes
| Concept | Description |
|---|---|
Session |
A Window Server session allows an application to control
and interrogate its windows, the events it wishes to receive, and
all other window groups connected to the Window Server. A session
is encapsulated by |
Events |
Applications function by waiting for events and handling
them. Common events are user input, and requests that windows be redrawn.
The Window Server session is used to request and obtain events. Most
events (such as key and point events) are encapsulated in A redraw event tells the application
what screen area needs redrawing. It is encapsulated in |
Window group |
A window group is special non-displayable type of window,
which can be considered as the root window of an application. Keyboard
events and focus are associated with it, and it can have a name, used
for such things as lists of running applications. A window group is
provided by |
Drawable windows |
Drawable windows allow applications to draw to the screen.
They have operations to control size, position, visibility, scrolling,
z-order, and parent/child relationships. In a standard drawable window,
areas that become invalid (e.g. when an overlaying window is removed)
must be redrawn by the client application. It is provided by These window types are derived from a sequence
of base classes, |
Graphics |
Applications draw to drawable windows through a windows graphics context provided by a windows graphics device. These implement the abstract graphics context and graphics device interfaces respectively, as defined in the Graphics API. The windows graphics context
is provided by |
Transparent windows |
Transparent windows enable you to display semi-transparent
bitmaps on a window. The window must be an |
Surfaces |
Applications (such as games and video) that use a rendering
API that can potentially benefit from hardware acceleration (depending
on hardware) can render directly to graphics surfaces. A surface
is a hardware independent buffer for holding an image or part of a
scene. Surfaces are identified using a 128 bit surface ID in a The Window Server delegates the composition of surfaces to a composition engine which has an adaptation part that enables device creators to take advantage of graphics processing hardware if it is available. However, this is largely transparent to application developers. See External Surfaces for more information. |
Direct Screen Access (DSA) |
DSA enables applications that require high frame rates (such as video and games) to bypass the Window Server and write to the screen directly. This avoids client-server communication and as a result is faster. However, some interaction with the Window Server is needed to prevent the application from drawing over other application's data. Support for direct screen access (DSA) is maintained for backward compatibility reasons, although Symbian recommends the use of external surfaces in preference to DSA. However, whereas on some earlier devices, applications might work without fully conforming to the rules of DSA, these rules are now necessarily enforced. See DSA Migration Guide for information about the guidelines applications must follow in order to be able to run correctly on a ScreenPlay device. |