3.2 KiB
Strucutral Overview
Foundation Components
There is a group os basic objects used widely throughout the codebase, which are useful building blocks in all major simulation elements.
Properties
Properties form a hierarchy of named, typed values. The hierarchy can be extended and modified at runtime by aircraft, simulation components and external protocols.
Listeners
Listeners are a callback interface SGPropertyChangeListener which can implemented
to receive notification when a property is modified, or its children added/removed.
Most commonly listeners are used to look for a user input or simulation event
changing some value (for example, a switch being turned from on to off), but it's
possible to monitor an entire sub-tree of properties.
It's important than listeners do not block or perform excessive amounts of work, since
the code modifying the property may not expect this. If non-trivial work needs to be
performed in response to a property changing, the listener should set a dirty flag
and a subsystem (etc) should check this in its update() method.
Tied properties
Tied properties allow a property to be defined by a C++ memory location (such as a class member variable) or by callback functions implementation the get/set operations. This is very convenient for the implementating code. However, by default it has the problematic side-effect that listeners do not correctly, since they have no way to be notified thst the underlying value (or result of invoking the getter) has changed.
It is therefore critical, when using tied properties, to manually invoke fireValueChanged
on dependent proeprties, when updating the internal state producing them. Care should be
taken to only fire the change, when the result of the getters would actually change; it is
not acceptable to simply call fireValueChanged each iteration, since any connected
listener will be run at the simulation rate.
Conditions
Conditions are predicates which can be evaluated on demand. They can contain logical operations such as 'and' and 'or', and comparisoms: for example checking that a property is equal to a constant value, or that a property is less than another property.
Conditions are widely used to drive other state from properties; for example enabling or disabling user-interface in a dialog, or activating an autopilot element.
Bindings
Bindings are a command invocation with associated arguments. In the simplest case
this might be invoking the property-set command with an argument of 42. At its
most complex, the command might invoke a Nasal script, supplied as part of the
binding, and substituing a runtime value into the Nasal logic.
Bindings are typically used in response to user actions, such as a menu item, key press or interacting with a cockpit control, to update some part of the simulator state.
Structural Components
Most code within FlightGear is organized into subsystems. Subsystems are isolated from each other; a well designed subsytem can be added, remove or restarting independently. Many subystems do not full conform to these goals, especially regarding runtime adding and removal, but an increasing number do support it.
=== Subsystems ===
=== Commands ===