An active object framework is a

callback Callback may refer to: * Callback (comedy), a joke which refers to one previously told * Callback (computer programming), executable code that is passed as a parameter to other code * Callback (telecommunications), the telecommunications event tha ...

-based form of multitasking for computer systems. Specifically, it is a form of

cooperative multitasking Cooperative multitasking, also known as non-preemptive multitasking, is a style of computer multitasking in which the operating system never initiates a context switch from a running process to another process. Instead, in order to run multiple ...

and is an important feature of the

Symbian Symbian is a discontinued mobile operating system A mobile operating system is an operating system for mobile phones, tablets, smartwatches, smartglasses, or other non-laptop personal mobile computing devices. While computers such as typic ...

operating system. Within the framework, active

objects Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...

may make requests of

asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal with ...

services (e.g. sending an

SMS Short Message/Messaging Service, commonly abbreviated as SMS, is a text messaging service component of most telephone, Internet and mobile device systems. It uses standardized communication protocols that let mobile devices exchange short text ...

message). When an asynchronous request is made, control is returned to the calling object immediately (i.e. without waiting for the call to complete). The caller may choose to do other things before it returns control back to the operating system, which typically schedules other tasks or puts the machine to sleep. When it makes the request, the calling object includes a reference to itself. When the asynchronous task completes, the

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...

identifies the thread containing the requesting active object, and wakes it up. An "active scheduler" in the thread identifies the object that made the request, and passes control back to that object. The implementation of active objects in

is based around each thread having a "request

semaphore Semaphore (; ) is the use of an apparatus to create a visual signal transmitted over distance. A semaphore can be performed with devices including: fire, lights, flags, sunlight, and moving arms. Semaphores can be used for telegraphy when arra ...

". This is incremented when a thread makes an asynchronous request, and decremented when the request is completed. When there are no outstanding requests, the thread is put to sleep. In practice there may be many active objects in a thread, each doing its own task. They can interact by requesting things of each other, and of active objects in other threads. They may even request things of themselves. This is an implementation of a very old idea that was developed to handle software interruptions in the 70s. The operating system was acting as the first object and the peripheral as the second one.

External links

developer.symbian.org
{{operating-system-stub Symbian OS