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| 3 | magnus | 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| 2 | <html><head> |
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| 3 | <title>liboop: How?</title> |
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| 4 | <link rel="stylesheet" type="text/css" href="style.css"> |
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| 5 | </head><body> |
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| 6 | |||
| 7 | <h2>Overview of liboop.</h2> |
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| 8 | |||
| 9 | <h4>The basic idea.</h4> |
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| 10 | |||
| 11 | Liboop is primarily an <em>interface definition</em>. It defines an interface |
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| 12 | which components may use to request notification when an <em>event</em> |
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| 13 | (activity on a file descriptor, the real-time clock reaches a certain value, |
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| 14 | a particular signal is received) occurs. The component which owns the event |
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| 15 | loop -- the component whose code is active when the system is idle -- |
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| 16 | implements the interface; it is an <em>event source</em>. Components which |
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| 17 | are interested in events register themselves with the event source; they are |
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| 18 | <em>event sinks</em>. Event sinks may themselves source other, higher-level |
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| 19 | events, but that is outside liboop's scope. |
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| 20 | |||
| 21 | <h4>Control flow.</h4> |
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| 22 | |||
| 23 | During initialization, the event source is created. At least one event sink |
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| 24 | is also created, and registered with the event source. Once initialization |
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| 25 | completes, control is transferred to the event source, which (at its core) |
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| 26 | waits for events, usually using a system function like select() or poll(). |
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| 27 | When an event occurs, the event source gives a <em>callback</em> to all the |
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| 28 | event sinks which registered interest in that event. |
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| 29 | <p> |
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| 30 | During callbacks, the event sinks react to the event as appropriate (usually |
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| 31 | performing some I/O, or at least modifying internal state). Event sinks for |
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| 32 | events which are no longer relevant may be unregistered; new event sinks may |
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| 33 | be registered for additional events. Each event sink, when it finishes, |
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| 34 | returns a value which tells the event source whether to continue processing |
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| 35 | events or whether to terminate. |
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| 36 | <p> |
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| 37 | While the event source must be fully reentrant (registration and deregistration |
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| 38 | may, and indeed usually are, performed within the context of an event), event |
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| 39 | sinks need not be; no event sink will be called while another event sink is |
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| 40 | active. |
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| 41 | <p> |
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| 42 | If no event sink instructs the event source to terminate, the event source |
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| 43 | continues waiting for events. Otherwise, the event source returns to its |
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| 44 | caller, which usually shuts down the system. |
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| 45 | |||
| 46 | <h4>The system event source.</h4> |
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| 47 | |||
| 48 | Liboop comes with a single "reference" implementation of an event source. |
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| 49 | This event source uses select() to dispatch events. Most programs built |
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| 50 | around liboop will probably use the standard system event source; legacy |
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| 51 | programs with their own event loop, or programs with specialized needs may |
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| 52 | implement their own event source. |
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| 53 | |||
| 54 | <h4>Adapters.</h4> |
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| 55 | |||
| 56 | Liboop supports <em>adapters</em> to enable legacy components to use the liboop |
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| 57 | interface. For example, many widget sets have their own event loop and their |
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| 58 | own mechanism for registering callbacks on timeouts and file descriptor |
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| 59 | activity; liboop uses <em>source adapters</em> that accept registration, |
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| 60 | register corresponding callbacks with the widget set's event loop, and route |
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| 61 | events appropriately. Such adapters let general-purpose liboop-based |
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| 62 | components work in an application based on that widget set. |
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| 63 | <p> |
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| 64 | Similarly, some components are designed to work in a non-blocking fashion, and |
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| 65 | they might be used with a <em>sink adapter</em> to work with liboop. An |
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| 66 | asynchronous DNS query package, for example, could work as a liboop sink that |
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| 67 | ultimately generates a higher-level "success" or "failure" callback to the |
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| 68 | invoking routine. |
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| 69 | |||
| 70 | <h4>Code.</h4> |
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| 71 | |||
| 72 | Liboop's abstract event source interface is implemented as a structure |
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| 73 | containing C function pointers. These functions accept a pointer to the |
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| 74 | structure as their first argument; sources are expected to include their |
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| 75 | own data (in whatever format) with the core function pointers. Callbacks |
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| 76 | are also C function pointers, with "void *" arguments to pass data. |
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| 77 | <p> |
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| 78 | For more about the liboop interface, see the <a href="ref">reference</a>. |
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| 79 | |||
| 80 | <hr><a href="">liboop home</a></body></html> |