Event driven programming

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1 Event driven programming Francesco Vespignani February 5, 2009

2 Today Event driven programming Matlab Sounds Matlab GUI

3 Execution Scripts are usually a set of instructions that have to be executed sequentially. For most purposes you are not interested in knowing exactly when specific computations are done but just they are executed in the right order. However an interpreter (e.g. Matlab) can decide to delay (or anticipate) specific execution steps in order to save resources (cpu time or memory). Matlab typically optimises some operations. System programming languages (e.g. C) typically do not provide such optimizations. In general lazy evaluation is a technique of delaying a computation until such time as the result of the computation is known to be needed (wikipedia).

4 File access Suppose you read or write small chunks of data from/to disk. Accessing disks is costly (time) with respect to working only with data that is present in the CPU and RAM. File access is typically bufferised in that reading-writing consists typically of (rather) large chunks. For this reason you may be sure that a file output is completed only when you give a fclose command. Before this data may be either be written on disk or be stored in a RAM buffer, waiting for more. Similar considerations hold for RAM memory allocation.

5 OS In modern computers more programs and services are allowed to run simultaneously on a same machine. In this case it is the O.S. (operative system) that shares execution resources (CPU) and memory (RAM). This can be implemented in many ways (multithreading) but from a user point of view does not matter too much. Different programs and services (Desktop environment, peripherals IO, internet services, user-level applications...) may have different priority within the access to resources. When RAM is full virtual RAMs on disks are automatically used by the O.S. (with detriment of performances).

6 Execution control In an interpreter environment such as Matlab we usually have very poor control over execution flow, both because internal Matlab. The Matlab application usually allows us to edit a file while a command is running. In some circumstances we need a precise control of the execution flow, for example when programming psychology experiments. For these purposes an exact timing of stimulation events (audio-video) and response events (keyboard, mouse button press). To do this use psychtoolbox.

7 Execution control Within sequential execution the organization of the code in functions is useful just for a better organization of the software and for re-usability. However the real power of functions and the reason we need such a construct emerge in the field of event-driven programming. Event-driven programming applies to all the situations where the execution sequence can not be determined by the programmers but it is determined by events that come from outside the program.

8 Event-driven problems Event-driven programming is useful for: Programming GUIs (menus, buttons, textbox...) it must be possible to program specific computation as a consequence of keyboard and mouse events focus. Services: programs that typically wait for other programs to perform specific functions (e.g. paper printing). Some of these services automatically run when the system starts-up but stay silent (without lot of CUP and memory consumption). Internet servers: communication protocols that act in response from requests coming from other computers. Psychology experiments where the PC has to performs some tasks (recording reaction times, creating specific stimulations) as a function of a specific behavior of the participant (e.g. modify the display when the gaze is directed to a specific point of the screen in an eye-movement experiment).

9 Event-driven elements Event loop: to build an event-driven program a loop that does nothing and waits for events is needed and must be run (matlab automatically runs one at startup). Callbacks: event driven programming consists in defining some initializations of the program (display of a GUI, opening a network connection), tell to the event-loop the events that have to captured and which actions have to be performed as a function of specific events and parameters of the event. Typically this is done by a binding of the event to a function. This type of functions, that respond to events, are called callbacks. Matlab has a built-in event-loops and specific methods to define callbacks. However the logic can be implemented in any language that has a delayed execution instruction and functions that allow to detect when specific events happens.

10 Examples Today we will see a GUI example of event driven programming: We will need to learn how: play a sound play a sound when pressing a button in a Matlab figure add a slidebar for a rating

11 Matlab Sounds

12 sound The sound function allows to play an array on the sound card of the PC: The same array can be written to files (e.g. by using wavwrite). Sounds on PC are clearly digitalized and are characterized by a sampling frequency (number of air pressure values per second) and bits (the memory size of these values). To create a pure tone (sin waveform) of frequuency f (in Hz) at the sampling fr4equency fs use the formula: a = sin([1:n]*f/fs*2*pi) A typical sampling frequency that works for nearly evry sound card is

13 Practice I hope you find a way to listen from you PC... See the examples in the website. Create a little function for creating pauses (silence). Concatenate different notes using the cat function. Create a structure that allows to play sounds using notes names (Do, Re, Mi...). Use pairlist structures: notesnames = struct( do,0, re,2); getfield(notesname, do ) Advanced: use a tapering function to avoid burst noise at sound onsets.

14 Matlab GUI

15 Handler When creating a figure with figure or other commands (such as plot or imagesc) a number is returned. This number is a handler, like the numbers returned by fopen commands. This is just a pointer that allows to further refer to the interactive graphical object created. Try the function get and set. Object s attributes can be changed with the set command and pairlist: copy of... AttributeName, value...

16 Callbacks Callbacks are just normal functions. Some objects of a GUI (as a Matlab figure) can have one (or more) function to be executed when specific actions on that object are done (e.g. button press, but also resizing etc.). To create new objects and menu there are two main commands: uimenu uicontrol Whiting these commands you can create new object (widget), change their properties, including the functions that are called upon activation of the widget (Callbacks).

17 Practice Try to change figure menus. Set up a figure with a couple of buttons (e.g. play, next, exit). Add a slidebar to record off-line judgement (e.g. higher/lower sound with respect of previous one). Set up Callbacks to record ratings and produce different sounds.

Review: Event Driven Programming & GUI. CSCE155N Matlab Fall 2016

Review: Event Driven Programming & GUI CSCE155N Matlab Fall 2016 1. Which of the following is not true about event-driven programming? A. An event source is a GUI component that could generate an event