0:44
So you would like to be able to use a graphic user interface to, you might like
to be able to use a graphic user interface to access the general purpose I/O pins.
Now, of course you're going to have to write some code,
write some pifying code for that.
But then once you've written that code, the user can interact with the pins,
make things happen on the pins,
and communicate with other devices by interacting with a graphic user interface.
So they'll see something on the screen, some kind of user interface, and click and
point and type and stuff like that and make something happen.
So that's the goal here to define some program that has a graphic user interface.
1:19
Now Raspberry Pi does it, Arduino doesn't.
So this is one of those things that's different,
that distinguishes Raspberry Pi from Arduino.
So the stuff we've seen for the last few lectures talking about accessing GPIO and
making pins high and low and pulse with modulation, and so on, all those
are things that the Arduino can do just as well, maybe better in a lot of cases.
But they don't really leverage the operating system at all.
1:45
So GUIs, graphic user interfaces, that's one of the things,
one of the advantages you get from having an operating system.
The operating system supports, you can talk to the HDMI, talk to the keyboard,
talk to the mouse and it can provide you with this graphic user interface, right?
And Linux allows that, right?
It supports that, so why not use that?
Right? So this is one of those things
that you can do with Raspberry Pi because it has this operating system support
that you couldn't do with regular or Arduino.
Not easily, when you're making a graphic user interface,
the user interacts with widgets.
Widgets are some kind of visual entities, something the user can interact with
what these are intuitively, you've seen these before buttons, menus, sliders,
scroll bars all those things you see on the screen that a user can mess with.
A drawing surface for drawing, you can put a drawing surface, and
a person can draw into that.
Any kind of graphic way User interacts with the screen.
Those are called widgets, so lots of different widgets, and
what you want to be able to do.
Well, for one thing, is to put these widgets on the screen in a Python program.
So write a Python program that puts widgets on the screen for you, and
allows users to interact.
2:55
Now, the execution of the code,
when you're talking about one of these graphic user interface programs it's
typically not completely controlled by the programmer but by the user, instead.
So what that means is the programmer writes code and
the code lists a bunch of lines of code that execute in a certain sequence.
Right, and with a regular Python program, everything, the order is completely
controlled by the programmer and the data the programmer receives.
But the programmer writes the sequence of instructions and
determines in what sequence they will execute, right?
And there are options, if-then statements, might execute this sequence or that.
But the programmer is in control of that by the way they write the code.
Now with a graphic user interface what happens is that the code that gets
executed is determined by what the user does in the interface.
That is if the user clicks on one button and then function gets called and
if user clicks on another button then another function get called.
So which function gets called isn't completely under
the control of the programmer.
In a regular Python program, you can have an if statement,
if this is true call function one else call function two.
Then the program determines one of these two things is gonna happen, but
with a graphic user interface these functions their
execution is controlled by the user and what they do.
Maybe they move a slider and then this function gets executed and so on.
That actually makes coding somewhat harder for
the programmer because you can't predict the control flow sequence as well, right?
You don't know exactly,
maybe the program's gonna press this button 100 times, right?
You don't know what random sequence they're gonna press, and
you have to accommodate all possible sequences.
So in general, it makes programming more difficult,
but you get a graphic user interface, and that's a good thing.
Okay, so often,
graphic user interface based programs are written as one big event loop, okay?
And this is simpler than it sounds, typically the program waits.
[LAUGH] It just sits there waiting for the user to interact with a widget, okay?
So it doesn't do anything it just sits there statically,
maybe it does some initial stuff.
But it puts a screen out there and it sits there and waits.
Until the user does something, type something and when the user isn't typing
anything, click anything, the code's not doing anything often.
So the user pushes a button or selects a menu item or
something, draws something, grabs a slider, does some event, okay.
5:11
Now when the event happens you do something in response to that event.
So when that event happens a function is called.
So when you say do something in response you typically mean execute a function.
So this so
in some place in your code in your Python code you're gonna have to associate
these events that the user could do with functions that you want to execute, right?
So somebody clicks on this button this function's executed.
They move this slider this function's executed and so on.
So you do something in response you call this function, and
you execute the function.
Then after your done executing the function, the function completes, and
then you wait for something else to do.
So you again wait until the user interacts again, and
then when the user interacts again you do the appropriate function.
So, that's the event loop.
You sit there, you wait for an event.
When the event happens you execute the appropriate function,
and then you wait for the next exam, okay.
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Ian HarrisProfessor
