How draw a line in a window with wxWidget with erlang? - erlang

Question

I try to draw a line in a window with wxWidget in Erlang. I tried:
wx:new(),
Frame = wxFrame:new(wx:null(), ?wxID_ANY, "Hello"),
wxDC:drawLine(50,50),
I get an error:
undefined function wxDC:drawLine/2
I read the documentation here, but I don't understand how to do this:
http://www.erlang.org/doc/man/wxDC.html#drawLine-3

Answer 1

X Windows programming isn't quite that simple, and I'm not quite sure how you are expecting to draw a line with parameters like [50, 50], that's at best a point, and a line needs 2 points, and wxDC:drawLine needs to know where to draw the line too, because you could have many frames.
You can create a frame like this, yes (used -1 instead of the macro because I'm using the shell here):
Wx = wx:new().
Frame = wxFrame:new(wx:null(), -1, "Hello").
Now the important bit, you can't just draw on the frame, you have to register a callback to handle REdrawing. This is because a frame can be covered up at any point by other windows, or because you minimise it, resize it, etc. In fact you don't necessarily need to handle a REdraw for all those cases, but you get the idea.
So, this is not the most efficient, because it performs a redraw regardless of the event, by responding to ANY paint event by drawing a line, but obviously that will do the job:
wxFrame:connect(Frame, paint, [{callback,
fun(_Evt, _Obj) ->
io:format("paint~n"),
DrawContext = wxPaintDC:new(Frame),
wxDC:drawLine(DrawContext, {50, 50}, {150,100}),
wxPaintDC:destroy(DrawContext)
end
}]).
I added the io:format in there so you can see that it is being called when you interact with the window, or some other window interacts with it, because without the io:format call it's a bit invisible in its effect, other than making sure there's always a line in your window.
I also used a draw context. I won't go into it here, I'm afraid it's just one of a load of things you need to learn about X Windows programming, but basically, just be aware for now, you need to have a draw context for your frame, and use that to actually draw with.
One last thing, you need to actually display the frame, if you want to see it, like this:
wxFrame:show(Frame).
Now you should see a window, with a line.

Answer 2

Michael's answer tells you how to do what you want to do, so let me address the confusion about the error
message.
In Erlang, functions that have the same name but different number of arguments are considered separate functions. The number of arguments is called the "arity", and is sometimes indicated as e.g. /2 after the function name.
Your code is calling wxDC:drawLine with two arguments, but you get an error saying that wxDC:drawLine/2 is undefined. In the documentation, you can see that the function you want is wxDC:drawLine/3, which takes three arguments (the first being the draw context, and the second and third being the points between which you want to draw a line).