# We're going to define a callback function that runs whenever the # a MouseButton which contains position and other information. # user clicks the mouse inside the canvas. # c.mouse.buttonpress is a `Reactive.Signal` that updates whenever the An important detail about a GtkReactive.Canvas object is that it contains a MouseHandler, accessible with c.mouse this object contains Reactive.Signal objects for mouse button press/release events, mouse movements, and scrolling: const drawing = Signal(false) # this will become true if we're actively dragging Now, let's make our application respond to mouse-clicks. We'll use Signals, so that our Canvas will be notified when there is new material to draw: const lines = Signal() # the list of lines that we'll drawД¬onst newline = Signal() # the in-progress line (will be added to list above) Consequently, we need a place to store user data. We're going to set this up so that a new line is started when the user clicks with the left mouse button when the user releases the mouse button, the line is finished and added to a list of previously-drawn lines. With this choice, if a user resizes the window by dragging its border, our lines will stay in the same relative position. Here we specified UserUnit units for our drawing and mouse-position units we chose these to be (0,0) for the top left and (1,1) for the bottom right. Cairo canvases can use two different coordinate systems: screen pixels, often called DeviceUnit, and a user-specified custom coordinate system, called UserUnit.all drawing occurs on a canvas object, which corresponds to a region of a window.

Let's begin by creating a window with a canvas in it: using Gtk.ShortNames, GtkReactive, Graphics, ColorsД¬ = canvas(UserUnit) # create a canvas with user-specified coordinatesДЄ few concepts from Cairo are important here: We can explore some of these tools by building a simple program for drawing lines. Aside from widgets, GtkReactive also adds canvas interactions, specifically handling of mouse clicks and scroll events.

Simple drawing program