# Gtk.jl clear canvas on key event

**URL:** <https://discourse.julialang.org/t/gtk-jl-clear-canvas-on-key-event/67757>\
**Category:** Visualization\
**Tags:** gtk\
**Created:** [September 6, 2021, 2:09pm UTC](https://discourse.julialang.org/t/gtk-jl-clear-canvas-on-key-event/67757 "2021-09-06T14:09:37Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![htmj](https://avatars.discourse-cdn.com/v4/letter/h/a88e4f/32.png) [@htmj](https://discourse.julialang.org/u/htmj)\
**Post date:** [September 6, 2021, 2:09pm UTC](https://discourse.julialang.org/t/gtk-jl-clear-canvas-on-key-event/67757/1 "2021-09-06T14:09:37Z")

</div>

Hi! I’m trying to make a simple drawing app, and i want to clear the canvas when space is pressed. I understood from this discussion [Resetting the canvas within Cairo.jl and Gtk.jl for an image selection viewing gui](https://discourse.julialang.org/t/resetting-the-canvas-within-cairo-jl-and-gtk-jl-for-an-image-selection-viewing-gui/24251) that you can just fill a rectangle over the canvas, but i can’t get it to work.

```julia
@guarded draw(c) do widget
    ctx = getgc(c)
    h = height(c)
    w = width(c)
    # Paint red rectangle
    for (xs,ys) in zip(xss,yss)
        for ((x0,y0),(x1,y1)) in zip(zip(xs[1:end-1],ys[1:end-1]),zip(xs[2:end],ys[2:end]))
            set_source_rgb(ctx,1, 0, 0)
            move_to(ctx,x0,y0)
            line_to(ctx,x1,y1)
            stroke(ctx)
        end
    end
end

signal_connect(win,"key_press_event") do widget, event
    # SPACE
    if event.keyval == 32
        
        ctx = getgc(c)
        set_source_rgb(ctx, 1, 0, 0)
        rectangle(ctx,0,0,600,600)
        stroke(ctx)
    end
end

```

---

<div class="post-metadata">

**Author:** ![htmj](https://avatars.discourse-cdn.com/v4/letter/h/a88e4f/32.png) [@htmj](https://discourse.julialang.org/u/htmj)\
**Post date:** [September 9, 2021, 2:03pm UTC](https://discourse.julialang.org/t/gtk-jl-clear-canvas-on-key-event/67757/2 "2021-09-09T14:03:14Z")

</div>

Solved it! I needed to make the rectangle inside the draw function, then call it from the key event:

```julia
@guarded draw(c) do widget
    ctx = getgc(c)
    h = height(c)
    w = width(c)
    # Paint red rectangle
    set_source_rgb(ctx,1,1,1)
    rectangle(ctx,0,0,600,600)
    fill(ctx)
    for (xs,ys) in zip(xss,yss)
        for ((x0,y0),(x1,y1)) in zip(zip(xs[1:end-1],ys[1:end-1]),zip(xs[2:end],ys[2:end]))
            set_source_rgb(ctx,1, 0, 0)
            move_to(ctx,x0,y0)
            line_to(ctx,x1,y1)
            stroke(ctx)
        end
    end
end

signal_connect(win,"key_press_event") do widget, event
    # SPACE
    if event.keyval == 32
        empty!(xss)
        empty!(yss)
        draw(c)
    end
end

```

---

<div class="post-metadata">

**Author:** ![htmj](https://avatars.discourse-cdn.com/v4/letter/h/a88e4f/32.png) [@htmj](https://discourse.julialang.org/u/htmj)\
**Post date:** [September 10, 2021, 2:18pm UTC](https://discourse.julialang.org/t/gtk-jl-clear-canvas-on-key-event/67757/3 "2021-09-10T14:18:48Z")

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I also realized now you can do the overwrite directly in the keycode function as long as you use reveal() at the end:

```
    elseif event.keyval == 122
        println(" (z was pressed)")
        ctx = getgc(c)
        set_source_rgb(ctx,1,1,1)
        rectangle(ctx,0,0,600,600)
        fill(ctx)
        reveal(c)
    end

```