added cairo_anim.nim example

examples/cairo_anim.nim

@@ -0,0 +1,44 @@
+# https://lists.cairographics.org/archives/cairo/2016-October/027791.html
+# Nim version of that plain cairo animation example
+
+import gintro/[gtk, glib, gobject, gio, cairo]
+import math
+
+const
+  NumPoints = 1000
+  Period = 100.0
+
+proc invalidateCb(w: Widget): bool =
+  queueDraw(w)
+  return SOURCE_CONTINUE
+
+proc sineToPoint(x, width, height: int): float =
+  math.sin(x.float * math.TAU / Period) * height.float * 0.5 + height.float * 0.5
+
+proc drawingAreaDrawCb(widget: DrawingArea; context: Context): bool =
+  var redrawNumber {.global.} : int
+  let width = getAllocatedWidth(widget)
+  let height = getAllocatedHeight(widget)
+  for i in 1 .. < NumPoints:
+    context.lineTo(i.float , sineToPoint(i + redrawNumber, width, height))
+  context.stroke
+  inc(redrawNumber)
+  return true # TRUE to stop other handlers from being invoked for the event. FALSE to propagate the event further.
+
+proc activate(app: Application) =
+  let window = newApplicationWindow(app)
+  window.title = "Drawing example"
+  window.defaultSize = (400, 400)
+  let drawingArea = newDrawingArea()
+  window.add(drawingArea)
+  showAll(window)
+  discard timeoutAdd(1000 div 60, invalidateCb, drawingArea)
+  connect(drawingArea, "draw", drawingAreaDrawCb)
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", activate)
+  discard run(app)
+
+main()
+