Implement hold_canvas context manager

Author: martinRenou

examples/numpy_heatmap.ipynb

@@ -7,7 +7,7 @@
    "outputs": [],
    "source": [
     "import numpy as np\n",
-    "from ipycanvas import Canvas"
+    "from ipycanvas import Canvas, hold_canvas"
    ]
   },
   {
@@ -44,7 +44,8 @@
    "source": [
     "n_pixels = 10\n",
     "\n",
-    "canvas = Canvas(size=(z.shape[0] * n_pixels, z.shape[1] * n_pixels))"
+    "canvas = Canvas(size=(z.shape[0] * n_pixels, z.shape[1] * n_pixels))\n",
+    "canvas"
    ]
   },
   {
@@ -56,34 +57,17 @@
     "min = np.min(z)\n",
     "max = np.max(z)\n",
     "\n",
-    "canvas.caching = True\n",
-    "r = 0\n",
-    "for row in z:\n",
-    "    c = 0\n",
-    "    for value in row:\n",
-    "        canvas.fill_style = colormap(value, min, max)\n",
-    "        canvas.fill_rect(r * n_pixels, c * n_pixels, n_pixels, n_pixels)\n",
-    "        \n",
-    "        c += 1\n",
-    "    r += 1"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "canvas"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "canvas.flush()"
+    "# Holding the canvas during the entire drawing process\n",
+    "with hold_canvas(canvas):\n",
+    "    r = 0\n",
+    "    for row in z:\n",
+    "        c = 0\n",
+    "        for value in row:\n",
+    "            canvas.fill_style = colormap(value, min, max)\n",
+    "            canvas.fill_rect(r * n_pixels, c * n_pixels, n_pixels, n_pixels)\n",
+    "\n",
+    "            c += 1\n",
+    "        r += 1"
    ]
   }
  ],

ipycanvas/__init__.py

@@ -4,7 +4,7 @@
 # Copyright (c) Martin Renou.
 # Distributed under the terms of the Modified BSD License.
 
-from .canvas import Canvas
-from ._version import __version__, version_info
+from .canvas import Canvas, hold_canvas  # noqa
+from ._version import __version__, version_info  # noqa
 
-from .nbextension import _jupyter_nbextension_paths
+from .nbextension import _jupyter_nbextension_paths  # noqa

ipycanvas/canvas.py

@@ -4,6 +4,8 @@
 # Copyright (c) Martin Renou.
 # Distributed under the terms of the Modified BSD License.
 
+from contextlib import contextmanager
+
 from ipywidgets import Color, DOMWidget
 
 from traitlets import Float, Tuple, Unicode, observe
@@ -133,3 +135,11 @@ def _send_command(self, command):
             self._commands_cache.append(command)
         else:
             self.send(command)
+
+
+@contextmanager
+def hold_canvas(canvas):
+    """Hold any drawing on the canvas, and perform only one draw command at the end."""
+    canvas.caching = True
+    yield
+    canvas.flush()