{
 "metadata": {
  "cell_tags": [
   [
    "<None>",
    null
   ]
  ],
  "name": "",
  "signature": "sha256:8cade57fabc6819dc950bc28502028554861fb1440d5d832922b95fd2b8bf25c"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "from __future__ import print_function # 2.7 compatability\n",
      "\n",
      "from IPython.html import widgets # Widget definitions\n",
      "from IPython.display import display # Used to display widgets in the notebook"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 1
    },
    {
     "cell_type": "heading",
     "level": 1,
     "metadata": {},
     "source": [
      "Traitlet Events"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "As mentioned in Part 1, the widget attributes are IPython traitlets.  Traitlets are eventful.  To handle  changes, the `on_trait_change` method of the widget can be used to register a callback.  The docstring for `on_trait_change` can be seen below.  Both the `name` and `remove` properties are optional."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print(widgets.Widget.on_trait_change.__doc__)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Setup a handler to be called when a trait changes.\n",
        "\n",
        "        This is used to setup dynamic notifications of trait changes.\n",
        "\n",
        "        Static handlers can be created by creating methods on a HasTraits\n",
        "        subclass with the naming convention '_[traitname]_changed'.  Thus,\n",
        "        to create static handler for the trait 'a', create the method\n",
        "        _a_changed(self, name, old, new) (fewer arguments can be used, see\n",
        "        below).\n",
        "\n",
        "        Parameters\n",
        "        ----------\n",
        "        handler : callable\n",
        "            A callable that is called when a trait changes.  Its\n",
        "            signature can be handler(), handler(name), handler(name, new)\n",
        "            or handler(name, old, new).\n",
        "        name : list, str, None\n",
        "            If None, the handler will apply to all traits.  If a list\n",
        "            of str, handler will apply to all names in the list.  If a\n",
        "            str, the handler will apply just to that name.\n",
        "        remove : bool\n",
        "            If False (the default), then install the handler.  If True\n",
        "            then unintall it.\n",
        "        \n"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Mentioned in the doc string, the callback registered can have 4 possible signatures:\n",
      "\n",
      "- callback()\n",
      "- callback(trait_name)\n",
      "- callback(trait_name, new_value)\n",
      "- callback(trait_name, old_value, new_value)\n",
      "\n",
      "Using this method, an example of how to output an IntSliderWiget's value as it is changed can be seen below."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "int_range = widgets.IntSliderWidget()\n",
      "display(int_range)\n",
      "\n",
      "def on_value_change(name, value):\n",
      "    print(value)\n",
      "\n",
      "int_range.on_trait_change(on_value_change, 'value')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 3
    },
    {
     "cell_type": "heading",
     "level": 1,
     "metadata": {},
     "source": [
      "Specialized Events"
     ]
    },
    {
     "cell_type": "heading",
     "level": 2,
     "metadata": {},
     "source": [
      "Button Click Event"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "The `ButtonWidget` is a special widget, like the `ContainerWidget` and `TabWidget`, that isn't used to represent a data type.  Instead the button widget is used to handle mouse clicks.  The `on_click` method of the `ButtonWidget` can be used to register function to be called when the button is clicked.  The docstring of the `on_click` can be seen below."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "print(widgets.ButtonWidget.on_click.__doc__)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Register a callback to execute when the button is clicked.\n",
        "\n",
        "        The callback will be called with one argument,\n",
        "        the clicked button widget instance.\n",
        "\n",
        "        Parameters\n",
        "        ----------\n",
        "        remove : bool (optional)\n",
        "            Set to true to remove the callback from the list of callbacks.\n"
       ]
      }
     ],
     "prompt_number": 4
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Button clicks are transmitted from the front-end to the back-end using custom messages.  By using the `on_click` method, a button that prints a message when it has been clicked is shown below."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "button = widgets.ButtonWidget(description=\"Click Me!\")\n",
      "display(button)\n",
      "\n",
      "def on_button_clicked(b):\n",
      "    print(\"Button clicked.\")\n",
      "\n",
      "button.on_click(on_button_clicked)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Button clicked.\n"
       ]
      },
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Button clicked.\n"
       ]
      },
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Button clicked.\n"
       ]
      }
     ],
     "prompt_number": 5
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Event handlers can also be used to create widgets.  In the example below, clicking a button spawns another button with a description equal to how many times the parent button had been clicked at the time."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def new_button(clicked):\n",
      "    button = widgets.ButtonWidget()\n",
      "    button.clicks = 0\n",
      "    clicked.clicks += 1\n",
      "    button.description = \"%d\" % clicked.clicks\n",
      "    display(button)\n",
      "    button.on_click(new_button)\n",
      "button = widgets.ButtonWidget(description = \"Start\")\n",
      "button.clicks = 0\n",
      "display(button)\n",
      "button.on_click(new_button)\n",
      "    "
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 6
    }
   ],
   "metadata": {}
  }
 ]
}