add /etc-based hook handler for joystick events

contrib/joystick-handler/README

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+Event handlers for Sense Hat joystick clicks.
+
+This event handler provides a UNIX-y interface to react to Sense Hat joystick
+events.
+
+A daemon (bin/joystick-handler) listens for joystick events using the evdev
+Python library. When an event is fired, hooks in the form of executable
+programs are executed and passed two command line arguments: STATE first (one
+of: press, hold, release) and KEY second (one of: up, down, left, right, push).
+
+Hooks are located in suitable /etc directories, e.g., /etc/joystick.d/press/up/
+and executed using run-parts (8). See etc/joystick.d/ for a sample /etc
+configuration directory hierarchy and etc/joystick.d/press/hello for a sample
+hook.
+
+A systemd unit file (etc/systemd/system/joystick-handler.service) is provided
+to start the joystick handler at boot and restart it if needed.
+
+ -- Stefano Zacchiroli <zack@upsilon.cc>  Mon, 22 Feb 2016 11:25:58 +0100

contrib/joystick-handler/bin/joystick-handler

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+#!/usr/bin/python
+
+# Event handler for RPi SenseHat joystick
+# Copyright (C) 2015 Stefano Zacchiroli <zack@upsilon.cc>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+#     * Redistributions of source code must retain the above copyright
+#       notice, this list of conditions and the following disclaimer.
+#
+#     * Redistributions in binary form must reproduce the above copyright
+#       notice, this list of conditions and the following disclaimer in the
+#       documentation and/or other materials provided with the distribution.
+#
+#     * Neither the name of the copyright holder nor the
+#       names of its contributors may be used to endorse or promote products
+#       derived from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+
+from sense_hat import SenseStick
+
+import os
+import subprocess
+
+
+HOOK_ROOT = '/etc/joystick.d'
+
+KEY_NAMES = {
+    SenseStick.KEY_UP:    'up',
+    SenseStick.KEY_DOWN:  'down',
+    SenseStick.KEY_LEFT:  'left',
+    SenseStick.KEY_RIGHT: 'right',
+    SenseStick.KEY_ENTER: 'push',
+}
+
+STATE_NAMES = {
+    SenseStick.STATE_PRESS:   'press',
+    SenseStick.STATE_HOLD:    'hold',
+    SenseStick.STATE_RELEASE: 'release',
+}
+
+
+def run_hooks(state, key):
+    hook_dir = os.path.join(HOOK_ROOT, state, key)
+    if os.path.isdir(hook_dir):
+        subprocess.call(['run-parts',
+                         '--arg', state,
+                         '--arg', key,
+                         hook_dir])
+
+
+def joystick_loop(j):
+    for event in j:
+        key_state = STATE_NAMES[event.state]
+        key_name = KEY_NAMES[event.key]
+        run_hooks(key_state, key_name)
+
+
+if __name__ == '__main__':
+    joystick_loop(SenseStick())

contrib/joystick-handler/etc/joystick.d/README

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+Event handlers for Sense Hat joystick clicks.
+
+Each subdir {press,hold,release}/{up,down,left,right,push}/ contains
+executable scripts that will be run by run-parts (8) when the joystick
+is pressed/released in the given position.
+
+Each script will receive 2 arguments: STATE, KEY.
+STATE is one of: press, hold, release.
+KEY is one of: up, down, left, right, push.

contrib/joystick-handler/etc/joystick.d/press/push/hello

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+#!/usr/bin/python
+
+from sense_hat import SenseHat
+import sys
+
+if __name__ == '__main__':
+    (_state, _key) = sys.argv[1:]  # ignored
+    sense = SenseHat()
+    sense.show_message("Hello, World!")

contrib/joystick-handler/etc/systemd/system/joystick-handler.service

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+[Unit]
+Description=Event handler for RPi SenseHat joystick
+
+[Service]
+ExecStart=/usr/local/bin/joystick-handler
+Restart=always
+
+[Install]
+WantedBy=multi-user.target