Keyboard class: add __getitem__.

Author: peterhinch

v3/docs/EVENTS.md

@@ -519,10 +519,16 @@ asyncio.run(main())
 Constructor mandatory args:
  * `rowpins` A list or tuple of initialised output pins.
  * `colpins` A list or tuple of initialised input pins (pulled down).
+
 Constructor optional keyword only args:
  * `buffer=bytearray(10)` Keyboard buffer.
  * `db_delay=50` Debounce delay in ms.
 
+ Magic method:
+ * `__getitem__(self, scan_code)` Return the state of a given pin. Enables code
+ that causes actions after a button press, for example on release or auto-repeat
+ while pressed.
+ 
 The `Keyboard` class is subclassed from [Ringbuf queue](./EVENTS.md#7-ringbuf-queue)
 enabling scan codes to be retrieved with an asynchronous iterator.
 
@@ -532,6 +538,13 @@ is not removed from the buffer, on overflow the oldest scan code is discarded.
 There is no limit on the number of rows or columns however if more than 256 keys
 are used, the `buffer` arg would need to be adapted to handle scan codes > 255.
 
+##### Application note
+
+Scanning of the keyboard occurs rapidly, and built-in pull-down resistors have a
+high value. If the capacitance between wires is high, spurious keypresses may be
+registed. To prevent this it is wise to add physical resistors between the input
+pins and gnd. A value in the region of 1KΩ to 5KΩ is recommended.
+
 ###### [Contents](./EVENTS.md#0-contents)
 
 # 7. Ringbuf Queue

v3/primitives/events.py

@@ -174,12 +174,15 @@ def __init__(self, rowpins, colpins, *, buffer=bytearray(10), db_delay=50):
         self.rowpins = rowpins
         self.colpins = colpins
         self.db_delay = db_delay  # Deounce delay in ms
+        self._state = 0  # State of all keys as bit array
         for opin in self.rowpins:  # Initialise output pins
             opin(0)
         asyncio.create_task(self.scan(len(rowpins) * len(colpins)))
 
+    def __getitem__(self, scan_code):
+        return bool(self._state & (1 << scan_code))
+
     async def scan(self, nbuttons):
-        prev = 0
         while True:
             await asyncio.sleep_ms(0)
             cur = 0
@@ -189,9 +192,9 @@ async def scan(self, nbuttons):
                     cur <<= 1
                     cur |= ipin()
                 opin(0)
-            if cur != prev:  # State change
-                pressed = cur & ~prev
-                prev = cur
+            if cur != self._state:  # State change
+                pressed = cur & ~self._state
+                self._state = cur
                 if pressed:  # Ignore button release
                     for v in range(nbuttons):  # Find button index
                         if pressed & 1: