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+Keyboard notifier
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+
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+One can use register_keyboard_notifier to get called back on keyboard
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+events (see kbd_keycode() function for details). The passed structure is
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+keyboard_notifier_param:
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+
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+- 'vc' always provide the VC for which the keyboard event applies;
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+- 'down' is 1 for a key press event, 0 for a key release;
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+- 'shift' is the current modifier state, mask bit indexes are KG_*;
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+- 'value' depends on the type of event.
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+
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+- KBD_KEYCODE events are always sent before other events, value is the keycode.
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+- KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.
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+ value is the keycode.
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+- KBD_UNICODE events are sent if the keycode -> keysym translation produced a
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+ unicode character. value is the unicode value.
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+- KBD_KEYSYM events are sent if the keycode -> keysym translation produced a
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+ non-unicode character. value is the keysym.
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+- KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.
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+ That permits one to inspect the resulting LEDs for instance.
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+
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+For each kind of event but the last, the callback may return NOTIFY_STOP in
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+order to "eat" the event: the notify loop is stopped and the keyboard event is
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+dropped.
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+
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+In a rough C snippet, we have:
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+
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+kbd_keycode(keycode) {
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+ ...
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+ params.value = keycode;
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+ if (notifier_call_chain(KBD_KEYCODE,¶ms) == NOTIFY_STOP)
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+ || !bound) {
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+ notifier_call_chain(KBD_UNBOUND_KEYCODE,¶ms);
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+ return;
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+ }
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+
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+ if (unicode) {
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+ param.value = unicode;
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+ if (notifier_call_chain(KBD_UNICODE,¶ms) == NOTIFY_STOP)
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+ return;
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+ emit unicode;
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+ return;
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+ }
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+
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+ params.value = keysym;
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+ if (notifier_call_chain(KBD_KEYSYM,¶ms) == NOTIFY_STOP)
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+ return;
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+ apply keysym;
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+ notifier_call_chain(KBD_POST_KEYSYM,¶ms);
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+}
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+
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+NOTE: This notifier is usually called from interrupt context.
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