feat: Added a flag to key press behavior to allow it to be inverted

app/dts/bindings/behaviors/zmk,behavior-key-press.yaml

@@ -6,3 +6,7 @@ description: Key press/release behavior
 compatible: "zmk,behavior-key-press"
 
 include: one_param.yaml
+
+properties:
+  invert-if-active:
+    type: boolean

app/src/behaviors/behavior_key_press.c

@@ -12,6 +12,7 @@
 
 #include <zmk/event_manager.h>
 #include <zmk/events/keycode_state_changed.h>
+#include <zmk/hid.h>
 #include <zmk/behavior.h>
 
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
@@ -37,18 +38,30 @@ static const struct behavior_parameter_metadata metadata = {
 
 #endif
 
+struct behavior_key_press_config {
+    bool invert_if_active;
+};
+
 static int behavior_key_press_init(const struct device *dev) { return 0; };
 
 static int on_keymap_binding_pressed(struct zmk_behavior_binding *binding,
                                      struct zmk_behavior_binding_event event) {
     LOG_DBG("position %d keycode 0x%02X", event.position, binding->param1);
-    return raise_zmk_keycode_state_changed_from_encoded(binding->param1, true, event.timestamp);
+    bool pressed = zmk_hid_is_pressed(binding->param1);
+    const struct behavior_key_press_config *cfg =
+        zmk_behavior_get_binding(binding->behavior_dev)->config;
+    return raise_zmk_keycode_state_changed_from_encoded(
+        binding->param1, !(cfg->invert_if_active & pressed), event.timestamp);
 }
 
 static int on_keymap_binding_released(struct zmk_behavior_binding *binding,
                                       struct zmk_behavior_binding_event event) {
     LOG_DBG("position %d keycode 0x%02X", event.position, binding->param1);
-    return raise_zmk_keycode_state_changed_from_encoded(binding->param1, false, event.timestamp);
+    bool pressed = zmk_hid_is_pressed(binding->param1);
+    const struct behavior_key_press_config *cfg =
+        zmk_behavior_get_binding(binding->behavior_dev)->config;
+    return raise_zmk_keycode_state_changed_from_encoded(
+        binding->param1, cfg->invert_if_active & !pressed, event.timestamp);
 }
 
 static const struct behavior_driver_api behavior_key_press_driver_api = {
@@ -60,7 +73,11 @@ static const struct behavior_driver_api behavior_key_press_driver_api = {
 };
 
 #define KP_INST(n)                                                                                 \
-    BEHAVIOR_DT_INST_DEFINE(n, behavior_key_press_init, NULL, NULL, NULL, POST_KERNEL,             \
+    static const struct behavior_key_press_config behavior_key_press_config_##n = {                \
+        DT_INST_PROP_OR(n, invert_if_active, false),                                               \
+    };                                                                                             \
+    BEHAVIOR_DT_INST_DEFINE(n, behavior_key_press_init, NULL, NULL,                                \
+                            &behavior_key_press_config_##n, POST_KERNEL,                           \
                             CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &behavior_key_press_driver_api);
 
 DT_INST_FOREACH_STATUS_OKAY(KP_INST)

app/tests/keypress/behavior_keymap.dtsi

@@ -8,7 +8,7 @@
 
         default_layer {
             bindings = <
-                &kp B &none
+                &kp B &kp B
                 &none &none
             >;
         };

app/tests/keypress/inverting-kp-press-release/events.patterns

@@ -0,0 +1 @@
+s/.*hid_listener_keycode_//p

app/tests/keypress/inverting-kp-press-release/keycode_events.snapshot

@@ -0,0 +1,10 @@
+pressed: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x05 implicit_mods 0x00 explicit_mods 0x00

app/tests/keypress/inverting-kp-press-release/native_posix_64.keymap

@@ -0,0 +1,20 @@
+#include "../behavior_keymap.dtsi"
+
+&kp {
+    invert-if-active;
+};
+
+&kscan {
+    events = <
+        ZMK_MOCK_PRESS(0,0,10)
+        ZMK_MOCK_PRESS(0,1,10)
+        ZMK_MOCK_RELEASE(0,0,10)
+        ZMK_MOCK_RELEASE(0,1,10)
+        ZMK_MOCK_PRESS(0,0,10)
+        ZMK_MOCK_PRESS(0,1,10)
+        ZMK_MOCK_RELEASE(0,1,10)
+        ZMK_MOCK_RELEASE(0,0,10)
+        ZMK_MOCK_PRESS(0,1,10)
+        ZMK_MOCK_RELEASE(0,1,10)
+    >;
+};

docs/docs/keymaps/behaviors/key-press.md

@@ -47,3 +47,50 @@ Example:
 ```dts
 &kp A
 ```
+
+### Configuration
+
+#### Inverting
+
+By default, ZMK will always send a keycode press event to the host on key press, and a keycode release event on key release.
+This is the case even when pressing multiple keys with the same keycode at once, which can cause some unusual interactions.
+For example, [toggling a key](./key-toggle.md) on and then pressing and releasing a key with the same keycode will result in the key press cancelling the toggle.
+
+The `invert-if-active` flag allows you to change this interaction. It makes the key press behavior invert its output events.
+In other words, if the keycode is pressed on key press, then it releases the keycode. If the keycode is released on key release, then it presses the keycode.
+
+You can set the flag by updating the existing behavior:
+
+```dts
+&kp {
+    invert-if-active;
+};
+
+/ {
+    keymap {
+        ...
+    };
+};
+```
+
+If you wish to use both versions in the same keymap, you can define a new key press behavior instead:
+
+```
+/ {
+    behaviors {
+        kpi: inverting_key_press {
+            compatible = "zmk,behavior-key-press";
+            #binding-cells = <1>;
+            display-name = "Inverting Key Press";
+            invert-if-active;
+        };
+        ...
+    };
+
+    keymap {
+        ...
+    };
+};
+```
+
+You would then use the normal behavior with `&kp` and the inverting behavior with `&kpi`.