Pin the scanner-state callback slot semantics

tests/components/ble_device_base/test_scanner_state_callback.cpp

@@ -0,0 +1,52 @@
+#include <gtest/gtest.h>
+
+#include <cstdint>
+
+#include "esphome/components/ble_device_base/ble_hub.h"
+
+namespace esphome::ble_device_base::testing {
+
+// Pins the ScannerStateCallback slot semantics, mirroring test_raw_callback:
+// a default-constructed slot is "no subscriber", a set slot delivers the
+// state, and a new registration replaces the old.
+namespace {
+
+struct CapturingSubscriber {
+  ScannerState last{ScannerState::IDLE};
+  int calls{0};
+
+  static void trampoline(void *self, ScannerState state) {
+    auto *sub = static_cast<CapturingSubscriber *>(self);
+    sub->last = state;
+    sub->calls++;
+  }
+};
+
+}  // namespace
+
+TEST(ScannerStateCallback, DefaultConstructedSlotIsNotSet) {
+  const ScannerStateCallback callback{};
+  EXPECT_FALSE(callback.is_set());
+}
+
+TEST(ScannerStateCallback, SubscriberSeesState) {
+  CapturingSubscriber subscriber;
+  ScannerStateCallback callback{&subscriber, CapturingSubscriber::trampoline};
+  ASSERT_TRUE(callback.is_set());
+  callback.invoke(ScannerState::RUNNING);
+  EXPECT_EQ(subscriber.calls, 1);
+  EXPECT_EQ(subscriber.last, ScannerState::RUNNING);
+}
+
+TEST(ScannerStateCallback, NewSubscriberReplacesOld) {
+  CapturingSubscriber first;
+  CapturingSubscriber second;
+  ScannerStateCallback callback{&first, CapturingSubscriber::trampoline};
+  callback = {&second, CapturingSubscriber::trampoline};
+  callback.invoke(ScannerState::STOPPED);
+  EXPECT_EQ(first.calls, 0);
+  EXPECT_EQ(second.calls, 1);
+  EXPECT_EQ(second.last, ScannerState::STOPPED);
+}
+
+}  // namespace esphome::ble_device_base::testing