[ble_device_base] Add platform-neutral GATT client contract (#18128)

tests/components/ble_device_base/test_gatt_client_contract.cpp

@@ -0,0 +1,66 @@
+// The GATT client contract compiles in no real build until a hub backend is
+// configured; this TU pins it on the host so the header cannot rot unseen.
+#define USE_BLE_GATT_CLIENT
+
+#include "esphome/components/ble_device_base/ble_gatt_client.h"
+
+#include <gtest/gtest.h>
+
+namespace esphome::ble_device_base::testing {
+
+class RecordingListener : public GattClientEventListener {
+ public:
+  void on_connection_state(bool connected, uint16_t mtu, int error) override { this->connected_ = connected; }
+  void on_service_discovery_done(int error) override { this->discovery_error_ = error; }
+  void on_read_result(uint16_t handle, const uint8_t *data, uint16_t len, int error) override {}
+  void on_write_result(uint16_t handle, int error) override {}
+  void on_notify_state(uint16_t handle, bool enabled, int error) override {}
+  void on_notify_data(uint16_t handle, const uint8_t *data, uint16_t len) override {}
+  bool connected_{false};
+  int discovery_error_{0};
+};
+
+class MinimalConnection : public BLEGattConnection {
+ public:
+  int connect(uint64_t address, uint8_t addr_type) override {
+    if (this->listener_ != nullptr)
+      this->listener_->on_connection_state(true, 517, 0);
+    return 0;
+  }
+  int disconnect() override { return 0; }
+  int discover_services() override {
+    if (this->listener_ != nullptr)
+      this->listener_->on_service_discovery_done(0);
+    return 0;
+  }
+  int read_characteristic(uint16_t handle) override { return GATT_ERR_NOT_CONNECTED; }
+  int write_characteristic(uint16_t handle, const uint8_t *data, uint16_t len, bool response) override { return 0; }
+  int read_descriptor(uint16_t handle) override { return 0; }
+  int write_descriptor(uint16_t handle, const uint8_t *data, uint16_t len) override { return 0; }
+  int notify_characteristic(uint16_t handle, bool enable) override { return 0; }
+  int update_connection_params(uint16_t min_interval, uint16_t max_interval, uint16_t latency,
+                               uint16_t timeout) override {
+    return 0;
+  }
+  GattServiceTable get_service_table() override { return {}; }
+  void release_services() override {}
+};
+
+TEST(BleGattClientContract, MinimalImplementerCompilesAndRoutesEvents) {
+  MinimalConnection connection;
+  RecordingListener listener;
+  connection.set_listener(&listener);
+  EXPECT_EQ(connection.connect(0xAABBCCDDEEFFULL, 0), 0);
+  EXPECT_TRUE(listener.connected_);
+  EXPECT_EQ(connection.discover_services(), 0);
+  EXPECT_EQ(listener.discovery_error_, 0);
+  EXPECT_EQ(connection.read_characteristic(1), GATT_ERR_NOT_CONNECTED);
+
+  // A default table is empty and safe to walk.
+  GattServiceTable table = connection.get_service_table();
+  EXPECT_EQ(table.service_count, 0);
+  EXPECT_EQ(table.characteristic_count, 0);
+  EXPECT_EQ(table.descriptor_count, 0);
+}
+
+}  // namespace esphome::ble_device_base::testing