feat: Add c bindings for FlagNotifier.

libs/client-sdk/include/launchdarkly/client_side/bindings/c/sdk.h

@@ -6,9 +6,11 @@
 #include <launchdarkly/bindings/c/context.h>
 #include <launchdarkly/bindings/c/data/evaluation_detail.h>
 #include <launchdarkly/bindings/c/export.h>
+#include <launchdarkly/bindings/c/listener_connection.h>
 #include <launchdarkly/bindings/c/memory_routines.h>
 #include <launchdarkly/bindings/c/status.h>
 #include <launchdarkly/bindings/c/value.h>
+
 #include <stddef.h>
 
 #ifdef __cplusplus
@@ -381,6 +383,74 @@ LDClientSDK_JsonVariationDetail(LDClientSDK sdk,
  */
 LD_EXPORT(void) LDClientSDK_Free(LDClientSDK sdk);
 
+typedef void (*FlagChangedCallbackFn)(char const* flag_key,
+                                      LDValue new_value,
+                                      LDValue old_value,
+                                      bool deleted,
+                                      void* user_data);
+
+/**
+ * Defines a feature flag listener which may be used to listen for flag changes.
+ * The struct should be initialized using LDFlagListener_Init before use.
+ */
+struct LDFlagListener {
+    /**
+     * Callback function which is invoked for flag changes.
+     *
+     * The provided pointers are only valid for the duration of the function
+     * call.
+     *
+     * @param flag_key The name of the flag that changed.
+     * @param new_value The new value of the flag. If there was not an new
+     * value, because the flag was deleted, then the LDValue will be of a null
+     * type. Check the deleted parameter to see if a flag was deleted.
+     * @param old_value The old value of the flag. If there was not an old
+     * value, for instance a newly created flag, then the Value will be of a
+     * null type.
+     * @param deleted True if the flag has been deleted.
+     */
+    FlagChangedCallbackFn FlagChanged;
+
+    /**
+     * UserData is forwarded into callback functions.
+     */
+    void* UserData;
+};
+
+/**
+ * Initializes a flag listener. Must be called before passing the listener
+ * to LDClientSDK_FlagNotifier_OnFlagChange.
+ *
+ * Create the struct, initialize the struct, set the FlagChanged handler
+ * and optionally UserData, and then pass the struct to
+ * LDClientSDK_FlagNotifier_OnFlagChange.
+ *
+ * @param listener Listener to initialize.
+ */
+LD_EXPORT(void) LDFlagListener_Init(struct LDFlagListener listener);
+
+/**
+ * Listen for changes for the specific flag.
+ *
+ * If the FlagChanged member of the listener struct is not set (NULL), then the
+ * function will not register a listener. In that case the return value
+ * will be NULL.
+ *
+ * @param sdk SDK. Must not be NULL.
+ * @param flag_key The unique key for the feature flag. Must not be NULL.
+ * @param listener The listener, whose FlagChanged callback will be invoked,
+ * when the flag changes. Must not be NULL.
+ *
+ * @return A LDListenerConnection. The connection can be freed using
+ * LDListenerConnection_Free and the listener can be disconnected using
+ * LDListenerConnection_Disconnect. NULL will be returned if the FlagChanged
+ * member of the listener struct is NULL.
+ */
+LD_EXPORT(LDListenerConnection)
+LDClientSDK_FlagNotifier_OnFlagChange(LDClientSDK sdk,
+                                              char const* flag_key,
+                                              struct LDFlagListener listener);
+
 #ifdef __cplusplus
 }
 #endif

libs/client-sdk/src/bindings/c/sdk.cpp

@@ -296,5 +296,37 @@ LD_EXPORT(void) LDClientSDK_Free(LDClientSDK sdk) {
     delete TO_SDK(sdk);
 }
 
+LD_EXPORT(LDListenerConnection)
+LDClientSDK_FlagNotifier_OnFlagChange(LDClientSDK sdk,
+                                      char const* flag_key,
+                                      struct LDFlagListener listener) {
+    LD_ASSERT_NOT_NULL(sdk);
+    LD_ASSERT_NOT_NULL(flag_key);
+
+    if (listener.FlagChanged) {
+        auto connection = TO_SDK(sdk)->FlagNotifier().OnFlagChange(
+            flag_key,
+            [listener](std::shared_ptr<launchdarkly::client_side::flag_manager::
+                                           FlagValueChangeEvent> event) {
+                listener.FlagChanged(
+                    event->FlagName().c_str(),
+                    reinterpret_cast<LDValue>(
+                        const_cast<Value*>(&event->NewValue())),
+                    reinterpret_cast<LDValue>(
+                        const_cast<Value*>(&event->OldValue())),
+                    event->Deleted(), listener.UserData);
+            });
+
+        return reinterpret_cast<LDListenerConnection>(connection.release());
+    }
+
+    return nullptr;
+}
+
+LD_EXPORT(void) LDFlagListener_Init(struct LDFlagListener listener) {
+    listener.FlagChanged = nullptr;
+    listener.UserData = nullptr;
+}
+
 // NOLINTEND cppcoreguidelines-pro-type-reinterpret-cast
 // NOLINTEND OCInconsistentNamingInspection

libs/client-sdk/tests/client_c_bindings_test.cpp

@@ -24,3 +24,44 @@ TEST(ClientBindings, MinimalInstantiation) {
 
     LDClientSDK_Free(sdk);
 }
+
+void ListenerFunction(char const* flag_key,
+                      LDValue new_value,
+                      LDValue old_value,
+                      bool deleted,
+                      void* user_data) {
+}
+
+// This test registers a listener. It doesn't use the listener, but it
+// will at least ensure 1.) Compilation, and 2.) Allow sanitizers to run.
+TEST(ClientBindings, RegisterFlagListener) {
+    LDClientConfigBuilder cfg_builder = LDClientConfigBuilder_New("sdk-123");
+    LDClientConfigBuilder_Offline(cfg_builder, true);
+
+    LDClientConfig config;
+    LDStatus status = LDClientConfigBuilder_Build(cfg_builder, &config);
+    ASSERT_TRUE(LDStatus_Ok(status));
+
+    LDContextBuilder ctx_builder = LDContextBuilder_New();
+    LDContextBuilder_AddKind(ctx_builder, "user", "shadow");
+
+    LDContext context = LDContextBuilder_Build(ctx_builder);
+
+    LDClientSDK sdk = LDClientSDK_New(config, context);
+
+    bool success = false;
+    LDClientSDK_Start(sdk, 3000, &success);
+    EXPECT_TRUE(success);
+
+    struct LDFlagListener listener{};
+    LDFlagListener_Init(listener);
+    listener.UserData =  const_cast<char*>("Potato");
+    listener.FlagChanged = ListenerFunction;
+
+    LDListenerConnection connection = LDClientSDK_FlagNotifier_OnFlagChange(sdk, "my-boolean-flag", listener);
+
+    LDListenerConnection_Disconnect(connection);
+
+    LDListenerConnection_Free(connection);
+    LDClientSDK_Free(sdk);
+}

libs/common/include/launchdarkly/bindings/c/listener_connection.h

@@ -0,0 +1,41 @@
+// NOLINTBEGIN modernize-use-using
+
+#pragma once
+
+#include <launchdarkly/bindings/c/export.h>
+
+#ifdef __cplusplus
+extern "C" {  // only need to export C interface if
+// used by C++ source code
+#endif
+
+/**
+ * Handle that represents a listener connection.
+ *
+ * To stop unregister a listener call LDListenerConnection_Disconnect.
+ * To free a connection listener call LDListenerConnection_Free.
+ *
+ * Freeing an LDListenerConnection does not disconnect the connection. If it is
+ * deleted, without being disconnected, then the listener will remain active
+ * until the associated SDK is freed.
+*/
+typedef struct _LDListenerConnection* LDListenerConnection;
+
+/**
+ * Disconnect a listener.
+ *
+ * @param connection The connection for the listener to disconnect.
+ * Must not be NULL.
+ */
+LD_EXPORT(void) LDListenerConnection_Disconnect(LDListenerConnection connection);
+
+/**
+ * Free a listener connection.
+ *
+ * @param connection The LDListenerConnection to free.
+ */
+LD_EXPORT(void) LDListenerConnection_Free(LDListenerConnection connection);
+
+#ifdef __cplusplus
+}
+#endif

libs/common/include/launchdarkly/connection.hpp

@@ -1,6 +1,6 @@
 #pragma once
 
-namespace launchdarkly::client_side {
+namespace launchdarkly {
 
 /**
  * Represents the connection of a listener.

libs/common/src/CMakeLists.txt

@@ -49,7 +49,8 @@ add_library(${LIBNAME} OBJECT
         config/persistence_builder.cpp
         bindings/c/data/evaluation_detail.cpp
         bindings/c/memory_routines.cpp
-        config/logging_builder.cpp)
+        config/logging_builder.cpp
+        bindings/c/listener_connection.cpp)
 
 add_library(launchdarkly::common ALIAS ${LIBNAME})
 

libs/common/src/bindings/c/listener_connection.cpp

@@ -0,0 +1,18 @@
+#include <launchdarkly/bindings/c/listener_connection.h>
+#include <launchdarkly/detail/c_binding_helpers.hpp>
+
+#include <launchdarkly/connection.hpp>
+
+#include <memory>
+
+#define TO_LC(ptr) (reinterpret_cast<launchdarkly::IConnection*>(ptr))
+
+LD_EXPORT(void)
+LDListenerConnection_Disconnect(LDListenerConnection connection) {
+    LD_ASSERT_NOT_NULL(connection);
+    TO_LC(connection)->Disconnect();
+}
+
+LD_EXPORT(void) LDListenerConnection_Free(LDListenerConnection connection) {
+    delete TO_LC(connection);
+}