Examples: add streamed parsing example

README.md

@@ -17,6 +17,25 @@ Streamed C++ [ULog](https://docs.px4.io/main/en/dev_log/ulog_file_format.html) r
 Check the [examples](examples) subdirectory.
 
 ## Include in a project
+### Using cmake FetchContent
+To use the library with `FetchContent` in cmake, add the following to the `CMakeLists.txt`:
+```cmake
+include(FetchContent)
+FetchContent_Declare(
+  ulog_cpp
+  GIT_REPOSITORY https://github.com/PX4/ulog_cpp.git
+  GIT_TAG v1.0.0
+  SYSTEM
+  EXCLUDE_FROM_ALL)
+
+FetchContent_MakeAvailable(ulog_cpp)
+
+target_link_libraries(YOUR_PROJECT PUBLIC
+        ulog_cpp::ulog_cpp
+)
+```
+
+### As a submodule
 To add the library as a submodule with cmake, use the following steps:
 ```shell
 git submodule add https://github.com/PX4/ulog_cpp.git ulog_cpp

examples/CMakeLists.txt

@@ -14,3 +14,8 @@ add_executable(ulog_writer ulog_writer.cpp)
 target_link_libraries(ulog_writer PUBLIC
 	ulog_cpp::ulog_cpp
 )
+
+add_executable(ulog_streamed_parsing ulog_streamed_parsing.cpp)
+target_link_libraries(ulog_streamed_parsing PUBLIC
+		ulog_cpp::ulog_cpp
+)

examples/ulog_data.cpp

@@ -9,6 +9,8 @@
 #include <ulog_cpp/data_container.hpp>
 #include <ulog_cpp/reader.hpp>
 
+// Example of how to use the typed data API for accessing topic data
+
 int main(int argc, char** argv)
 {
   if (argc < 2) {

examples/ulog_info.cpp

@@ -13,6 +13,8 @@
 #include <ulog_cpp/reader.hpp>
 #include <variant>
 
+// Example of how to access the different data messages (info, logging, parameters, subscriptions)
+
 int main(int argc, char** argv)
 {
   if (argc < 2) {

examples/ulog_streamed_parsing.cpp

@@ -0,0 +1,186 @@
+/****************************************************************************
+ * Copyright (c) 2025 PX4 Development Team.
+ * SPDX-License-Identifier: BSD-3-Clause
+ ****************************************************************************/
+
+#include <cinttypes>
+#include <fstream>
+#include <iostream>
+#include <string>
+#include <ulog_cpp/data_container.hpp>
+#include <ulog_cpp/reader.hpp>
+
+// Example for parsing and processing an ULog file in real-time, without keeping the whole file
+// in memory
+
+class TopicSubscription {
+ public:
+  virtual ~TopicSubscription() = default;
+
+  virtual void handleData(const ulog_cpp::TypedDataView& data) = 0;
+};
+
+// A topic we are interested in
+class VehicleStatus : public TopicSubscription {
+ public:
+  explicit VehicleStatus(const std::shared_ptr<ulog_cpp::Subscription>& subscription)
+  {
+    _timestamp_field = subscription->field("timestamp");
+    _nav_state_field = subscription->field("nav_state");
+    // Optional field (e.g. when a message changes)
+    if (subscription->fieldMap().find("armed_state") != subscription->fieldMap().end()) {
+      _armed_state_field = subscription->field("armed_state");
+    }
+  }
+
+  void handleData(const ulog_cpp::TypedDataView& data) override
+  {
+    const auto timestamp = data[*_timestamp_field].as<std::uint64_t>();
+    const auto nav_state = data[*_nav_state_field].as<std::uint32_t>();
+    uint8_t armed_state = 0;
+    if (_armed_state_field) {
+      armed_state = data[*_nav_state_field].as<std::uint8_t>();
+    }
+    printf("vehicle_status: t: %" PRId64 ": nav_state: %" PRIu32 ", armed_state: %" PRId8 "\n",
+           timestamp, nav_state, armed_state);
+  }
+
+ private:
+  std::shared_ptr<ulog_cpp::Field> _timestamp_field;
+  std::shared_ptr<ulog_cpp::Field> _nav_state_field;
+  std::shared_ptr<ulog_cpp::Field> _armed_state_field;
+};
+
+class ULogDataHandler : public ulog_cpp::DataContainer {
+ public:
+  ULogDataHandler() : ulog_cpp::DataContainer(ulog_cpp::DataContainer::StorageConfig::Header) {}
+
+  void error(const std::string& msg, bool is_recoverable) override
+  {
+    printf("Parsing error: %s\n", msg.c_str());
+  }
+
+  void headerComplete() override { ulog_cpp::DataContainer::headerComplete(); }
+
+  void messageInfo(const ulog_cpp::MessageInfo& message_info) override
+  {
+    DataContainer::messageInfo(message_info);
+    if (message_info.isMulti()) {
+      // Multi messages might be continued, but we only know with the next message, so we keep it
+      // stored and append if needed. We assume that continued multi messages are not interleaved
+      // with other messages.
+      if (message_info.isContinued()) {
+        if (_current_multi_message.field().name() == message_info.field().name()) {
+          // Append to previous
+          _current_multi_message.valueRaw().insert(_current_multi_message.valueRaw().end(),
+                                                   message_info.valueRaw().begin(),
+                                                   message_info.valueRaw().end());
+        }
+      } else {
+        finishCurrentMultiMessage();
+        _current_multi_message = message_info;
+      }
+    } else {
+      finishCurrentMultiMessage();
+      messageInfoComplete(message_info);
+    }
+  }
+  void parameter(const ulog_cpp::Parameter& parameter) override
+  {
+    finishCurrentMultiMessage();
+    DataContainer::parameter(parameter);
+  }
+  void addLoggedMessage(const ulog_cpp::AddLoggedMessage& add_logged_message) override
+  {
+    finishCurrentMultiMessage();
+    DataContainer::addLoggedMessage(add_logged_message);
+    if (_subscriptions_by_message_id.find(add_logged_message.msgId()) !=
+        _subscriptions_by_message_id.end()) {
+      throw ulog_cpp::ParsingException("Duplicate AddLoggedMessage message ID");
+    }
+
+    auto format_iter = messageFormats().find(add_logged_message.messageName());
+    if (format_iter == messageFormats().end()) {
+      throw ulog_cpp::ParsingException("AddLoggedMessage message format not found");
+    }
+
+    auto ulog_subscription = std::make_shared<ulog_cpp::Subscription>(
+        add_logged_message, std::vector<ulog_cpp::Data>{}, format_iter->second);
+
+    if (add_logged_message.messageName() == "vehicle_status" && add_logged_message.multiId() == 0) {
+      auto subscription = std::make_shared<VehicleStatus>(ulog_subscription);
+      _subscriptions_by_message_id.insert(
+          {add_logged_message.msgId(), SubscriptionData{ulog_subscription, subscription}});
+    }
+  }
+  void logging(const ulog_cpp::Logging& logging) override
+  {
+    finishCurrentMultiMessage();
+    DataContainer::logging(logging);
+  }
+  void data(const ulog_cpp::Data& data) override
+  {
+    finishCurrentMultiMessage();
+    const auto iter = _subscriptions_by_message_id.find(data.msgId());
+    if (iter != _subscriptions_by_message_id.end()) {
+      const ulog_cpp::TypedDataView data_view(data, *iter->second.ulog_subscription->format());
+      iter->second.subscription->handleData(data_view);
+    }
+  }
+
+ private:
+  struct SubscriptionData {
+    std::shared_ptr<ulog_cpp::Subscription> ulog_subscription;
+    std::shared_ptr<TopicSubscription> subscription;
+  };
+
+  void finishCurrentMultiMessage()
+  {
+    if (!_current_multi_message.field().name().empty()) {
+      messageInfoComplete(_current_multi_message);
+      _current_multi_message.field() = {};
+    }
+  }
+  void messageInfoComplete(const ulog_cpp::MessageInfo& message_info)
+  {
+    if (message_info.field().definitionResolved()) {
+      printf("Info message: %s\n", message_info.field().name().c_str());
+    }
+  }
+
+  std::map<uint16_t, SubscriptionData> _subscriptions_by_message_id;
+  ulog_cpp::MessageInfo _current_multi_message{"",
+                                               ""};  ///< Keep this stored for continued messages
+};
+
+int main(int argc, char** argv)
+{
+  if (argc < 2) {
+    printf("Usage: %s <file.ulg>\n", argv[0]);
+    return -1;
+  }
+  FILE* file = fopen(argv[1], "rb");
+  if (!file) {
+    printf("opening file failed\n");
+    return -1;
+  }
+  uint8_t buffer[4096];
+  int bytes_read;
+  const auto data_container = std::make_shared<ULogDataHandler>();
+  ulog_cpp::Reader reader{data_container};
+  while ((bytes_read = fread(buffer, 1, sizeof(buffer), file)) > 0) {
+    try {
+      reader.readChunk(buffer, bytes_read);
+    } catch (const ulog_cpp::ExceptionBase& exception) {
+      printf("Failed to parse ulog file: %s\n", exception.what());
+      return -1;
+    }
+    if (data_container->hadFatalError()) {
+      printf("Failed to parse ulog file\n");
+      return -1;
+    }
+  }
+  fclose(file);
+
+  return 0;
+}

examples/ulog_writer.cpp

@@ -8,6 +8,8 @@
 #include <thread>
 #include <ulog_cpp/simple_writer.hpp>
 
+// Example of how to create an ULog file with timeseries, printf messages and parameters
+
 using namespace std::chrono_literals;
 
 namespace {

ulog_cpp/messages.cpp

@@ -68,6 +68,10 @@ MessageInfo::MessageInfo(const uint8_t* msg, bool is_multi) : _is_multi(is_multi
     _field = Field(info->key_value_str, info->key_len);
     initValues(info->key_value_str + info->key_len, info->msg_size - info->key_len - 1);
   }
+  if (_field.type().type != Field::BasicType::NESTED) {
+    // For basic types we can resolve the definition immediately
+    _field.resolveDefinition(0);
+  }
 }
 void MessageInfo::initValues(const char* values, int len)
 {
@@ -468,6 +472,10 @@ ParameterDefault::ParameterDefault(const uint8_t* msg)
   _field = Field(param_default->key_value_str, param_default->key_len);
   initValues(param_default->key_value_str + param_default->key_len,
              param_default->msg_size - param_default->key_len - 2);
+  if (_field.type().type != Field::BasicType::NESTED) {
+    // For basic types we can resolve the definition immediately
+    _field.resolveDefinition(0);
+  }
 }
 void ParameterDefault::initValues(const char* values, int len)
 {