Add Advanced Usage/Utility Classes section in Building Graph in C++

PiperOrigin-RevId: 508776246

docs/framework_concepts/building_graphs_cpp.md

@@ -156,8 +156,124 @@ And you don't need to duplicate names and tags (`InferenceCalculator`,
 `TENSORS`, `MODEL`) or introduce dedicated constants here and there - those
 details are localized to `RunInference` function.
 
-Tip: extracting `RunInference` and similar functions to dedicated units (e.g.
+Tip: extracting `RunInference` and similar functions to dedicated modules (e.g.
 inference.h/cc which depends on the inference calculator) enables reuse in
 graphs construction code and helps automatically pull in calculator dependencies
 (e.g. no need to manually add `:inference_calculator` dep, just let your IDE
 include `inference.h` and build cleaner pull in corresponding dependency).
+
+#### Utility Classes
+
+And surely, it's not only about functions, in some cases it's beneficial to
+introduce utility classes which can help making your graph construction code
+more readable and less error prone.
+
+MediaPipe offers `PassThroughCalculator` calculator, which is simply passing
+through its inputs:
+
+```
+input_stream: "float_value"
+input_stream: "int_value"
+input_stream: "bool_value"
+
+output_stream: "passed_float_value"
+output_stream: "passed_int_value"
+output_stream: "passed_bool_value"
+
+node {
+  calculator: "PassThroughCalculator"
+  input_stream: "float_value"
+  input_stream: "int_value"
+  input_stream: "bool_value"
+  // The order must be the same as for inputs (or you can use explicit indexes)
+  output_stream: "passed_float_value"
+  output_stream: "passed_int_value"
+  output_stream: "passed_bool_value"
+}
+```
+
+Let's see the straightforward C++ construction code to create the above graph:
+
+```c++
+CalculatorGraphConfig BuildGraph() {
+  Graph graph;
+
+  // Graph inputs.
+  Stream<float> float_value = graph.In(0).SetName("float_value").Cast<float>();
+  Stream<int> int_value = graph.In(1).SetName("int_value").Cast<int>();
+  Stream<bool> bool_value = graph.In(2).SetName("bool_value").Cast<bool>();
+
+  auto& pass_node = graph.AddNode("PassThroughCalculator");
+  float_value.ConnectTo(pass_node.In("")[0]);
+  int_value.ConnectTo(pass_node.In("")[1]);
+  bool_value.ConnectTo(pass_node.In("")[2]);
+  Stream<float> passed_float_value = pass_node.Out("")[0].Cast<float>();
+  Stream<int> passed_int_value = pass_node.Out("")[1].Cast<int>();
+  Stream<bool> passed_bool_value = pass_node.Out("")[2].Cast<bool>();
+
+  // Graph outputs.
+  passed_float_value.SetName("passed_float_value").ConnectTo(graph.Out(0));
+  passed_int_value.SetName("passed_int_value").ConnectTo(graph.Out(1));
+  passed_bool_value.SetName("passed_bool_value").ConnectTo(graph.Out(2));
+
+  // Get `CalculatorGraphConfig` to pass it into `CalculatorGraph`
+  return graph.GetConfig();
+}
+```
+
+While `pbtxt` representation maybe error prone (when we have many inputs to pass
+through), C++ code looks even worse: repeated empty tags and `Cast` calls. Let's
+see how we can do better by introducing a `PassThroughNodeBuilder`:
+
+```c++
+class PassThroughNodeBuilder {
+ public:
+  explicit PassThroughNodeBuilder(Graph& graph)
+      : node_(graph.AddNode("PassThroughCalculator")) {}
+
+  template <typename T>
+  Stream<T> PassThrough(Stream<T> stream) {
+    stream.ConnectTo(node_.In(index_));
+    return node_.Out(index_++).Cast<T>();
+  }
+
+ private:
+  int index_ = 0;
+  GenericNode& node_;
+};
+```
+
+And now graph construction code can look like:
+
+```c++
+CalculatorGraphConfig BuildGraph() {
+  Graph graph;
+
+  // Graph inputs.
+  Stream<float> float_value = graph.In(0).SetName("float_value").Cast<float>();
+  Stream<int> int_value = graph.In(1).SetName("int_value").Cast<int>();
+  Stream<bool> bool_value = graph.In(2).SetName("bool_value").Cast<bool>();
+
+  PassThroughNodeBuilder pass_node_builder(graph);
+  Stream<float> passed_float_value = pass_node_builder.PassThrough(float_value);
+  Stream<int> passed_int_value = pass_node_builder.PassThrough(int_value);
+  Stream<bool> passed_bool_value = pass_node_builder.PassThrough(bool_value);
+
+  // Graph outputs.
+  passed_float_value.SetName("passed_float_value").ConnectTo(graph.Out(0));
+  passed_int_value.SetName("passed_int_value").ConnectTo(graph.Out(1));
+  passed_bool_value.SetName("passed_bool_value").ConnectTo(graph.Out(2));
+
+  // Get `CalculatorGraphConfig` to pass it into `CalculatorGraph`
+  return graph.GetConfig();
+}
+```
+
+Now you can't have incorrect order or index in your pass through construction
+code and save some typing by guessing the type for `Cast` from the `PassThrough`
+input.
+
+Tip: the same as for the `RunInference` function, extracting
+`PassThroughNodeBuilder` and similar utility classes into dedicated modules
+enables reuse in graph construction code and helps to automatically pull in the
+corresponding calculator dependencies.