Move stream API loopback to third_party.

PiperOrigin-RevId: 559037020

mediapipe/framework/api2/stream/BUILD

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+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])
+
+cc_library(
+    name = "loopback",
+    hdrs = ["loopback.h"],
+    deps = [
+        "//mediapipe/calculators/core:previous_loopback_calculator",
+        "//mediapipe/framework/api2:builder",
+        "//mediapipe/framework/api2:port",
+    ],
+    alwayslink = 1,
+)

mediapipe/framework/api2/stream/loopback.h

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+#ifndef MEDIAPIPE_FRAMEWORK_API2_STREAM_LOOPBACK_H_
+#define MEDIAPIPE_FRAMEWORK_API2_STREAM_LOOPBACK_H_
+
+#include <functional>
+#include <utility>
+
+#include "mediapipe/framework/api2/builder.h"
+#include "mediapipe/framework/api2/port.h"
+
+namespace mediapipe::api2::builder {
+
+// Returns a pair of two values:
+// - A stream with loopback data. Such stream, for each new packet in @tick
+//   stream, provides a packet previously calculated within the graph.
+// - A function to define/set loopback data producing stream.
+//   NOTE:
+//     * function must be called and only once, otherwise graph validation will
+//       fail.
+//     * calling function after graph is destroyed results in undefined behavior
+//
+// The function wraps `PreviousLoopbackCalculator` into a convenience function
+// and allows graph input to be processed together with some previous output.
+//
+// -------
+//
+// Example:
+//
+// ```
+//
+//   Graph graph;
+//   Stream<...> tick = ...; // E.g. main input can surve as a tick.
+//   auto [prev_data, set_loopback_fn] = GetLoopbackData<int>(tick, graph);
+//   ...
+//   Stream<int> data = ...;
+//   set_loopback_fn(data);
+//
+// ```
+template <class DataT, class TickT>
+std::pair<Stream<DataT>, std::function<void(Stream<DataT>)>> GetLoopbackData(
+    Stream<TickT> tick, mediapipe::api2::builder::Graph& graph) {
+  auto& prev = graph.AddNode("PreviousLoopbackCalculator");
+  tick.ConnectTo(prev.In("MAIN"));
+  return {prev.Out("PREV_LOOP").template Cast<DataT>(),
+          [prev_ptr = &prev](Stream<DataT> data) {
+            // TODO: input stream info must be specified, but
+            // builder api doesn't support it at the moment. As a workaround,
+            // input stream info is added by GraphBuilder as a graph building
+            // post processing step.
+            data.ConnectTo(prev_ptr->In("LOOP"));
+          }};
+}
+
+}  // namespace mediapipe::api2::builder
+
+#endif  // MEDIAPIPE_FRAMEWORK_API2_STREAM_LOOPBACK_H_

mediapipe/framework/api2/stream/loopback_test.cc

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+#include "mediapipe/framework/api2/stream/loopback.h"
+
+#include "mediapipe/framework/api2/builder.h"
+#include "mediapipe/framework/api2/node.h"
+#include "mediapipe/framework/api2/port.h"
+#include "mediapipe/framework/port/gmock.h"
+#include "mediapipe/framework/port/gtest.h"
+#include "mediapipe/framework/port/parse_text_proto.h"
+
+namespace mediapipe::api2::builder {
+namespace {
+
+class TestDataProducer : public NodeIntf {
+ public:
+  static constexpr Input<float> kLoopbackData{"LOOPBACK_DATA"};
+  static constexpr Output<float> kProducedData{"PRODUCED_DATA"};
+  MEDIAPIPE_NODE_INTERFACE(TestDataProducer, kLoopbackData, kProducedData);
+};
+
+TEST(LoopbackTest, GetLoopbackData) {
+  Graph graph;
+
+  Stream<int> tick = graph.In("TICK").Cast<int>();
+
+  auto [data, set_loopback_data_fn] = GetLoopbackData<float>(tick, graph);
+
+  auto& producer = graph.AddNode<TestDataProducer>();
+  data.ConnectTo(producer[TestDataProducer::kLoopbackData]);
+  Stream<float> data_to_loopback(producer[TestDataProducer::kProducedData]);
+
+  set_loopback_data_fn(data_to_loopback);
+
+  // PreviousLoopbackCalculator configuration is incorrect here and should be
+  // updated when corresponding b/175887687 is fixed.
+  // Use mediapipe::aimatter::GraphBuilder to fix back edges in the graph.
+  EXPECT_THAT(graph.GetConfig(),
+              testing::EqualsProto(
+                  mediapipe::ParseTextProtoOrDie<CalculatorGraphConfig>(R"pb(
+                    node {
+                      calculator: "PreviousLoopbackCalculator"
+                      input_stream: "LOOP:__stream_2"
+                      input_stream: "MAIN:__stream_0"
+                      output_stream: "PREV_LOOP:__stream_1"
+                    }
+                    node {
+                      calculator: "TestDataProducer"
+                      input_stream: "LOOPBACK_DATA:__stream_1"
+                      output_stream: "PRODUCED_DATA:__stream_2"
+                    }
+                    input_stream: "TICK:__stream_0"
+                  )pb")));
+}
+
+}  // namespace
+}  // namespace mediapipe::api2::builder