[glass, wpilib] Rewrite Mechanism2d (#3281)

Substantially improves Mechanism2d by moving it to NetworkTables and adding
a robot API to create the mechanism elements, instead of requiring a JSON file.

Co-authored-by: Peter Johnson <johnson.peter@gmail.com>

wpilibc/src/main/native/include/frc/smartdashboard/MechanismObject2d.h

@@ -0,0 +1,88 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include <networktables/NetworkTable.h>
+#include <wpi/StringMap.h>
+#include <wpi/Twine.h>
+
+namespace frc {
+
+/**
+ * Common base class for all Mechanism2d node types.
+ *
+ * To append another node, call Append with the type of node and its
+ * construction parameters. None of the node types are designed to be
+ * constructed directly, and are owned by their parent node/container - obtain
+ * pointers from the Append function or similar factory methods.
+ *
+ * @see Mechanism2d.
+ */
+class MechanismObject2d {
+  friend class Mechanism2d;
+
+ protected:
+  explicit MechanismObject2d(const wpi::Twine& name);
+
+  /**
+   * Update all entries with new ones from a new table.
+   *
+   * @param table the new table.
+   */
+  virtual void UpdateEntries(std::shared_ptr<NetworkTable> table) = 0;
+
+  mutable wpi::mutex m_mutex;
+
+ public:
+  virtual ~MechanismObject2d() = default;
+
+  /**
+   * Retrieve the object's name.
+   *
+   * @return the object's name relative to its parent.
+   */
+  const std::string& GetName() const;
+
+  /**
+   * Append a Mechanism object that is based on this one.
+   *
+   * @param name the name of the new object.
+   * @param args constructor arguments of the object type.
+   * @return the constructed and appended object, useful for variable
+   * assignments and call chaining.
+   * @throw if an object with the given name already exists.
+   */
+  template <typename T, typename... Args,
+            typename =
+                std::enable_if_t<std::is_convertible_v<T*, MechanismObject2d*>>>
+  T* Append(wpi::StringRef name, Args&&... args) {
+    std::scoped_lock lock(m_mutex);
+    auto& obj = m_objects[name];
+    if (obj) {
+      throw std::runtime_error(("MechanismObject names must be unique! `" +
+                                name + "` was inserted twice!")
+                                   .str());
+    }
+    obj = std::make_unique<T>(name, std::forward<Args>(args)...);
+    T* ex = static_cast<T*>(obj.get());
+    if (m_table) {
+      ex->Update(m_table->GetSubTable(name));
+    }
+    return ex;
+  }
+
+ private:
+  std::string m_name;
+  wpi::StringMap<std::unique_ptr<MechanismObject2d>> m_objects;
+  std::shared_ptr<NetworkTable> m_table;
+  void Update(std::shared_ptr<NetworkTable> table);
+};
+}  // namespace frc