[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>

wpilibcExamples/src/main/cpp/examples/Mechanism2d/cpp/Robot.cpp

@@ -0,0 +1,73 @@
+// 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.
+
+#include <frc/AnalogPotentiometer.h>
+#include <frc/Encoder.h>
+#include <frc/Joystick.h>
+#include <frc/PWMSparkMax.h>
+#include <frc/TimedRobot.h>
+#include <frc/simulation/SimHooks.h>
+#include <frc/smartdashboard/Mechanism2d.h>
+#include <frc/smartdashboard/MechanismLigament2d.h>
+#include <frc/smartdashboard/SmartDashboard.h>
+#include <units/angle.h>
+
+/**
+ * This sample program shows how to use Mechanism2d - a visual representation of
+ * arms, elevators, and other mechanisms on dashboards; driven by a node-based
+ * API.
+ *
+ * <p>Ligaments are based on other ligaments or roots, and roots are contained
+ * in the base Mechanism2d object. Use pointers for nodes, and beware not to let
+ * the container out of scope - the appended nodes will be recursively
+ * destructed!
+ */
+class Robot : public frc::TimedRobot {
+  static constexpr double kMetersPerPulse = 0.01;
+  static constexpr double kElevatorMinimumLength = 0.5;
+
+ public:
+  void RobotInit() override {
+    m_elevatorEncoder.SetDistancePerPulse(kMetersPerPulse);
+
+    // publish to dashboard
+    frc::SmartDashboard::PutData("Mech2d", &m_mech);
+  }
+
+  void RobotPeriodic() override {
+    // update the dashboard mechanism's state
+    m_elevator->SetLength(kElevatorMinimumLength +
+                          m_elevatorEncoder.GetDistance());
+    m_wrist->SetAngle(units::degree_t(m_wristPotentiometer.Get()));
+  }
+
+  void TeleopPeriodic() override {
+    m_elevatorMotor.Set(m_joystick.GetRawAxis(0));
+    m_wristMotor.Set(m_joystick.GetRawAxis(1));
+  }
+
+ private:
+  frc::PWMSparkMax m_elevatorMotor{0};
+  frc::PWMSparkMax m_wristMotor{1};
+  frc::Encoder m_elevatorEncoder{0, 1};
+  frc::AnalogPotentiometer m_wristPotentiometer{1, 90};
+  frc::Joystick m_joystick{0};
+
+  // the main mechanism object
+  frc::Mechanism2d m_mech{3, 3};
+  // the mechanism root node
+  frc::MechanismRoot2d* m_root = m_mech.GetRoot("climber", 3, 3);
+  // MechanismLigament2d objects represent each "section"/"stage" of the
+  // mechanism, and are based off the root node or another ligament object
+  frc::MechanismLigament2d* m_elevator =
+      m_root->Append<frc::MechanismLigament2d>("elevator", 1, 90_deg);
+  frc::MechanismLigament2d* m_wrist =
+      m_elevator->Append<frc::MechanismLigament2d>("wrist", 0.5, 90_deg);
+};
+
+#ifndef RUNNING_FRC_TESTS
+int main() {
+  return frc::StartRobot<Robot>();
+}
+#endif

wpilibcExamples/src/main/cpp/examples/examples.json

@@ -51,6 +51,14 @@
     "gradlebase": "cpp",
     "commandversion": 2
   },
+  {
+    "name":"Mechanism2d",
+    "foldername":"Mechanism2d",
+    "gradlebase":"cpp",
+    "description":"An example usage of Mechanism2d to display mechanism states on a dashboard.",
+    "tags":[],
+    "commandversion": 2
+  },
   {
     "name": "Solenoids",
     "description": "Demonstrate controlling a single and double solenoid from Joystick buttons.",