Simulation GUI: Add 2D field view (#2261)

The field image and robot image can be loaded or just a wireframe used.
The robot can be moved and rotated with a mouse click + drag.
The robot position is settable in robot code via the Field2d class.

wpilibj/src/main/java/edu/wpi/first/wpilibj/simulation/Field2d.java

@@ -0,0 +1,98 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+import edu.wpi.first.hal.SimDevice;
+import edu.wpi.first.hal.SimDouble;
+import edu.wpi.first.wpilibj.geometry.Pose2d;
+import edu.wpi.first.wpilibj.geometry.Rotation2d;
+import edu.wpi.first.wpilibj.geometry.Translation2d;
+
+/**
+ * 2D representation of game field (for simulation).
+ *
+ * <p>In non-simulation mode this simply stores and returns the robot pose.
+ *
+ * <p>The robot pose is the actual location shown on the simulation view.
+ * This may or may not match the robot's internal odometry.  For example, if
+ * the robot is shown at a particular starting location, the pose in this
+ * class would represent the actual location on the field, but the robot's
+ * internal state might have a 0,0,0 pose (unless it's initialized to
+ * something different).
+ *
+ * <p>As the user is able to edit the pose, code performing updates should get
+ * the robot pose, transform it as appropriate (e.g. based on simulated wheel
+ * velocity), and set the new pose.
+ */
+public class Field2d {
+  public Field2d() {
+    m_device = SimDevice.create("Field2D");
+    if (m_device != null) {
+      m_x = m_device.createDouble("x", false, 0.0);
+      m_y = m_device.createDouble("y", false, 0.0);
+      m_rot = m_device.createDouble("rot", false, 0.0);
+    }
+  }
+
+  /**
+   * Set the robot pose from a Pose object.
+   *
+   * @param pose 2D pose
+   */
+  public void setRobotPose(Pose2d pose) {
+    if (m_device != null) {
+      Translation2d translation = pose.getTranslation();
+      m_x.set(translation.getX());
+      m_y.set(translation.getY());
+      m_rot.set(pose.getRotation().getDegrees());
+    } else {
+      m_pose = pose;
+    }
+  }
+
+  /**
+   * Set the robot pose from x, y, and rotation.
+   *
+   * @param xMeters X location, in meters
+   * @param yMeters Y location, in meters
+   * @param rotation rotation
+   */
+  public void setRobotPose(double xMeters, double yMeters, Rotation2d rotation) {
+    if (m_device != null) {
+      m_x.set(xMeters);
+      m_y.set(yMeters);
+      m_rot.set(rotation.getDegrees());
+    } else {
+      m_pose = new Pose2d(xMeters, yMeters, rotation);
+    }
+  }
+
+  /**
+   * Get the robot pose.
+   *
+   * @return 2D pose
+   */
+  public Pose2d getRobotPose() {
+    if (m_device != null) {
+      return new Pose2d(m_x.get(), m_y.get(), Rotation2d.fromDegrees(m_rot.get()));
+    } else {
+      return m_pose;
+    }
+  }
+
+  private Pose2d m_pose;
+
+  @SuppressWarnings("MemberName")
+  private final SimDevice m_device;
+
+  @SuppressWarnings("MemberName")
+  private SimDouble m_x;
+
+  @SuppressWarnings("MemberName")
+  private SimDouble m_y;
+
+  private SimDouble m_rot;
+}