[glass] Add more widgets (#2947)

This adds the following widgets:
- Speed Controller
- Gyroscope
- Command
- Subsystem
- PIDController
- Scheduler

glass/src/libnt/native/cpp/NTSubsystem.cpp

@@ -0,0 +1,45 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "glass/networktables/NTSubsystem.h"
+
+using namespace glass;
+
+NTSubsystemModel::NTSubsystemModel(wpi::StringRef path)
+    : NTSubsystemModel(nt::GetDefaultInstance(), path) {}
+
+NTSubsystemModel::NTSubsystemModel(NT_Inst instance, wpi::StringRef path)
+    : m_nt(instance),
+      m_name(m_nt.GetEntry(path + "/.name")),
+      m_defaultCommand(m_nt.GetEntry(path + "/.default")),
+      m_currentCommand(m_nt.GetEntry(path + "/.command")) {
+  m_nt.AddListener(m_name);
+  m_nt.AddListener(m_defaultCommand);
+  m_nt.AddListener(m_currentCommand);
+  Update();
+}
+
+void NTSubsystemModel::Update() {
+  for (auto&& event : m_nt.PollListener()) {
+    if (event.entry == m_name) {
+      if (event.value && event.value->IsString())
+        m_nameValue = event.value->GetString();
+    } else if (event.entry == m_defaultCommand) {
+      if (event.value && event.value->IsString())
+        m_defaultCommandValue = event.value->GetString();
+    } else if (event.entry == m_currentCommand) {
+      if (event.value && event.value->IsString()) {
+        m_currentCommandValue = event.value->GetString();
+      }
+    }
+  }
+}
+
+bool NTSubsystemModel::Exists() {
+  return m_nt.IsConnected() &&
+         nt::GetEntryType(m_defaultCommand) != NT_UNASSIGNED;
+}