[command] Add NetworkButton (#2373)

Closes #2371

wpilibNewCommands/src/main/java/edu/wpi/first/wpilibj2/command/button/NetworkButton.java

@@ -0,0 +1,53 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2008-2020 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+package edu.wpi.first.wpilibj2.command.button;
+
+import edu.wpi.first.networktables.NetworkTable;
+import edu.wpi.first.networktables.NetworkTableEntry;
+import edu.wpi.first.networktables.NetworkTableInstance;
+
+/**
+ * A {@link Button} that uses a {@link NetworkTable} boolean field.
+ */
+public class NetworkButton extends Button {
+  private final NetworkTableEntry m_entry;
+
+  /**
+   * Creates a NetworkButton that commands can be bound to.
+   *
+   * @param entry The entry that is the value.
+   */
+  public NetworkButton(NetworkTableEntry entry) {
+    m_entry = entry;
+  }
+
+  /**
+   * Creates a NetworkButton that commands can be bound to.
+   *
+   * @param table The table where the networktable value is located.
+   * @param field The field that is the value.
+   */
+  public NetworkButton(NetworkTable table, String field) {
+    this(table.getEntry(field));
+  }
+
+  /**
+   * Creates a NetworkButton that commands can be bound to.
+   *
+   * @param table The table where the networktable value is located.
+   * @param field The field that is the value.
+   */
+  public NetworkButton(String table, String field) {
+    this(NetworkTableInstance.getDefault().getTable(table), field);
+  }
+
+  @Override
+  public boolean get() {
+    return m_entry.getInstance().isConnected() && m_entry.getBoolean(false);
+  }
+}