[commands] Add Trigger.onChange() (#6390)

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

@@ -49,6 +49,32 @@ public class Trigger implements BooleanSupplier {
     this(CommandScheduler.getInstance().getDefaultButtonLoop(), condition);
   }
 
+  /**
+   * Starts the command when the condition changes.
+   *
+   * @param command the command to start
+   * @return this trigger, so calls can be chained
+   */
+  public Trigger onChange(Command command) {
+    requireNonNullParam(command, "command", "onChange");
+    m_loop.bind(
+        new Runnable() {
+          private boolean m_pressedLast = m_condition.getAsBoolean();
+
+          @Override
+          public void run() {
+            boolean pressed = m_condition.getAsBoolean();
+
+            if (m_pressedLast != pressed) {
+              command.schedule();
+            }
+
+            m_pressedLast = pressed;
+          }
+        });
+    return this;
+  }
+
   /**
    * Starts the given command whenever the condition changes from `false` to `true`.
    *

wpilibNewCommands/src/main/native/cpp/frc2/command/button/Trigger.cpp

@@ -13,6 +13,34 @@ using namespace frc2;
 
 Trigger::Trigger(const Trigger& other) = default;
 
+Trigger Trigger::OnChange(Command* command) {
+  m_loop->Bind(
+      [condition = m_condition, previous = m_condition(), command]() mutable {
+        bool current = condition();
+
+        if (previous != current) {
+          command->Schedule();
+        }
+
+        previous = current;
+      });
+  return *this;
+}
+
+Trigger Trigger::OnChange(CommandPtr&& command) {
+  m_loop->Bind([condition = m_condition, previous = m_condition(),
+                command = std::move(command)]() mutable {
+    bool current = condition();
+
+    if (previous != current) {
+      command.Schedule();
+    }
+
+    previous = current;
+  });
+  return *this;
+}
+
 Trigger Trigger::OnTrue(Command* command) {
   m_loop->Bind(
       [condition = m_condition, previous = m_condition(), command]() mutable {

wpilibNewCommands/src/main/native/include/frc2/command/button/Trigger.h

@@ -57,6 +57,23 @@ class Trigger {
 
   Trigger(const Trigger& other);
 
+  /**
+   * Starts the command when the condition changes.
+   *
+   * @param command the command to start
+   * @return this trigger, so calls can be chained
+   */
+  Trigger OnChange(Command* command);
+
+  /**
+   * Starts the command when the condition changes. Moves command ownership to
+   * the button scheduler.
+   *
+   * @param command the command to start
+   * @return this trigger, so calls can be chained
+   */
+  Trigger OnChange(CommandPtr&& command);
+
   /**
    * Starts the given command whenever the condition changes from `false` to
    * `true`.