Merge "Improve documentation of VirtualTouchEvent"

2 files changed, 15 insertions, 0 deletions

diff --git a/core/java/android/hardware/input/VirtualTouchEvent.java b/core/java/android/hardware/input/VirtualTouchEvent.java
index ad51eb894e22..9b1e6e84a022 100644
--- a/core/java/android/hardware/input/VirtualTouchEvent.java
+++ b/core/java/android/hardware/input/VirtualTouchEvent.java
@@ -33,6 +33,11 @@ import java.lang.annotation.RetentionPolicy;
  * The pointer id, tool type, action, and location are required; pressure and main axis size are
  * optional.
  *
+ * Note: A VirtualTouchEvent with ACTION_CANCEL can only be created with TOOL_TYPE_PALM (and vice
+ * versa). Events are injected into the uinput kernel module, which has no concept of cancelling
+ * an action. The only way to state the intention that a pointer should not be handled as a pointer
+ * is to change its tool type to TOOL_TYPE_PALM.
+ *
  * @hide
  */
 @SystemApi
@@ -186,6 +191,10 @@ public final class VirtualTouchEvent implements Parcelable {
 
         /**
          * Creates a {@link VirtualTouchEvent} object with the current builder configuration.
+         *
+         * @throws IllegalArgumentException if one of the required arguments is missing or if
+         * ACTION_CANCEL is not set in combination with TOOL_TYPE_PALM. See
+         * {@link VirtualTouchEvent} for a detailed explanation.
          */
         public @NonNull VirtualTouchEvent build() {
             if (mToolType == TOOL_TYPE_UNKNOWN || mPointerId == MotionEvent.INVALID_POINTER_ID
diff --git a/core/tests/coretests/src/android/hardware/input/VirtualTouchEventTest.java b/core/tests/coretests/src/android/hardware/input/VirtualTouchEventTest.java
index 3f504a00773c..100aba5ab362 100644
--- a/core/tests/coretests/src/android/hardware/input/VirtualTouchEventTest.java
+++ b/core/tests/coretests/src/android/hardware/input/VirtualTouchEventTest.java
@@ -136,6 +136,12 @@ public class VirtualTouchEventTest {
                 .build());
     }
 
+    /**
+     * The combination of TOOL_TYPE_PALM with anything else than ACTION_CANCEL should throw an
+     * exception. This is due to an underlying implementation detail. See documentation of {@link
+     * VirtualTouchEvent}
+     * for details.
+     */
     @Test
     public void touchEvent_palmUsedImproperly() {
         assertThrows(IllegalArgumentException.class, () -> new VirtualTouchEvent.Builder()