Add snippets for XR alpha04 hands (#530)

* Add snippet that demonstrates how to detect a secondary hand

* Add snippet to demonstrate how to detect a basic stop gesture

Author: devbridie

xr/src/main/java/com/example/xr/arcore/Hands.kt

@@ -16,6 +16,7 @@
 
 package com.example.xr.arcore
 
+import android.app.Activity
 import androidx.activity.ComponentActivity
 import androidx.lifecycle.lifecycleScope
 import androidx.xr.arcore.Hand
@@ -28,8 +29,10 @@ import androidx.xr.runtime.SessionConfigureSuccess
 import androidx.xr.runtime.math.Pose
 import androidx.xr.runtime.math.Quaternion
 import androidx.xr.runtime.math.Vector3
+import androidx.xr.runtime.math.toRadians
 import androidx.xr.scenecore.GltfModelEntity
 import androidx.xr.scenecore.scene
+import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.launch
 
 @Suppress("RestrictedApi") // b/416288516 - session.config and session.configure() are incorrectly restricted
@@ -65,6 +68,16 @@ fun ComponentActivity.collectHands(session: Session) {
     }
 }
 
+fun secondaryHandDetection(activity: Activity, session: Session) {
+    fun detectGesture(handState: Flow<Hand.State>) {}
+    // [START androidxr_arcore_hand_handedness]
+    val handedness = Hand.getHandedness(activity.contentResolver)
+    val secondaryHand = if (handedness == Hand.Handedness.LEFT) Hand.right(session) else Hand.left(session)
+    val handState = secondaryHand?.state ?: return
+    detectGesture(handState)
+    // [END androidxr_arcore_hand_handedness]
+}
+
 fun ComponentActivity.renderPlanetAtHandPalm(leftHandState: Hand.State) {
     val session: Session = null!!
     val palmEntity: GltfModelEntity = null!!
@@ -106,3 +119,27 @@ fun ComponentActivity.renderPlanetAtFingerTip(rightHandState: Hand.State) {
     indexFingerEntity.setPose(Pose(position, rotation))
     // [END androidxr_arcore_hand_entityAtIndexFingerTip]
 }
+
+private fun detectPinch(session: Session, handState: Hand.State): Boolean {
+    // [START androidxr_arcore_hand_pinch_gesture]
+    val thumbTip = handState.handJoints[HandJointType.THUMB_TIP] ?: return false
+    val thumbTipPose = session.scene.perceptionSpace.transformPoseTo(thumbTip, session.scene.activitySpace)
+    val indexTip = handState.handJoints[HandJointType.INDEX_TIP] ?: return false
+    val indexTipPose = session.scene.perceptionSpace.transformPoseTo(indexTip, session.scene.activitySpace)
+    return Vector3.distance(thumbTipPose.translation, indexTipPose.translation) < 0.05
+    // [END androidxr_arcore_hand_pinch_gesture]
+}
+
+private fun detectStop(session: Session, handState: Hand.State): Boolean {
+    // [START androidxr_arcore_hand_stop_gesture]
+    val threshold = toRadians(angleInDegrees = 30f)
+    fun pointingInSameDirection(joint1: HandJointType, joint2: HandJointType): Boolean {
+        val forward1 = handState.handJoints[joint1]?.forward ?: return false
+        val forward2 = handState.handJoints[joint2]?.forward ?: return false
+        return Vector3.angleBetween(forward1, forward2) < threshold
+    }
+    return pointingInSameDirection(HandJointType.INDEX_PROXIMAL, HandJointType.INDEX_TIP) &&
+        pointingInSameDirection(HandJointType.MIDDLE_PROXIMAL, HandJointType.MIDDLE_TIP) &&
+        pointingInSameDirection(HandJointType.RING_PROXIMAL, HandJointType.RING_TIP)
+    // [END androidxr_arcore_hand_stop_gesture]
+}