Change MotionPredictor API to only support a single device

Before this CL, the MotionPredictor API was inconvenient to use. The
MotionPredictor.predict returned a list of MotionEvents, with one
MotionEvent per-device.

The goal was to force applications to consider multi-device streams.
However, even if the apps use this API, there's no way they can
currently test this behaviour, since the multi-device feature is not yet
available.

For the multi-device streams feature, the current plan is to provide a
new View callback to get these raw streams. In those streams, the
per-device MotionEvents would continue to look the same. However, the
events may now be interleaved between different devices. For example, a
sequence like this would be possible: DOWN(deviceId=2) ->
DOWN(deviceId=3) -> MOVE(deviceId=2).

That means that the app will likely have to do per-device bookkeeping
anyways. So the app might as well just create a per-device
MotionPredictor object, as well.

Bug: 167946763
Test: (cd frameworks/native/services/inputflinger && atest)
Change-Id: I34b8dc02d4f995146cb3eed33888ae34abde29d6

6 files changed, 34 insertions, 47 deletions

diff --git a/apct-tests/perftests/core/src/android/input/MotionPredictorBenchmark.kt b/apct-tests/perftests/core/src/android/input/MotionPredictorBenchmark.kt
index fabf8892b544..9482591c65b5 100644
--- a/apct-tests/perftests/core/src/android/input/MotionPredictorBenchmark.kt
+++ b/apct-tests/perftests/core/src/android/input/MotionPredictorBenchmark.kt
@@ -33,7 +33,6 @@ import androidx.test.filters.LargeTest
 import androidx.test.ext.junit.runners.AndroidJUnit4
 
 import org.junit.After
-import org.junit.Assert.assertEquals
 import org.junit.Assert.assertTrue
 import org.junit.Before
 import org.junit.Rule
@@ -132,8 +131,7 @@ class MotionPredictorBenchmark {
             predictor.record(moveEvent)
             val predictionTime = eventTime + eventInterval
             val predicted = predictor.predict(predictionTime.toNanos())
-            assertEquals(1, predicted.size)
-            assertTrue(predicted[0].eventTime <= (predictionTime + offset).toMillis())
+            assertTrue(predicted.eventTime <= (predictionTime + offset).toMillis())
         }
     }
 
diff --git a/core/api/current.txt b/core/api/current.txt
index 2b1cb0e4ab65..259f1e9d4c53 100644
--- a/core/api/current.txt
+++ b/core/api/current.txt
@@ -51443,7 +51443,7 @@ package android.view {
   public final class MotionPredictor {
     ctor public MotionPredictor(@NonNull android.content.Context);
     method public boolean isPredictionAvailable(int, int);
-    method @NonNull public java.util.List<android.view.MotionEvent> predict(long);
+    method @Nullable public android.view.MotionEvent predict(long);
     method public void record(@NonNull android.view.MotionEvent);
   }
 
diff --git a/core/java/android/view/MotionPredictor.java b/core/java/android/view/MotionPredictor.java
index 4d32efea9081..7d452f954fb0 100644
--- a/core/java/android/view/MotionPredictor.java
+++ b/core/java/android/view/MotionPredictor.java
@@ -17,14 +17,11 @@
 package android.view;
 
 import android.annotation.NonNull;
+import android.annotation.Nullable;
 import android.content.Context;
 
 import libcore.util.NativeAllocationRegistry;
 
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.List;
-
 /**
  * Calculate motion predictions.
  *
@@ -68,9 +65,12 @@ public final class MotionPredictor {
 
     /**
      * Record a movement so that in the future, a prediction for the current gesture can be
-     * generated. Ensure to add all motions from the gesture of interest to generate correct
-     * predictions.
+     * generated. Only gestures from one input device at a time should be provided to an instance of
+     * MotionPredictor.
+     *
      * @param event The received event
+     *
+     * @throws IllegalArgumentException if an inconsistent MotionEvent stream is sent.
      */
     public void record(@NonNull MotionEvent event) {
         if (!isPredictionEnabled()) {
@@ -80,28 +80,24 @@ public final class MotionPredictor {
     }
 
     /**
-     * Get predicted events for all gestures that have been provided to {@link #record}.
-     * If events from multiple devices were sent to 'record', this will produce a separate
-     * {@link MotionEvent} for each device. The returned list may be empty if no predictions for
-     * any of the added events/devices are available.
+     * Get a predicted event for the gesture that has been provided to {@link #record}.
      * Predictions may not reach the requested timestamp if the confidence in the prediction results
      * is low.
      *
      * @param predictionTimeNanos The time that the prediction should target, in the
      * {@link android.os.SystemClock#uptimeMillis} time base, but in nanoseconds.
      *
-     * @return A list of predicted motion events, with at most one for each device observed by
-     * {@link #record}. Be sure to check the historical data in addition to the latest
-     * ({@link MotionEvent#getX getX()}, {@link MotionEvent#getY getY()}) coordinates for smooth
-     * prediction curves. An empty list is returned if predictions are not supported, or not
-     * possible for the current set of gestures.
+     * @return The predicted motion event, or `null` if predictions are not supported, or not
+     * possible for the current gesture. Be sure to check the historical data in addition to the
+     * latest ({@link MotionEvent#getX getX()}, {@link MotionEvent#getY getY()}) coordinates for
+     * smooth prediction curves.
      */
-    @NonNull
-    public List<MotionEvent> predict(long predictionTimeNanos) {
+    @Nullable
+    public MotionEvent predict(long predictionTimeNanos) {
         if (!isPredictionEnabled()) {
-            return Collections.emptyList();
+            return null;
         }
-        return Arrays.asList(nativePredict(mPtr, predictionTimeNanos));
+        return nativePredict(mPtr, predictionTimeNanos);
     }
 
     private boolean isPredictionEnabled() {
@@ -129,7 +125,7 @@ public final class MotionPredictor {
 
     private static native long nativeInitialize(int offsetNanos);
     private static native void nativeRecord(long nativePtr, MotionEvent event);
-    private static native MotionEvent[] nativePredict(long nativePtr, long predictionTimeNanos);
+    private static native MotionEvent nativePredict(long nativePtr, long predictionTimeNanos);
     private static native boolean nativeIsPredictionAvailable(long nativePtr, int deviceId,
             int source);
     private static native long nativeGetNativeMotionPredictorFinalizer();
diff --git a/core/jni/android_view_MotionPredictor.cpp b/core/jni/android_view_MotionPredictor.cpp
index 2c232fadbbc5..de3e81c7088b 100644
--- a/core/jni/android_view_MotionPredictor.cpp
+++ b/core/jni/android_view_MotionPredictor.cpp
@@ -20,7 +20,6 @@
 #include <input/MotionPredictor.h>
 
 #include "android_view_MotionEvent.h"
-#include "core_jni_converters.h"
 #include "core_jni_helpers.h"
 
 /**
@@ -30,14 +29,6 @@
 
 namespace android {
 
-// ----------------------------------------------------------------------------
-
-static struct {
-    jclass clazz;
-} gMotionEventClassInfo;
-
-// ----------------------------------------------------------------------------
-
 static void release(void* ptr) {
     delete reinterpret_cast<MotionPredictor*>(ptr);
 }
@@ -57,14 +48,20 @@ static void android_view_MotionPredictor_nativeRecord(JNIEnv* env, jclass clazz,
                                                       jobject event) {
     MotionPredictor* predictor = reinterpret_cast<MotionPredictor*>(ptr);
     MotionEvent* motionEvent = android_view_MotionEvent_getNativePtr(env, event);
-    predictor->record(*motionEvent);
+
+    android::base::Result<void> result = predictor->record(*motionEvent);
+    if (!result.ok()) {
+        jniThrowException(env, "java/lang/IllegalArgumentException",
+                          result.error().message().c_str());
+    }
 }
 
 static jobject android_view_MotionPredictor_nativePredict(JNIEnv* env, jclass clazz, jlong ptr,
                                                           jlong predictionTimeNanos) {
     MotionPredictor* predictor = reinterpret_cast<MotionPredictor*>(ptr);
-    return toJavaArray(env, predictor->predict(static_cast<nsecs_t>(predictionTimeNanos)),
-                       gMotionEventClassInfo.clazz, &android_view_MotionEvent_obtainFromNative);
+    return android_view_MotionEvent_obtainFromNative(env,
+                                                     predictor->predict(static_cast<nsecs_t>(
+                                                             predictionTimeNanos)));
 }
 
 static jboolean android_view_MotionPredictor_nativeIsPredictionAvailable(JNIEnv* env, jclass clazz,
@@ -84,15 +81,13 @@ static const std::array<JNINativeMethod, 5> gMotionPredictorMethods{{
          (void*)android_view_MotionPredictor_nativeGetNativeMotionPredictorFinalizer},
         {"nativeRecord", "(JLandroid/view/MotionEvent;)V",
          (void*)android_view_MotionPredictor_nativeRecord},
-        {"nativePredict", "(JJ)[Landroid/view/MotionEvent;",
+        {"nativePredict", "(JJ)Landroid/view/MotionEvent;",
          (void*)android_view_MotionPredictor_nativePredict},
         {"nativeIsPredictionAvailable", "(JII)Z",
          (void*)android_view_MotionPredictor_nativeIsPredictionAvailable},
 }};
 
 int register_android_view_MotionPredictor(JNIEnv* env) {
-    jclass motionEventClazz = FindClassOrDie(env, "android/view/MotionEvent");
-    gMotionEventClassInfo.clazz = MakeGlobalRefOrDie(env, motionEventClazz);
     return RegisterMethodsOrDie(env, "android/view/MotionPredictor", gMotionPredictorMethods.data(),
                                 gMotionPredictorMethods.size());
 }
diff --git a/tests/Input/src/com/android/test/input/MotionPredictorTest.kt b/tests/Input/src/com/android/test/input/MotionPredictorTest.kt
index 8b1b06fe40e8..24a567130ff0 100644
--- a/tests/Input/src/com/android/test/input/MotionPredictorTest.kt
+++ b/tests/Input/src/com/android/test/input/MotionPredictorTest.kt
@@ -124,14 +124,12 @@ class MotionPredictorTest {
         predictor.record(moveEvent)
 
         val predicted = predictor.predict(Duration.ofMillis(8).toNanos())
-        assertEquals(1, predicted.size)
-        val event = predicted[0]
-        assertNotNull(event)
+        assertNotNull(predicted)
 
         // Prediction will happen for t=12 (since it is the next input interval after the requested
         // time, 8, plus the model offset, 1).
-        assertEquals(12, event.eventTime)
-        assertEquals(30f, event.x, /*delta=*/5f)
-        assertEquals(60f, event.y, /*delta=*/15f)
+        assertEquals(12, predicted!!.eventTime)
+        assertEquals(30f, predicted.x, /*delta=*/5f)
+        assertEquals(60f, predicted.y, /*delta=*/15f)
     }
 }
diff --git a/tests/MotionPrediction/src/test/motionprediction/DrawingView.kt b/tests/MotionPrediction/src/test/motionprediction/DrawingView.kt
index f529bf77f32a..229d0c8da6e9 100644
--- a/tests/MotionPrediction/src/test/motionprediction/DrawingView.kt
+++ b/tests/MotionPrediction/src/test/motionprediction/DrawingView.kt
@@ -97,8 +97,8 @@ class DrawingView(context: Context, attrs: AttributeSet) : View(context, attrs)
         }
 
         // Draw predictions. Convert to nanos and hardcode to +20ms into the future
-        val predictionList = predictor.predict(eventTime * 1000000 + 20000000)
-        for (prediction in predictionList) {
+        val prediction = predictor.predict(eventTime * 1000000 + 20000000)
+        if (prediction != null) {
             val realEvents = events.get(prediction.deviceId)!!
             drawLine(canvas, realEvents[realEvents.size - 1], prediction, predictionPaint)
         }