Fix initialization of AnimatorSet

Bug: 289165914

When I previously changed the initialization code for AnimatorSet,
I had simplified a section because the initialization contained
all of the complexity. I later reverted the change to how I
initialized the child animators, but did not revert a part of
the code.

This reverts the change to initialization during start.

Test: broken animation sample, new test
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:466f64288de4052960d188526dc5bfce70ff3ca1)
Merged-In: I7333865f677f42964b023bcd9ee768ff4ec0137d
Change-Id: I7333865f677f42964b023bcd9ee768ff4ec0137d

core/java/android/animation/AnimatorSet.java

@@ -1346,8 +1346,26 @@ public final class AnimatorSet extends Animator implements AnimationHandler.Anim
         }
         // Set the child animators to the right end:
         if (mShouldResetValuesAtStart) {
-            initChildren();
-            skipToEndValue(!mReversing);
+            if (isInitialized()) {
+                skipToEndValue(!mReversing);
+            } else if (mReversing) {
+                // Reversing but haven't initialized all the children yet.
+                initChildren();
+                skipToEndValue(!mReversing);
+            } else {
+                // If not all children are initialized and play direction is forward
+                for (int i = mEvents.size() - 1; i >= 0; i--) {
+                    if (mEvents.get(i).mEvent == AnimationEvent.ANIMATION_DELAY_ENDED) {
+                        Animator anim = mEvents.get(i).mNode.mAnimation;
+                        // Only reset the animations that have been initialized to start value,
+                        // so that if they are defined without a start value, they will get the
+                        // values set at the right time (i.e. the next animation run)
+                        if (anim.isInitialized()) {
+                            anim.skipToEndValue(true);
+                        }
+                    }
+                }
+            }
         }
 
         if (mReversing || mStartDelay == 0 || mSeekState.isActive()) {

core/tests/coretests/src/android/animation/AnimatorSetActivityTest.java

@@ -24,6 +24,7 @@ import static org.junit.Assert.assertTrue;
 import android.util.Property;
 import android.view.View;
 
+import androidx.annotation.NonNull;
 import androidx.test.annotation.UiThreadTest;
 import androidx.test.filters.SmallTest;
 import androidx.test.rule.ActivityTestRule;
@@ -36,6 +37,8 @@ import org.junit.Rule;
 import org.junit.Test;
 
 import java.util.ArrayList;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
 
 @SmallTest
 public class AnimatorSetActivityTest {
@@ -613,6 +616,68 @@ public class AnimatorSetActivityTest {
         });
     }
 
+    @Test
+    public void initAfterStartNotification() throws Throwable {
+        Property<int[], Integer> property = new Property<>(Integer.class, "firstValue") {
+            @Override
+            public Integer get(int[] target) {
+                throw new IllegalStateException("Shouldn't be called");
+            }
+
+            @Override
+            public void set(int[] target, Integer value) {
+                target[0] = value;
+            }
+        };
+        int[] target = new int[1];
+        ObjectAnimator animator1 = ObjectAnimator.ofInt(target, property, 0, 100);
+        ObjectAnimator animator2 = ObjectAnimator.ofInt(target, property, 0, 100);
+        ObjectAnimator animator3 = ObjectAnimator.ofInt(target, property, 0, 100);
+        animator1.setDuration(10);
+        animator2.setDuration(10);
+        animator3.setDuration(10);
+        AnimatorSet set = new AnimatorSet();
+        set.playSequentially(animator1, animator2, animator3);
+        final int[] values = new int[4];
+        animator2.addListener(new AnimatorListenerAdapter() {
+            @Override
+            public void onAnimationStart(@NonNull Animator animation, boolean isReverse) {
+                values[0] = target[0];
+                animator2.setIntValues(target[0], target[0] + 100);
+            }
+
+            @Override
+            public void onAnimationEnd(@NonNull Animator animation, boolean isReverse) {
+                values[1] = target[0];
+            }
+        });
+        animator3.addListener(new AnimatorListenerAdapter() {
+            @Override
+            public void onAnimationStart(@NonNull Animator animation, boolean isReverse) {
+                values[2] = target[0];
+                animator3.setIntValues(target[0], target[0] + 100);
+            }
+
+            @Override
+            public void onAnimationEnd(@NonNull Animator animation, boolean isReverse) {
+                values[3] = target[0];
+            }
+        });
+        final CountDownLatch latch = new CountDownLatch(1);
+        set.addListener(new AnimatorListenerAdapter() {
+            @Override
+            public void onAnimationEnd(@NonNull Animator animation, boolean isReverse) {
+                latch.countDown();
+            }
+        });
+        mActivityRule.runOnUiThread(() -> set.start());
+        assertTrue(latch.await(1, TimeUnit.SECONDS));
+        assertEquals(100, values[0]);
+        assertEquals(200, values[1]);
+        assertEquals(200, values[2]);
+        assertEquals(300, values[3]);
+    }
+
     /**
      * Check that the animator list contains exactly the given animators and nothing else.
      */