Send UNIX timestamp along with JS touch events, instead of systemUptime

Summary:
We want to be able to instrument touch processing delays in JS, which does not have access to systemUptime; therefore we want a UNIX timestamp, which JS has access to and can compare to the touch time.

It only matters that there is relative consistency between multiple touch events in JS, which is still the case; so this should have no impact on product code.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D26705429

fbshipit-source-id: 0f2db726048fcab9a30e830970d7d8a8d2eae446

Author: JoshuaGross

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/Event.java

@@ -38,6 +38,15 @@ public abstract class Event<T extends Event> {
   private long mTimestampMs;
   private int mUniqueID = sUniqueID++;
 
+  // Android native Event times use 'uptimeMillis', and historically we've used `uptimeMillis`
+  // throughout this Event class as the coalescing key for events, and for other purposes.
+  // To get an accurate(ish) absolute UNIX time for the event, we store the initial clock time here.
+  // uptimeMillis can then be added to this to get an accurate UNIX time.
+  // However, we still default to uptimeMillis: you must explicitly request UNIX time if you want
+  // that; see `getUnixTimestampMs`.
+  public static final long sInitialClockTimeUnixOffset =
+      SystemClock.currentTimeMillis() - SystemClock.uptimeMillis();
+
   protected Event() {}
 
   @Deprecated
@@ -58,8 +67,10 @@ protected void init(int viewTag) {
   protected void init(int surfaceId, int viewTag) {
     mSurfaceId = surfaceId;
     mViewTag = viewTag;
-    mTimestampMs = SystemClock.uptimeMillis();
     mInitialized = true;
+
+    // This is a *relative* time. See `getUnixTimestampMs`.
+    mTimestampMs = SystemClock.uptimeMillis();
   }
 
   /** @return the view id for the view that generated this event */
@@ -80,6 +91,11 @@ public final long getTimestampMs() {
     return mTimestampMs;
   }
 
+  /** @return the time at which the event happened as a UNIX timestamp, in milliseconds. */
+  public final long getUnixTimestampMs() {
+    return sInitialClockTimeUnixOffset + mTimestampMs;
+  }
+
   /** @return false if this Event can *never* be coalesced */
   public boolean canCoalesce() {
     return true;

ReactAndroid/src/main/java/com/facebook/react/uimanager/events/TouchesHelper.java

@@ -64,7 +64,7 @@ private static WritableArray createsPointersArray(
       touch.putDouble(LOCATION_Y_KEY, PixelUtil.toDIPFromPixel(locationY));
       touch.putInt(TARGET_SURFACE_KEY, surfaceId);
       touch.putInt(TARGET_KEY, reactTarget);
-      touch.putDouble(TIMESTAMP_KEY, event.getTimestampMs());
+      touch.putDouble(TIMESTAMP_KEY, event.getUnixTimestampMs());
       touch.putDouble(POINTER_IDENTIFIER_KEY, motionEvent.getPointerId(index));
       touches.pushMap(touch);
     }