Fix timestamp of touch events should use system startup time

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -421,16 +421,24 @@ - (void)loadView {
   _scrollView.reset(scrollView);
 }
 
+- (flutter::PointerData)generatePointerDataForFake {
+  flutter::PointerData pointer_data;
+  pointer_data.Clear();
+  pointer_data.kind = flutter::PointerData::DeviceKind::kTouch;
+  // `UITouch.timestamp` is defined as seconds since system startup. Synthesized events can get this
+  // time with `NSProcessInfo.systemUptime`. See
+  // https://developer.apple.com/documentation/uikit/uitouch/1618144-timestamp?language=objc
+  pointer_data.time_stamp = [[NSProcessInfo processInfo] systemUptime] * kMicrosecondsPerSecond;
+  return pointer_data;
+}
+
 static void SendFakeTouchEvent(FlutterEngine* engine,
                                CGPoint location,
                                flutter::PointerData::Change change) {
   const CGFloat scale = [UIScreen mainScreen].scale;
-  flutter::PointerData pointer_data;
-  pointer_data.Clear();
+  flutter::PointerData pointer_data = [[engine viewController] generatePointerDataForFake];
   pointer_data.physical_x = location.x * scale;
   pointer_data.physical_y = location.y * scale;
-  pointer_data.kind = flutter::PointerData::DeviceKind::kTouch;
-  pointer_data.time_stamp = [[NSDate date] timeIntervalSince1970] * kMicrosecondsPerSecond;
   auto packet = std::make_unique<flutter::PointerDataPacket>(/*count=*/1);
   pointer_data.change = change;
   packet->SetPointerData(0, pointer_data);
@@ -759,14 +767,9 @@ - (void)flushOngoingTouches {
     // touches to the framework so nothing gets orphaned.
     for (NSNumber* device in _ongoingTouches.get()) {
       // Create fake PointerData to balance out each previously started one for the framework.
-      flutter::PointerData pointer_data;
-      pointer_data.Clear();
-
-      // Use current time.
-      pointer_data.time_stamp = [[NSDate date] timeIntervalSince1970] * kMicrosecondsPerSecond;
+      flutter::PointerData pointer_data = [self generatePointerDataForFake];
 
       pointer_data.change = flutter::PointerData::Change::kCancel;
-      pointer_data.kind = flutter::PointerData::DeviceKind::kTouch;
       pointer_data.device = device.longLongValue;
       pointer_data.pointer_identifier = 0;
 

shell/platform/darwin/ios/framework/Source/FlutterViewControllerTest.mm

@@ -7,6 +7,7 @@
 
 #include "flutter/fml/platform/darwin/message_loop_darwin.h"
 #import "flutter/lib/ui/window/platform_configuration.h"
+#include "flutter/lib/ui/window/pointer_data.h"
 #import "flutter/lib/ui/window/viewport_metrics.h"
 #import "flutter/shell/platform/darwin/common/framework/Headers/FlutterBinaryMessenger.h"
 #import "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
@@ -138,6 +139,7 @@ - (void)startKeyBoardAnimation:(NSTimeInterval)duration;
 - (void)ensureViewportMetricsIsCorrect;
 - (void)invalidateDisplayLink;
 - (void)addInternalPlugins;
+- (flutter::PointerData)generatePointerDataForFake;
 @end
 
 @interface FlutterViewControllerTest : XCTestCase
@@ -1092,4 +1094,17 @@ - (void)testMouseSupport API_AVAILABLE(ios(13.4)) {
       dispatchPointerDataPacket:std::make_unique<flutter::PointerDataPacket>(0)];
 }
 
+- (void)testFakeEventTimeStamp {
+  FlutterViewController* vc = [[FlutterViewController alloc] initWithEngine:self.mockEngine
+                                                                    nibName:nil
+                                                                     bundle:nil];
+  XCTAssertNotNil(vc);
+
+  flutter::PointerData pointer_data = [vc generatePointerDataForFake];
+  int64_t current_micros = [[NSProcessInfo processInfo] systemUptime] * 1000 * 1000;
+  int64_t interval_micros = current_micros - pointer_data.time_stamp;
+  const int64_t tolerance_millis = 2;
+  XCTAssertTrue(interval_micros / 1000 < tolerance_millis,
+                @"PointerData.time_stamp should be equal to NSProcessInfo.systemUptime");
+}
 @end