Repository: flutter
Issue: OverscrollNotification does not work on iOS
Labels: platform-ios,framework,f: material design,f: scrolling,f: cupertino,d: api docs,customer: alibaba,has reproducible steps,P2,workaround available,found in release: 3.7,found in release: 3.10,team-ios,triaged-ios

## Steps to Reproduce

1. Add an OverscrollNotification to a ListView, take Gallery as an example
2. Scroll down to the bottom of the list
3. There's no notification callback on iOS, while OK on Android. 

Modify the Gallery code as an example,

list_demo.dart  line 237
The code after modified:
```dart
body: new Scrollbar(
        child: new NotificationListener<OverscrollNotification>(child: new ListView(
          padding: new EdgeInsets.symmetric(vertical: _dense ? 4.0 : 8.0),
          children: listTiles.toList(),
        ),
          onNotification: (OverscrollNotification notification) {
            print("====shubin debug: notification ${notification.overscroll}");
            if (notification.overscroll > 0 ) {
              print("====shubin debug: notification>0 ${notification.overscroll}");
            }
            return false;
          },

        ),
      ),
```

The code before modified:
```dart
body: new Scrollbar(
        child: new ListView(
          padding: new EdgeInsets.symmetric(vertical: _dense ? 4.0 : 8.0),
          children: listTiles.toList(),
        ),
   
        ),
      ),
```

To solve this problem, we're using a alternative way on iOS like below. But is the above a problem?
```dart
void _handleScrollNotification() {
  if (widget.controller.position.pixels >
      widget.controller.position.maxScrollExtent) {
    if (widget.adapter.hasMore()) {
      widget.adapter.loadMore();
    }
  }
}
```


## Logs

There are logs like "====shubin debug: notification" on Android, but failed on iOS.

Flutter doctor -v:

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
✓] Flutter (Channel beta, v0.3.2, on Mac OS X 10.12.6 16G1314, locale zh-Hans-HK)
    • Flutter version 0.3.2 at /Users/guoyou/projects/flutter
    • Framework revision 44b7e7d3f4 (4 weeks ago), 2018-04-20 01:02:44 -0700
    • Engine revision 09d05a3891
    • Dart version 2.0.0-dev.48.0.flutter-fe606f890b

[✓] Android toolchain - develop for Android devices (Android SDK 27.0.3)
    • Android SDK at /Users/guoyou/Library/Android/sdk
    • Android NDK at /Users/guoyou/Library/Android/sdk/ndk-bundle
    • Platform android-27, build-tools 27.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)
    • All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 9.2)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 9.2, Build version 9C40b
    • ios-deploy 1.9.2
    • CocoaPods version 1.4.0

[✓] Android Studio (version 3.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 24.2.1
    • Dart plugin version 173.4700
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)

[✓] IntelliJ IDEA Community Edition (version 2017.2.3)
    • IntelliJ at /Applications/IntelliJ IDEA CE.app
    • Flutter plugin installed
    • Dart plugin version 172.3968.27

[✓] Connected devices (3 available)
    • Android SDK built for x86 • emulator-5554                            • android-x86 • Android 8.0.0 (API 26) (emulator)
    • iPhone (3)                • cd8bf03b9894bcc00eb25fdae9473ff0b2df095a • ios         • iOS 10.3.2
    • iPhone X                  • 8D4A21F9-E417-43CE-8B40-52CEAB6B7D04     • ios         • iOS 11.2 (simulator)

```