acceptGesture method Null safety

Implementation

@override
void acceptGesture(int pointer) {
  assert(!_acceptedActivePointers.contains(pointer));
  _acceptedActivePointers.add(pointer);
  if (_state != _DragState.accepted) {
    _state = _DragState.accepted;
    final OffsetPair delta = _pendingDragOffset;
    final Duration timestamp = _lastPendingEventTimestamp!;
    final Matrix4? transform = _lastTransform;
    final Offset localUpdateDelta;
    switch (dragStartBehavior) {
      case DragStartBehavior.start:
        _initialPosition = _initialPosition + delta;
        localUpdateDelta = Offset.zero;
        break;
      case DragStartBehavior.down:
        localUpdateDelta = _getDeltaForDetails(delta.local);
        break;
    }
    _pendingDragOffset = OffsetPair.zero;
    _lastPendingEventTimestamp = null;
    _lastTransform = null;
    _checkStart(timestamp, pointer);
    if (localUpdateDelta != Offset.zero && onUpdate != null) {
      final Matrix4? localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
      final Offset correctedLocalPosition = _initialPosition.local + localUpdateDelta;
      final Offset globalUpdateDelta = PointerEvent.transformDeltaViaPositions(
        untransformedEndPosition: correctedLocalPosition,
        untransformedDelta: localUpdateDelta,
        transform: localToGlobal,
      );
      final OffsetPair updateDelta = OffsetPair(local: localUpdateDelta, global: globalUpdateDelta);
      final OffsetPair correctedPosition = _initialPosition + updateDelta; // Only adds delta for down behaviour
      _checkUpdate(
        sourceTimeStamp: timestamp,
        delta: localUpdateDelta,
        primaryDelta: _getPrimaryValueFromOffset(localUpdateDelta),
        globalPosition: correctedPosition.global,
        localPosition: correctedPosition.local,
      );
    }
    // This acceptGesture might have been called only for one pointer, instead
    // of all pointers. Resolve all pointers to `accepted`. This won't cause
    // infinite recursion because an accepted pointer won't be accepted again.
    resolve(GestureDisposition.accepted);
  }
}