recommendDeferredLoading method Null safety

bool recommendDeferredLoading(
  1. double velocity,
  2. ScrollMetrics metrics,
  3. BuildContext context

Provides a heuristic to determine if expensive frame-bound tasks should be deferred.

The velocity parameter must not be null, but may be positive, negative, or zero.

The metrics parameter must not be null.

The context parameter must not be null. It normally refers to the BuildContext of the widget making the call, such as an Image widget in a ListView.

This can be used to determine whether decoding or fetching complex data for the currently visible part of the viewport should be delayed to avoid doing work that will not have a chance to appear before a new frame is rendered.

For example, a list of images could use this logic to delay decoding images until scrolling is slow enough to actually render the decoded image to the screen.

The default implementation is a heuristic that compares the current scroll velocity in local logical pixels to the longest side of the window in physical pixels. Implementers can change this heuristic by overriding this method and providing their custom physics to the scrollable widget. For example, an application that changes the local coordinate system with a large perspective transform could provide a more or less aggressive heuristic depending on whether the transform was increasing or decreasing the overall scale between the global screen and local scrollable coordinate systems.

The default implementation is stateless, and simply provides a point-in- time decision about how fast the scrollable is scrolling. It would always return true for a scrollable that is animating back and forth at high velocity in a loop. It is assumed that callers will handle such a case, or that a custom stateful implementation would be written that tracks the sign of the velocity on successive calls.

Returning true from this method indicates that the current scroll velocity is great enough that expensive operations impacting the UI should be deferred.


bool recommendDeferredLoading(double velocity, ScrollMetrics metrics, BuildContext context) {
  assert(velocity != null);
  assert(metrics != null);
  assert(context != null);
  if (parent == null) {
    final double maxPhysicalPixels = WidgetsBinding.instance!.window.physicalSize.longestSide;
    return velocity.abs() > maxPhysicalPixels;
  return parent!.recommendDeferredLoading(velocity, metrics, context);