Functions to move to dart:core from dart:ui

[Hixie]

These are functions and types that we have in dart:ui (Flutter's core library) that really should just be in dart:core:

• lerpDouble, which should really just be a static method on num:
  https://master-api.flutter.dev/flutter/dart-ui/lerpDouble.html
• clampDouble, which should really just be a method on double:
  https://master-api.flutter.dev/flutter/dart-ui/clampDouble.html
• hashValues and hashList, which should really be in dart:core somewhere:
  https://master-api.flutter.dev/flutter/dart-ui/hashValues.html
• VoidCallback, widely used in Flutter code, also defined in dart:html
  https://master-api.flutter.dev/flutter/dart-ui/VoidCallback.html

[sethladd]

cc @floitschG

[lrhn]

The lerpDouble function could be put on double (or num) as

double interpolate(num a, num b) => a + (b - a) * this.toDouble();

(it should return double, not num).

I'm more concerned about the hash functions.
The 373 (* 37 + x)* is a classic, so we can at least say that we are no worse than everybody else, but it's not great. It's quick, though.
We have other hash implementations spread around the source code that we could centralize and expose.

[Hixie]

Just to be clear, the algorithms used in the Flutter hash functions today are horrible; we'd love to have better ones if you take these functions over. We're planning on changing the implementations of the ones we use anyway (flutter/flutter#859). It's the interface we're interested in moving over to core Dart, not the precise implementations.

If lerpDouble made it over that would be great. We currently have lerp functions on many of our classes (Point, Rect, Color, Border, BoxDecoration, etc) but since there's no function on "num" for it, we have to call this function instead when we need to lerp a double. Calling it "interpolate" would be fine, we'd change our functions to match (though note that "lerp" is a pretty widely used term of art).

[kasperpeulen]

I often see people use those hash functions in Dart libraries:

https://github.com/google/quiver-dart/blob/master/lib/src/core/hash.dart

[floitschG]

We discussed the lerp function, and it seems simpler if this stays a helper function. It's a common operation in drawing/3D, but is otherwise not much used. Maybe we can get it through extension methods (no promises).