Fix regression for strange values of slider using precision property

[Amitpatil215]

Description

Added precision property to handle decimal places of slider's value.

Root cause of the issue

lets say,

min: 0,
max: 100,
division:100,
value: 0.07 // value sent to _lerp()

flutter/packages/flutter/lib/src/material/slider.dart

Lines 600 to 603 in 9b2d32b

	double _lerp(double value) {
	assert(value >= 0.0);
	assert(value <= 1.0);
	return value * (widget.max - widget.min) + widget.min;

According to above method we end up having

0.07 * (100 - 0) + 0

expected value: 7.0
actual value : 7.000000000000001

And this value is shown to user, that's why we having regression in slider's label.

Refer issue on Dart-lang repo

dart-lang/sdk#23315

Related issue

fixes #73787

Test

added test Slider respects precision value

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

[pedromassango]

What if precision is null? will we still face the same issue?

I suggest to fix the issue directly instead of introduce a new property. FYI, this was already working without any additional property so it still should be posible to fix the issue without any additional property.

Maybe just round the resulting value?

[Amitpatil215]

Hi @pedromassango , Thanks for the review!

Maybe just round the resulting value?

Example

Slider(
	value: value,
	label: '$value',
	min: 0,
	max: 10,
	divisions: 20,
	onChanged: (v) => setState(() {
		value = v;
		print(value);
		}),
 	),

User wants 20 divisions of 10 that means 0.5 as a single division. If we round the value it end up giving us values 0, 1.0, 2.0

What if precision is null? will we still face the same issue?

Thanks for pointing out this. We could set precision to 0 (Which is equivalent to round) as a default. and modify assertion to

assert(precision != null && (precision >= 0 && precision <= 20))

This approach may affect many users as it is a breaking change. (That's the only reason I made it null by default).

Why I chosen to add precision property workaround?

instead of rounding it up internally to certain precision to 2,3, or foo. Cause we never know what exact precision user wants. So I tried to give it a bit customizable approach.

If I'm wrong anywhere please point it out to me.

Meanwhile I try to figure out some other workarounds to fix this issue without introducing new property.

Thanks!

[gspencergoog]

I agree with @pedromassango, it seems possible to fix this without introducing a new parameter, since it used to print with less precision (that math will always result in a rounding error in the mantissa for IEEE doubles, so it had to be printing in lower precision before).

Maybe figure out why the regression happened, and see if that sheds some light? Probably you could fix the formatting code in the label, rather than modifying the value.

Also, even if you end up with this method, it's probably a lot faster to do something like this, instead of creating strings and parsing them.

import 'dart:math' as math;
// ...
final double factor = math.pow(10, precision);
final double lerpValue = (factor * _lerp(value)).roundToDouble() / factor;

[gspencergoog]

(sorry, I wrote the above comment months ago, I just hadn't submitted it)

[HansMuller]

CC @JoseAlba @clocksmith

[flutter-dashboard]

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[gspencergoog]

@Amitpatil215 Are you still planning to work on this PR? If not, could you please close it?