A simple way to cache values that result from rather expensive operations.

Renan

Last update: Nov 11, 2022

Related tags

Utilities cached_value

Overview

cached_value

A simple way to cache values that result from rather expensive operations.

It is useful to cache values that:

Are computed from other values in a consistent way;
Can be changed given known and unknown conditions;
Should not be computed on every access (like a getter);

Installation

Add to pubspec.yaml:

    dependencies:
       cached_value: <most recent version>

Find the most recent version on pub.

Usage

There are to types of cache to choose: simple and dependent.

Simple cache

A simple cache is only invalidated manually.

  int factorial(int n) {
    if (n < 0) throw ('Negative numbers are not allowed.');
    return n <= 1 ? 1 : n * factorial(n - 1);
  }

  int originalValue = 1;
  // One can also use CachedValue.simple
  final factorialCache = CachedValue(() => factorial(originalValue));
  print(factorialCache.value); // 1

  originalValue = 6;

  print(factorialCache.value); // 1
  print(factorialCache.isValid) // true, invalid only when invalidate is called
  
  // mark as invalid
  factorialCache.invalidate();
  print(factorialCache.isValid); // false

  print(factorialCache.value); // 720
  print(factorialCache.isValid); // true

It can be refreshed manually via the refresh method:

  // ...
  originalValue = 12;
  factorialCache.refresh();

  print(factorialCache.value); // 12

Dependent cache

A dependent cache is marked as invalid if its dependency value has changed.

  int factorial(int n) {
    if (n < 0) throw ('Negative numbers are not allowed.');
    return n <= 1 ? 1 : n * factorial(n - 1);
  }
  
  int originalValue = 1;
  final factorialCache = CachedValue.dependent(
    on: () => originalValue,
    compute: () => factorial(originalValue),
  );
  print(factorialCache.value); // 1
  print(factorialCache.isValid); // true
  
  // update value
  originalValue = 6;
  print(factorialCache.isValid); // false

  print(factorialCache.value); // 720
  print(factorialCache.isValid); // true

The dependency callback on is called on every value access. So it is recommended to keep it as declarative as possible.

// Avoid this:
final someCache = CachedValue.dependent(
  on: () => someExpensiveOperation(originalValue),
  compute: () => someCacheComputation(originalValue),
);

More docs

There is more detailed docs on the API documentation.

Motivation

Some imperative APIs such as the canvas paint on Flutter render objects of Flame's components may benefit from values that can be stored and reused between more than a single frame (or paint).

In some very specific cases, I found very convenient to store some objects across frames, like Paint and TextPainter instances.

Example on a render object:

class BlurredRenderObject extends RenderObject {

  // ...

  double _blurSigma = 0.0;
  double get blurSigma => _blurSigma;
  set blurSigma(double value) {
    _blurSigma = blurSigma;
    markNeedsPaint();
  }

  // ...

  @override
  void paint(PaintingContext context, Offset offset) {
    final canvas = context.canvas;


    final paint = Paint()..maskFilter = MaskFilter.blur(
        BlurStyle.normal, blurSigma
    );
    canvas.drawRect(Rect.fromLTWH(0, 0, 100, 100), paint);
  }
}

Can be changed to:

class BlurredRenderObject extends RenderObject {

  // ...

  double _blurSigma = 0.0;
  double get blurSigma => _blurSigma;
  set blurSigma(double value) {
    _blurSigma = blurSigma;
    markNeedsPaint();
  }

  // Add cache:
  late final paintCache = CachedValue.dependent(
    on: () => blurSigma,
    compute: () =>
        Paint()..maskFilter = MaskFilter.blur(BlurStyle.normal, blurSigma),
  );

  // ...

  @override
  void paint(PaintingContext context, Offset offset) {
    final canvas = context.canvas;

    // use cache:
    final paint = paintCache.value;
    canvas.drawRect(Rect.fromLTWH(0, 0, 100, 100), paint);
  }
}

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Comments

Add ttl, update docs and move to declarative API

Move to an API that allows composability:

Simple:

// this is unchanged, not a shorthand anymore, the default:
CachedValue(() => factorial(originalValue));

// from:
CachedValue.simple(() => factorial(originalValue));

// to:
CachedValue(() => factorial(originalValue));

Dependent:

// from:
final factorialCache = CachedValue.dependent(
  on: () => originalValue,
  compute: () => factorial(originalValue),
);

// to:
final factorialCache = CachedValue(
  () => factorial(originalValue),
).withDependency(() => originalValue);

opened by renancaraujo 0

[Request] Add large cache option

So I really like where this package is going.

Something I'd like to see is to have dependant cache hold multiple values.

I'd like to build a cache of multiple values, and only compute if that value is new.

For example

string getName(int id) {
    switch(id) {
        case 0: 
            return "zero";
       case 1:
            return "one";
      ....
    }
} 

... 
int id = 0;
final factorialCacheFirst = CachedValue.dependent(
    on: () => id,
    compute: () => getName(id),
  ); // computes

final factorialCacheSecond = CachedValue.dependent(
    on: () => id,
    compute: () => getName(id),
  ); // returns cached value

id = 1;
final factorialCacheThird = CachedValue.dependent(
    on: () => id,
    compute: () => getName(id),
  ); // computes

id = 0;
final factorialCacheSecond = CachedValue.dependent(
    on: () => id,
    compute: () => getName(id),
  ); // returns cached value

id = 1;
final factorialCacheThird = CachedValue.dependent(
    on: () => id,
    compute: () => getName(id),
  ); // computes

The summary is I'd love to cache a function's return value for a variety of inputs.

opened by cadaniel 1

A simple way to cache values that result from rather expensive operations.

Related tags

Overview

cached_value

Installation

Usage

Simple cache

Dependent cache

More docs

Motivation

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Owner

Renan

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