it-depends

Lightweight dependency tracking / caching library for JavaScript

About the library

it-depends library is the attempt to create a lightweight dependency tracking library that helps you organize your JavaScript code using the approach to build a model proposed in KnockoutJS

it-depends does not worry about UI - it is solely for managing dependencies between data/model elements.

The key point in using the library is to define as less state values as possible and define the rest of the model as computed values, based on them.

All computed values are lazy and cacheable. It means that if you define the computed value based on other values it is not calculated immediately.

The calculation function will be called on the first attempt to read the value.

The calculation function will not be called immediately after the value of any dependency is changed. You must read the value again to trigger the execution of calculation function.

When to use?

Your code has quite a little state and a lot of values that can be computed from this state.

Your aim is to have easy to test, debug and maintain self-describing pieces of code without side effects. You are OK with building code that way (it looks similar to Functional Programming).

You have some state that should be calculated on demand only and you do not want to clutter your code with additional caching logic.

You use a framework that is responsible for rendering the model on the screen.

Limitation(to be removed): the framework is built in Angular way - it makes decisions when it is better to query data from the model by itself and does not require you to notify that some value has changed (see #3, #5 - when they will be implemented, the limitation will be removed)

History

I created the library when I was refactoring one of complex screens in an AngularJS application.

The corresponding UI code contained logic to handle the checked/unchecked state of checkboxes mixed with the quite complex business rules for calculating whether the checkbox is enabled or disabled (based on the checked/unchecked state).

The code was slow because for any (even unrelated to checkboxes) change on UI Angular restarted $digest loop and required the checked and enabled states. I preferred to do not use $scope.$watch to update values on change in checked/unchecked state, because I did not want to clutter my scope with implementation details of intermediate values, I wanted to always have some exact state (without intermediate transition states, when some values were updated and some of them - not) and I did not want to manage dependencies manually.

I refactored code to extract these calculation functions into small pieces, implicitly declared dependencies between them, so the framework became able to handle both dependencies and caching for me.

After that I allowed AngularJS to query my $scope so that it could get actual values from observable values and cached computed.

Installation and usage

NodeJS

npm install it-depends

In your application include the module and use it:

var itDepends = require('it-depends');

// your code goes here:
var firstName = itDepends.value('James');
...

AMD/RequireJS

Download the latest release

Place where it is suitable according to your AMD/RequireJS configuration

Use it as a dependency of AMD module:

define(['it-depends'], function(itDepends) {
    // your code goes here:
    var firstName = itDepends.value('James');
    ...
});

Browser globals

Download the latest release

Place where it is suitable

Include it into the page:

<script src="js/ext/it-depends.js"></script>
<script src="js/your-application.js"></script>

You will be able to use itDepends global variable in your-application.js:

// your code goes here:
var firstName = itDepends.value('James');
...

API

`itDepends.value(initialValue)`

Creates observable value object.

Parameters:

initialValue (optional, any value, undefined by default) - the value to be stored in the observable when created.

Returns:

the observable value object.

`observableValue()`

Reads the current value of observable value object.

Returns:

the current value of observable value object.

`observableValue.write(newValue)`

Updates the current value of observable value object.

Parameters:

newValue (mandatory, any value) - the new value to write to observable value object.

Returns:

void

`itDepends.computed(calculator)`

Creates computed value object.

Parameters:

calculator (mandatory, function (...parameters: any[]) -> any) - the function that will be called later to (re)calculate the value of computed. Gets called when you request the value for the first time, or when you request the value when some of dependencies (values/computeds) was changed. Should return(calculate) the current value of the computed value object. Must not have side-effects. Calculator function can take parameters. In this case the resulting computed behaves as a set of elementary computeds bound to each distinct set of arguments.

Returns:

the computed value object.

`computedValue(...parameters)`

Reads the current value of computed value object for the given set of parameters. calculator will be called if it is the first call or if a change was made to some of the dependencies (values/computeds) called from calculator previous time. Otherwise the cached current value will be returned. During the call dependencies (values/computeds) used in the calculator will be recorded and stored in the list of dependencies.

Parameters:

parameters (varadic, any[]) - parameters that will be passed to the calculator. Calculator will be called only once for each unique set of parameters unless dependencies are changed.

Returns:

the current value of computed value object for the given parameters.

`itDepends.promiseValue(promise, initialValue)`

Creates promise value wrapper object.

Parameters:

promise (mandatory, Promise) - the promise object that is the source of the value.
initialValue (optional, any, undefined by default) - the value to be stored in the promise value when created.

Returns:

the promise value object filled with the initialValue or undefined if none specified. Depending on the concrete Promise implementation can be filled with the value of a Promise if it was resolved already.

`promiseValue()`

Reads the current value of promise value wrapper object.

Returns:

the current value of promise value wrapper object: initialValue of an object or the value that was used to resolve the Promise.

`itDepends.onChange(callback)`

Creates the subscription on a change to any observable value.

Parameters:

callback (mandatory, function (changed: observableValue, from: any, to: any) -> void ) - the function that will be called immediately when a change is made to any observable value. The callback receives the changed observableValue, old and new value of observable object.

Returns:

the subscription object that can be used to control the subscription.

`observableValue.onChange(callback)`

Creates the subscription on a change to this observable value.

Parameters:

callback (mandatory, function (changed: observableValue, from: any, to: any) -> void ) - the function that will be called immediately when a change is made to this observable value. The callback receives the changed observableValue, old and new value of observable object.

Returns:

the subscription object that can be used to control the subscription.

`subscription.disable()`

Disables subscription, so change notifications will stop coming any more, unless it is enabled again with enable method.

Returns:

void

`subscription.enable()`

Enables subscription if it was disabled previously, so change notifications will start coming again.

Returns:

void

Example code (Try it in Tonic)

var firstName = itDepends.value('James');
var lastName  = itDepends.value('Bond');

var fullName = itDepends.computed(function(){
    return 'Hello, ' + firstName() + ' ' + lastName();
});

console.log(fullName()); // Hello, James Bond

firstName.write('Jack');

console.log(fullName()); // Hello, Jack Bond

Contributing

I will be glad if you will join the development.

There are lot of things you can help me with. Here are few of them:

Enhance documentation / write samples
Raise/discuss/prepare PRs for improvements/issues
Add more unit tests
Help me with publishing to different repositories, automate it: npm, Bower, NuGet
Move to TypeScript / write typings
Make library known to publicity

Contribution Guide

Development environment prerequisites:

NodeJS, npm
Gulp. Install with: npm install --global gulp-cli

Workflow:

Create a fork
Clone your git repository
Run npm install
Run gulp, all tests should be green, build should work
Make a code change
Run gulp, all tests should be green, build should work, fix if failed
Commit, push
Create PR (pull request)

Author

Sergey Gerasimov

License

MSPL (Microsoft Public License)