sahara

An inversion-of-control container for managing dependencies. Supports constructor, property and method injection

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Sahara

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Sahara is an inversion of control container. It supports constructor, property and method injection, parsing the method signature automatically to determine dependencies. It also supports interception for function calls.

Topics

Installation

Install using NPM: npm install sahara

Usage

API

All of these are explained in mind-numbing detail below.

sahara.Container = function() {};
Container.prototype = {
    registerType: function(ctor[, options]) {},
    registerType: function(ctor[, key, lifetime, injection, injection...]) {},

    registerInstance: function(instance[, options]) {},
    registerInstance: function(instance[, key, lifetime, injection, injection...]) {},

    registerFactory: (factory[, options]) {},
    registerFactory: (factory[, key, lifetime, injection, injection...]) {},

    isRegistered: function(key) {},

    resolve: function(key, callback) {}
    resolveSync: function(key) {},
    tryResolveSync: function(key) {},

    inject: function(instance, key, callback) {}
    injectSync: function(instance[, key]) {},

    intercept: function(matcher, callHandler[, callHandler...]) {},

    createChildContainer: function() {}
};

sahara.inject = {
    property: function(name, key) {},
    propertyValue: function(name, value) {},
    method: function(name, args) {}
};

sahara.lifetime = {
    transient: function() {},
    memory: function() {},
    external: function(manager) {}
};

Of note: there are two ways to register something in the container. You can pass an options object, or you can specify everything explicitly in the arguments. So these are equivalent:

var options = {
    key: 'foo',
    lifetime: lifetime.memory(),
    injections: [ inject.property('foo', 'bar'), inject.method('doStuff', [ 'arg1', 'arg2' ]) ]
};

container.registerInstance({}, options);
container.registerInstance({}, options.key, options.lifetime, options.injections[0], options.injections[1]);

Registering an instance

Sahara is simply a container for objects and object dependencies. In the simplest case, you shove an object into it using registerInstance() and retrieve it using resolveSync() (or, asynchronously, resolve()):

var Container = require('sahara').Container,
    container = new Container();

var myObject = { oh: 'hi mark' };
container.registerInstance(myObject, 'MyObject');
var instance = container.resolveSync('MyObject');
console.log(myObject === instance); //true, they are literally the same instance

But that's not abundantly interesting. The magic comes when you want to inject your myObject instance somewhere else. Like into another object via a constructor argument:

function AnotherObject(/** MyObject */theObject) {
    this.obj = theObject;
}

container.registerType(AnotherObject);

var anotherObject = container.resolveSync(AnotherObject);
console.log(anotherObject.obj); // { oh: 'hai mark' }

This will dynamically create an instance of AnotherObject, using the container to resolve its dependencies (in this case, it's dependency was of type MyObject).

Registering a type

You might have noticed there was a little bit of hand-waving going on up above. When registering a type, Sahara will do some fancy regular expression voodoo to ferret out the constructor signature, as well as the types of the constructor arguments.

Since JavaScript isn't even close to a statically typed language, and doesn't have a reflection API, we have to use doc comments to specify the types. Specifically, they must be inline doc comments.

function Foo(message) { this.message = message; }
function Bar(/** Foo */foo) { this.foo = foo; }
function Baz(/** Bar */bar) { this.bar = bar; }

container
    .registerInstance(new Foo('oh hi mark'))
    .registerType(Bar)
    .registerType(Baz)
    .resolveSync(Baz)
    .bar.foo.message; //"oh hi mark"

NOTE: When using registerType, you MUST register a function that is a constructor.

Named functions

By default, Sahara will use the name of the constructor as the resolution key. As a means of convenience (as you can see by most of the examples on this page), you can also pass the constructor to the resolveSync() function instead of the resolution key.

If you pass a constructor to resolveSync(), it'll use ctor.name to deduce the resolution key.

Otherwise, you need to pass in a key property in the options argument to the register*() methods. Alternatively, as a means of convenience, you can also just pass a straight-up string as the second argument in lieu of the options object, and it will be used as the key.

function Foo() {}

//the following registrations are equivalent
container.registerType(Foo);
container.registerType(Foo, { key: 'Foo' });
container.registerType(Foo, 'Foo');

//the following resolutions are equivalent
container.resolveSync(Foo); //uses Foo.name
container.resolveSync('Foo');

When registering instances, it'll try and use instance.constructor.name to get the resolution key.

function Foo() {}

var instance = new Foo();

//the following registrations are equivalent
container.registerInstance(instance)
container.registerInstance(instance, { key: 'Foo' });
container.registerInstance(instance, 'Foo');

Anonymous functions

If you don't have a named function, you can also register an anonymous function, but you must provide a resolution key for it:

var foo = function() {};
container.registerType(foo, 'MySpecialName');
var fooInstance = container.resolveSync('MySpecialName');

//with an instance
container.registerInstance(fooInstance, 'AnotherSpecialName');
var sameInstance = container.resolveSync('AnotherSpecialName');

Registering a factory

In some cases you'll want to defer the creation of an object until it's absolutely needed. You can do this by using container.registerFactory(). Your factory function should take in one argument, the container.

This is mostly used as a replacement for registerInstance(), but for the times when you don't want to create the instance immediately.

Note that the key option is required when using registerFactory().

function Foo() {}

container.registerFactory(function(container) {
    return new Foo();
}, 'MyKey');

container.resolveSync('MyKey');

Determining if something is registered

Use the container.isRegistered() function.

function foo() {}

container.registerType(foo);

console.log(container.isRegistered(foo));   //true
console.log(container.isRegistered('foo')); //true
console.log(container.isRegistered('bar')); //false

Asynchronous resolution

All examples given are synchronous. However, if you need to resolve something asynchronously, you can use the resolve(key, callback) function. Note that everything (from resolution, to injection, to object building) will be asynchronous, so bear that in mind.

This is most relevant for registerFactory(), because you either have to be very careful, or make sure your factory function can handle both async and synchronous code paths.

Here is an asynchronous example:

function createThingAsync(container, callback) {
    setTimeout(function() {
        callback(null, {});
    }, 1000);
}

new Container()
    .registerFactory(createThingAsync, 'Thing')
    .resolve('Thing', function(err, thing) {
        if (err) {
            console.error(err);
            return;
        }

        //do something with thing
    });

But if you try to resolve Thing synchronously, nothing will be returned. If you need that flexibility, you'll have to account for both cases in your factory function:

function createThingMaybeAsync(container, callback) {
    var theThing = { oh: 'hi mark' };
    if (callback) {
        //asynchronous code path
        callback(null, theThing);
        return;
    }

    //synchronous code path
    return theThing;
}

It's best to not mix the idioms, though, since some things just can't be done synchronously.

Cyclic dependencies

...are bad.

function Foo(/** Bar */bar) {}
function Bar(/** Foo */foo) {}

container.registerType(Foo); //throws "Cyclic dependency from Foo to Bar"

Lifetime management

But wait, there's more configuration! To give you more fine-grained control over the objects that Sahara creates, you can specify a Lifetime. This basically tells Sahara how to store the object, in effect governing the lifetime of the object.

The default lifetime is the TransientLifetime, which means that every time you call resolve() Sahara will pretend like it's never seen this type before and run the object building sequence each time.

The MemoryLifetime will store the object instance in memory, and reuse that instance every time the container attempts to resolve that type. This is useful when you have an object with an expensive construction time (e.g. a database connection) that you would want to reuse for the duration of your script.

var lifetime = require('sahara').lifetime;

function DbConnection() {
    this.client = mysql.connect({...});
}

container.registerType(DbConnection, { lifetime: lifetime.memory() });

The ExternallyManagedLifetime puts the onus of object management on the client (i.e. you). This is useful for managing something that should exist temporarily.

var manager = new sahara.ObjectManager(),
    container = new sahara.Container();

container.registerInstance({ foo: 'bar' }, 'Foo', sahara.lifetime.external(manager));
container.registerInstance({ foo: 'baz' }, 'Bar', sahara.lifetime.external(manager));

You can also provide a collection to the ObjectManager, and all of the managed instances will be stored in that object.

var items = {
        foo: 'bar';
    },
    manager = new sahara.ObjectManager(items);

container.registerInstance('hello world', 'Foo', sahara.lifetime.external(manager));

var instance = container.resolveSync('Foo');
console.log(items);
/*
{ foo: 'bar',
  '95ae7c41-b413-48a9-9712-5d1756d6cf92': 'hello world' }
*/

To purge the ObjectManager's stored values, call purge():

manager.purge();

The ObjectManager is an EventEmitter and emits add when a new object is added, and purge when the manager is purged.

Injection

By default, Sahara performs constructor injection. That is, it resolves dependencies that are specified in the constructor. What if you have dependencies that are not in the constructor? Well, there are a few ways to alleviate that problem as well.

NOTE: cyclic dependencies are not detected when performing property and method injection, because I haven't figured out a clever way of doing it yet. So if you see a "maximum stack size exceeded" error, you probably created a cyclic dependency.

Property injection

Property injection simply sets the value of a property on an object when it is resolve()'d.

There are two ways to do it. You can simply give the value of the property:

var inject = sahara.inject;

function Foo() {
    this.value = 'foo';
}

container.registerType(Foo, { injections: [ inject.propertyValue('value', 'bar') ] });
console.log(container.resolveSync(Foo).value); //"bar"

Or you can have the container resolve the type. In this case, you must specify the property's type:

function Foo() {
    this.value = 'foo';
}
function Bar() {
    this.toString = function() { return 'I am Bar'; };
}

container
    .registerType(Foo, { injections: [ inject.property('value', 'Bar') ] })
    .registerType(Bar);
console.log(container.resolveSync(Foo).value); //"I am Bar"

Method injection

Method injection invokes the specified method with optional arguments during resolution. Again, you can specify the arguments explicitly:

function Foo() {
    this.value = 'foo';
    this.setValue = function(newValue) {
        this.value = newValue;
    };
}

container.registerType(Foo, { injections: [ inject.method('setValue', [ 'bar' ]) ] });
console.log(container.resolveSync(Foo).value); //"bar"

Or you can let the container resolve the method's arguments. To accomplish this, simply omit the optional array of arguments to inject.method(). Note that this uses the same procedure as container.registerType(), so you'll need to specify the types of each parameter with a comment.

function Foo() {
    this.value = 'foo';
    this.setValue = function(/** TheNewValue */newValue) {
        this.value = newValue;
    };
}

container
    .registerType(Foo, { injections: [ inject.method('setValue') ] })
    .registerInstance('This is the new value', 'TheNewValue');
console.log(container.resolveSync(Foo).value); //"This is the new value"

Manual injection

The container also provides a method for performing injection on an already existing object. This can be useful if an object was created without using the container.

function Foo() {
    this.value = 'foo';
}

container.registerType(Foo, { injections: [ inject.propertyValue('value', 'bar') ] });

var instance = new Foo();
console.log(instance.value); //"foo"
container.injectSync(instance);
console.log(instance.value); //"bar"

//specify the key explicitly
container.injectSync(instance, 'Foo');

The async way:

container.inject(instance, 'Foo', function(err) {
    if (err) {
        console.error(err);
    }

    //injection completed successfully
});

Interception

Interception is a means of intercepting a function call and doing something before or after. For example, you could modify the return value, nullify an error, perform extra logging, etc.

Limitations

Interception in JavaScript is accomplished by defining a non-writable property that wraps a function call. So, if your method is not configurable then it will not be intercepted.

Note that currently only types can be intercepted. This may be expanded to other registrations in the future.

Usage

There are three components to configuring registration:

  1. Define a matcher predicate, which will determine when a function call should be intercepted
  2. Define a call handler, which will be executed when the function is called
  3. Call container.intercept()
    • container.intercept().sync() for synchronous interception
    • container.intercept().async() for asynchronous interception

NOTE: interception is configured internally during object construction, which occurs when you call resolve() or resolveSync(). So make sure interception has been configured prior to resolving the type.

You should also make use of the memory lifetime where appropriate, as determining when a function should be intercepted can add a bit of time to the creation of the object. If the memory lifetime is used, that performance hit will only occur once.

Matchers

The matcher predicate is a function that takes in two arguments:

  1. instance - the object instance
  2. methodName - the name of the method that is being invoked

If the matcher returns true, then the method will be intercepted.

//intercept every possible function
function always() {
    return true;
}

//only intercept the validate() method
function onlyValidate(instance, methodName) {
    return methodName === 'validate';
}

//only intercept validate() on the UserRepository
function conditionalValidate(instance, methodName) {
    return instance instanceof UserRepository && methodName === 'validate';
}

Call handlers

The call handler is invoked when the intercepted function is called. It is a function that takes two arguments:

  1. context - An object representing the current state of the invocation
    • context.instance - the object instance
    • context.methodName - the name of the method being invoked
    • context.arguments - an array of arguments passed to the method
    • context.error - the error that was raised during invocation
    • context.returnValue - the value to be returned by the method
  2. next - Invoke the next handler in the chain

You MUST call next() once and only once somewhere in your call handler, to make sure the handler chain completes. If you have multiple call handlers defined, it will invoke the next one. Otherwise, it will invoke the original method. However, if context.error is set, then the original method will not be invoked.

//log the signature of the method and return value
function logMethodCalls(context, next) {
    var message = context.instance.constructor.name + '.' + context.methodName + '(' +
        context.arguments.map(function(arg) { return arg.toString(); }).join(',') + ')';

    console.log(message);
    next();
    console.log(message + ': ' + (context.error || context.returnValue));
}

//modify the return value
function ohHiMark(context, next) {
    next();
    context.returnValue = 'oh hi mark';
}

//force an error
function alwaysErrors(context, next) {
    next();
    context.error = new Error('NOPE.');
}

//nullify an error
function noErrors(context, next) {
    next();
    context.error = null;
}

//modify function arguments
function addFoo(context, next) {
    context.arguments[0] = 'foo';
    next();
}

Putting it all together

function Foo() {
    this.bar = function(message) {
        console.log(message);
    };
}

function matchBar(instance, methodName) {
    return methodName === 'bar';
}

var container = new Container()
    .registerType(Foo)
    .intercept(matchBar, logMethodCalls, addFoo).sync();

var foo = container.resolveSync(Foo);
foo.bar();

Signature shortcuts

The matcher argument accepts other things besides a function.

  • If you give it a string, it will only intercept methods with that name
  • If you give it an array, the matcher[0] is the type, and matcher[1] is the method name (matcher[1] is optional)
  • If you give it something that's not a function, array or string, it will convert it to a boolean, and either match everything or nothing.
    • truthy values (!!value === true) match everything
    • falsey values (!!value === false) match nothing

So the example above could be configured more easily:

container = new Container()
    .registerType(Foo)
    .intercept('bar', logMethodCalls, addFoo).sync();

Or even better:

container = new Container()
    .registerType(Foo)
    .intercept([ Foo, 'bar' ], logMethodCalls, addFoo).sync();

Asynchronous interception

All you've seen so far is synchronous interception. Asynchronous interception is a little tricky. Since Sahara simply wraps the original function call, it needs to assume some things for async functions:

  1. The last argument is the callback
  2. The callback uses the standard node convention: callback(err, returnValue)

Sahara will try to gracefully handle situations where the callback is not given, or optional arguments are omitted. If you're not doing anything too weird, you should be perfectly fine. So even if you have a function defined like this that has a variable arity, Sahara will still do the right thing:

function(options, callback) {
    if (typeof(options) === 'function') {
        callback = options;
        options = {};
    }
    if (!callback) {
        callback = arguments[arguments.length - 1];
    }

    //do stuff
    callback(err, result);
}

Your call handler may also change slightly. If you want to do something after calling next(), you can pass an optional callback to next(). So our logging call handler from above becomes:

function logMethodCallsAsync(context, next) {
    var message = context.instance.constructor.name + '.' + context.methodName + '(' +
        context.arguments.map(function(arg) { return arg.toString(); }).join(',') + ')';

    console.log(message);
    next(function(done) {
        console.log(message + ': ' + (context.error || context.returnValue));
        done(); // <-- this is important!
    });
}

If you don't need to do anything after the function is executed, simply call next() with no arguments.

var count = 0;
function incrementCounter(context, next) {
    count++;
    next();
}

And finally, when configuring interception, use the .async() chain:

function Foo() {
    this.bar = function(message, callback) {
        console.log(message);
        callback();
    };
}

var container = new Container()
    .registerType(Foo)
    .intercept(Foo, matchBar, logMethodCallsAsync).async();

//note that aysnc interception is only for asynchronous methods.
//so you can still resolve synchronously and intercept asynchronously.

container.resolveSync(Foo).bar('hello world', function(err, result) {
    //...
});

It's important to note that matchers are applied to all registrations, so keep that in mind when mixing async and sync registrations. Sahara will ensure that only async call handlers or only sync call handlers will be used per function call, but since it doesn't know if the function itself is async, it'll simply assume that the first match determines whether it's asynchronous or not.

For example, don't do this:

function Foo() {
    this.bar = function() {};
}

function asyncHandler(context, next) {
    next(function() {
        console.log('yay!');
    });
}

var container = new Container()
    .registerType(Foo)
    .intercept('bar', asyncCallHandler).async();

//synchronous call is going to use an async handler
container.resolve(Foo).bar();

Creating child containers

Occasionally the need arises to create a new container that inherits all of the configurations from another container. This can be accomplished with the createChildContainer() function.

function Foo() {}

var parent = new Container().registerType(Foo),
    child = parent.createChildContainer();

var fooInstance = child.resolveSync(Foo); // instance of Foo

Anything you do on the parent container will not affect the state of the child container, and vice versa. They are completely independent.

If you want the child container to inherit the events as well, pass true to createChildContainer().

child = parent.createChildContainer(true);

Events

Events emitted by a Container instance:

  • registering - when a type/instance/factory is being registered
    1. arguments[0]: the registration key
    2. arguments[1]: the registration type (type, instance or factory)
  • resolving - when an object is being resolved
    1. arguments[0]: the registration key
  • resolved - when an object has been resolved
    1. arguments[0]: the registration key
    2. arguments[1]: the resolved object

Events emitted by an ObjectBuilder instance:

  • building - when an object is being built
    1. arguments[0]: metadata for the type: { args: [], ctor: Function, name: 'name' }
  • built - when an object has been built
    1. arguments[0]: metadata for the type (see above)
    2. arguments[1]: the object instance
  • intercepting - when a method is being intercepted
    1. arguments[0]: the object instance
    2. arguments[1]: the name of the method being intercepted

Example:

var container = new Container();

container
    .on('registering', function(key) { console.log(key + ' is being registered'); })
    .on('resolving', function(key) { console.log(key + ' is being resolved'); })
    .on('resolved', function(key) { console.log(key + ' has been resolved'); });

container.builder
    .on('building', function(info) { console.log('building ' + info.name); })
    .on('built', function(info) { console.log('built ' + info.name); })
    .on('intercepting', function(instance, methodName) {
        console.log('intercepting ' + instance.constructor.name + '.' + methodName);
    });

Development

git clone git@github.com:tmont/sahara.git
cd sahara
npm install
npm test
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