Documentation: Dependency Trees

The module instances are organized in dependency trees. You can see a dependency tree as a set of instances that logically belong together. The main tree always has the identifier root.

Take a mongodb database as an example. You hav a connector class for a single collection. But now you have multiple modules that require connection to different collections. Of course you now could start to connect to all of you needed collections in you collection connector. But wouldn't it be easier to build this generic? This is what is possible with dependency trees. You just define the injections in your, lets say controllers, with an identifier that matches the collection you need and your connector will know about this (for a little example look below). The module will have the current tree available in the parameter module.exports.tree. Now there is a little caveat. It is called dependency tree for a reason. When a module is instantiated cdi will try to inject instances of that tree only if not advised differently. You can force this by setting the tree identifier in the inject definition of the exports (see Module Structure for details).

example

src/speed.js:

class Speed {
    constructor() {
        this.speed = 0;
    }

    setSpeed(speed) {
        this.speed = speed;
    }
}
module.exports = Speed;

src/wheel.js:

class Wheel {
    constructor(size, speed) {
        this.size = size;
        this.position = module.exports.tree;
        this.speed = speed;
    }
}
module.exports = Wheel;
module.exports.inject = ['config:bike:wheels.size', 'Speed:root'];

src/bike.js:

class Bike {
    constructor(frontWheel, backWheel) {
        this.wheels = {frontWheel, backWheel};
    }
}
module.exports = Bike;
module.exports.inject = ['Wheel:front', 'Wheel:back'];

main.js:

const path = require('path');
const CDI = require('cdi');

const bikeConfig = {
    wheels: {
        size: 26
    }
};

let cdi = new CDI({
    moduleSrc: [path.join(__dirname, 'src')],
    cacheFile: path.join(__dirname, 'cache', 'cdi.json'),
    configurations: {
        bike: bikeConfig
    }
});

let myBike = cdi.getInstance('Bike');
// now you got your bike with two injected wheels, which got the size information injected 
// and share one common speed instance