npm package: @girders-elements/config
Description: Girders elements packages that provides the posibility to define a custom configuration
Author: netcetera
Latest version: 1.0.0-alpha.26

# Config 

Girders Element's `config` package is a small library that can be used to create **layered configuration** setup useful when building platforms. 

A *configuration* consists of one or more **feature configuration objects** (ex. translations, theme etc). 

## Layers 

Configurations are distributed into **layers**. Each layer has its own set of **featurre configuration objects**.
Layers are organized in a hirearchy, with every layer overriding the configuration object of its parents. Layers are useful is situations when a base package (e.g. the platforms) needs to provide *reasonable defaults* for the configuration that still can be overriden but the package's user (e.g. an app).

## Profiles

However, we often need to change certain configuration values based on the
context where it is used. Examples of this could be:

- one server URL is used in development, another in production
- one font is used for android, another for iOS and a third for web

To achieve this, we also introduce *profiles*. A profile is a named set of configuration
entries that, when activated, will override the  configuration entires specified elsewhere (as a default in the current layer or the layer below).

More than one profile may be active at any given time. The list of active profiles'
order is significant because it determines the lookup procedure. This list is provided on application starup.

There always exists a profile called **default** which contains all the entries
that were not specified in any other profile. This profile is *always active*
and is at the bottom of the lookup list.


## Example Feature Configuration Object 

```javascript

const theme = {
   backgoundColor: red,         // configuration value in the default profile
   profiles: {                  // profiles
     dev: {                     // dev
         backgoundColor: white  // dev config value
     },
     android: {                 // android
        backgoundColor: blue    // android config value
     }
   }
}

```


## Usage

### Installation

Simply add the package to your `package.json`:

```
$ yarn add @girders-elements/config
```

or 

```
$ npm install --save @girders-elements/config
```

### Defining layers

Calling the `define` method on configuration creates a root layer from a given configuraton object and returns the created layer.
Calling `define` on an *existing layer* also creates a new layer *over the existing one*. The configuration in the new layer overrides the one in the existing.


```javascript
import config from '@girders-elements/config'

import videoRoot from './videoRoot'
import translationsRoot from './translationsRoot'

import video1 from './video1'
import translations1 from './translations1'

import video2 from './video2'
import translations2 from './translations2'

// root layer is defined
const rootLayer = config.define ({
    video: videoRoot,
    translations: translationsRoot
})

// layer1 is defined, with rootLayer as the layer below
const layer1 = rootLayer.define ({
    video: video1,
    translations: translations1
})

// layer2 is defined, with layer1 as the layer below
const layer2 = layer1.define ({
    video: video2,
    translations: translations2
})
```

#### Initializing layers

Calling the init method on a layer computes the configuration for the given layer sequence and for the activated profiles.

The sequence consists of all parent layers to the one being initialized, down to and including the root layer.
After initialization, the computed configuration values are available for all the members of the layer chain, as immutable values.
Once a configuration is computed the layers are 'frozen', and they can neither be re-initialized nor modified.

```javascript
// intializing layer2 computes the configuration and makes it available to its layer sequence (rootLayer -> layer1 -> layer2)
layer2.init(profiles)

// the same computed configuration is available for all layers in the sequence
// layer2.video === layer1.video === rootLayer.video
// layer2.translations === layer1.translations === rootLayer.translations

// throws - layers are frozen and cannot be re-initialized
rootLayer.init()
layer1.init()
layer2.init()

// throws - computed configuration cannot be modified
layer1.video.playerID = newPlayerID
```

### Examples

#### Multiple layers, one feature, one profile


```javascript
  const layerOneConfig = {
      config1: {
        a: 1,
        b: 2,
        profiles: {
          dev: {
            a: 5,
            b: 6
          }
        }
      }
    }
    const layerTwoConfig = {
      config1: {
        b: 3
      }
    }
    const layerThreeConfig = {
      config1: {
        profiles: {
          dev: {
            b: 10
          }
        }
      }
    }
    init('dev')
```

##### Config1 resolution:

###### Layer 3:

**dev** profile: { b: 10}

**default** profile: { }

**final**: { b: 10 }

###### Layer 2:

**dev** profile: {}

**default** profile: { b: 3 }

**final**: { b: 3 }

###### Layer 1:
**dev** profile: { a: 5, b: 6 }

**default** profile: { a: 1, b: 2 }

**final**: { a: 5, b: 6 }

###### Final:

**Layer 2**: { a: 16, b: 14 }

**Layer 1**: { a: 1, b: 7, c: 3 }

**Final**:  { a: 16, b: 14, c: 3 }

##### Multiple layers, features and profiles

* Features - config1 and config2
* Profiles - dev, prod, ios, android
* Layers - layer1 and layer2

```javascript

  const layerOneConfig = {
      config1: {
        a: 1,
        b: 2,
        c: 3,
        profiles: {
          prod: {
            b: 6
          },
          ios: {
            b: 7
          },
          android: {
            a: 8
          }
        }
      }
    }

    const layerTwoConfig = {
      config1: {
        a: 10,
        b: 11,
        profiles: {
          dev: {
            b: 14
          },
          prod: {
            a: 15
          },
          ios: {
            a: 16
          },
          android: {
            b: 17
          }
        }
      },
      config2: {
        c: 5,
        profiles: {
          dev: {
            c: 10
          },
          prod: {
            c: 1
          },
          ios: {
            c: 2
          },
          android: {
            c: 3
          }
        }
      }
    }

    const expectedConfig1 = {
      a: 16,
      b: 14,
      c: 3
    }

    const expectedConfig2 = {
      c: 10
    }

    init(['dev', 'ios'])

```



##### Config1 resolution:

###### Layer 2:
**dev** profile: { b: 14 }

**ios** profile: { a: 16 }

**default** profile: { a: 10, b: 11 }

**final**: { a: 16, b: 14 }


###### Layer 1:

**dev** profile: {}

**ios** profile: { b: 7 }

**default** profile: { a: 1, b: 2, c: 3 }

**final**: { a: 1, b: 7, c: 3 }

###### Final:

**Layer 2**: { a: 16, b: 14 }

**Layer 1**: { a: 1, b: 7, c: 3 }

**Final**:  { a: 16, b: 14, c: 3 }


##### Config2 resolution:
##### Layer 2:
**dev** profile: { c: 10 }

**ios** profile: { c: 2 }

**default** profile: { c: 5 }

**final**: { c: 10 }


###### Layer 1:

**No config**

**final**: { }

###### Final:

**Layer 2**: { c: 10 }

**Layer 1**: { }

**Final**:  { c: 10 }