Expose sourceOfConfig to macro config mergers

[mydea]

Currently, the merging strategy for @embroider/macros configs will just merge all given objects together. This means that the last config to be registered will "win", if a property is defined in more than one place.

Since the app's own config will be called first, this means that an app cannot overwrite configuration that is set in an addon.

This PR changes this by keeping a private __priority property on the config (which is stripped out at the end) by which we order the configs, if appropriate. The ordering works like this:

0. setOwnConfig will always have the lowest priority
1. setConfig called from an addon (e.g. setting config for another addon)
2. setConfig called from the host app

The following example shows the output hierarchy:

// addon-1/index.js
setOwnConfig: {
  value1: 'default',
  value2: 'default',
  value3: 'default'
}

// addon-2/index.js
setConfig: {
  'addon-1': {
    value1: 'addon',
    value2: 'addon'
  }
}

// my-app/ember-cli-build.js
setConfig: {
  'addon-1': {
    value1: 'app'
  }
}

// Output from addon-1:
assert.deepEquals(
  getOwnConfig(), 
  { 
    value1: 'app', 
    value2: 'addon', 
    value3: 'default'
  }
);

I tried to build it in an as backwards-compatible way as possible.

Closes #885

[ef4]

Thanks for working on this. I know you were trying not to introduce API changes but I think it's worth having some small changes here so that custom mergers can see exactly where each config is coming from. That way, your change becomes just a change to the default merger.

• add changes on top of yours that expose a new sourceOfConfig method to mergers
• rebased on master

[ef4]

I'm tagging this as breaking only because the type signature for config has changed from unknown to object (because it must be a valid WeakMap key).

[mydea]

Great!
BTW while working on this, I noticed that it is currently not possible to actually set a custom merger - is that on purpose? There is no reference to useMerger anywhere in the code, as far as I see.

[ef4]

It's possible, it's just not documented or used in any examples yet.

const { MacrosConfig } = require('@embroider/macros/node');
MacrosConfig.for(appInstance).useMerger(__filename, ...);

The appInstance is available to an addon's index.js as this._findHost() and to an app's ember-cli-build.js as the actual app local variable.

[mydea]

Ah, very nice - I didn't know that! Great to know if I ever get into the position again that I need some custom handling 👍