feat(transloco): support nested array of scopes

[dmitry-stepanenko]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/transloco/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Right now transloco is able to resolve only 1-level array of scopes. This works fine with straightforward approach when using { provide: TRANSLOCO_SCOPE, useValue: scope, multi: true }. However, it might be needed to pass an array of scopes within single provider.

What is the new behavior?

This change allows to use the following syntax

{ 
  provide: TRANSLOCO_SCOPE,
  useValue: ['scope1', 'scope2']
  multi: true
}

In my scenario there's a big amount of scopes that we're using. They're provided either as component or module providers. And there's a limitation related to Angular's injection hierarchy: it resolves ElementInjectors before ModuleInjectors and if TRANSLOCO_SCOPE is provided on the ElementInjector level, it won't grab module injectors at all. To overcome this I want to grab all parent scopes and provide them explicitly

export function provideLanguageScopes(
  scope: LanguageScope,
  ...otherScopes: LanguageScope[]
): Provider {
  const scopeSet = new Set(otherScopes.concat(scope));
  return {
    provide: TRANSLOCO_SCOPE,
    useFactory: (parentScopes: LanguageScope[] | null) => {
      parentScopes && parentScopes.flat(Infinity).forEach(p => scopeSet.add(p));
      return [...scopeSet.values()];
    },
    multi: true,
    deps: [[new SkipSelf(), new Optional(), TRANSLOCO_SCOPE]],
  };
}

This approach requires transloco to be able to work with nested arrays of scopes. This PR does not introduce any changes to existing behavior, only enhances it so that it is possible to provide arrays under TRANSLOCO_SCOPE

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Setting yes as there's a change to MaybeArray interface, that might not be compatible with all implementations

Other information

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[dmitry-stepanenko]

I believe this was done unintentionally, scope resolver is not able to process array of scopes. There's no compilation error only because of isScopeObject type guard, that is also not supposed to handle array of scopes

[dmitry-stepanenko]

to be able to use Array.prototype.flat()

[dmitry-stepanenko]

@shaharkazaz will you be able to look at this? An issue on our side appears to be quite serious. Appreciated in advance 🙏