feat(assert-injector): add assertInjector

closes #15

This PR adds a utility function `assertInjector` that abstracts `assertInInjectionContext` and guarantees an `Injector` after the utility is invoked

```ts
function injectDummy(injector?: Injector) {
  injector = assertInjector(injectDummy, injector);
  // ^ injector is guaranteed to be an Injector
  runInInjectionContext(injector, () => {
    // always run in a particular Injector's context
  })
}
```

libs/ngxtension/assert-injector/README.md

@@ -0,0 +1,3 @@
+# ngxtension/assert-injector
+
+Secondary entry point of `ngxtension`. It can be used by importing from `ngxtension/assert-injector`.

libs/ngxtension/assert-injector/ng-package.json

@@ -0,0 +1,5 @@
+{
+	"lib": {
+		"entryFile": "src/index.ts"
+	}
+}

libs/ngxtension/assert-injector/src/assert-injector.spec.ts

@@ -0,0 +1,39 @@
+import { Injector, runInInjectionContext } from '@angular/core';
+import { TestBed } from '@angular/core/testing';
+import { createInjectionToken } from 'libs/ngxtension/create-injection-token/src/create-injection-token';
+import { assertInjector } from './assert-injector';
+
+describe(assertInjector.name, () => {
+	const [injectNumber, provideNumber] = createInjectionToken(() => 1);
+
+	function injectDummy(injector?: Injector) {
+		injector = assertInjector(injectDummy, injector);
+		return runInInjectionContext(injector, () => injectNumber());
+	}
+
+	it('given no custom injector, when run in injection context, then return value', () => {
+		TestBed.runInInjectionContext(() => {
+			const value = injectDummy();
+			expect(value).toEqual(1);
+		});
+	});
+
+	it('given no custom injector, when run outside injection context, then throw', () => {
+		expect(() => injectDummy()).toThrowError(
+			/injectDummy\(\) can only be used within an injection context/i
+		);
+	});
+
+	it('given a custom injector, when run in that injector context without providing number, then throw', () => {
+		expect(() => injectDummy(Injector.create({ providers: [] }))).toThrowError(
+			/No provider for InjectionToken/i
+		);
+	});
+
+	it('given a custom injector, when run in that injector context and providing number, then return value', () => {
+		const value = injectDummy(
+			Injector.create({ providers: [provideNumber(2)] })
+		);
+		expect(value).toEqual(2);
+	});
+});

libs/ngxtension/assert-injector/src/assert-injector.ts

@@ -0,0 +1,9 @@
+import { Injector, assertInInjectionContext, inject } from '@angular/core';
+
+export function assertInjector(
+	fn: Function,
+	injector: Injector | undefined | null
+): Injector {
+	!injector && assertInInjectionContext(fn);
+	return injector ?? inject(Injector);
+}

libs/ngxtension/assert-injector/src/index.ts

@@ -0,0 +1 @@
+export const greeting = 'Hello World!';

libs/ngxtension/project.json

@@ -47,7 +47,9 @@
 					"libs/ngxtension/resize/**/*.ts",
 					"libs/ngxtension/resize/**/*.html",
 					"libs/ngxtension/create-injection-token/**/*.ts",
-					"libs/ngxtension/create-injection-token/**/*.html"
+					"libs/ngxtension/create-injection-token/**/*.html",
+					"libs/ngxtension/assert-injector/**/*.ts",
+					"libs/ngxtension/assert-injector/**/*.html"
 				]
 			}
 		},

tsconfig.base.json

@@ -16,6 +16,9 @@
 		"baseUrl": ".",
 		"paths": {
 			"ngxtension": ["libs/ngxtension/src/index.ts"],
+			"ngxtension/assert-injector": [
+				"libs/ngxtension/assert-injector/src/index.ts"
+			],
 			"ngxtension/create-injection-token": [
 				"libs/ngxtension/create-injection-token/src/index.ts"
 			],