refactor state handling to make application available independent of the state

tests/unit/application/ApplicationFactory.spec.ts

@@ -0,0 +1,60 @@
+import 'mocha';
+import { expect } from 'chai';
+import { ApplicationFactory, ApplicationGetter } from '@/application/ApplicationFactory';
+import { ApplicationStub } from '../stubs/ApplicationStub';
+
+describe('ApplicationFactory', () => {
+    describe('ctor', () => {
+        it('throws if getter is undefined', () => {
+            // arrange
+            const expectedError = 'undefined getter';
+            const getter = undefined;
+            // act
+            const act = () => new SystemUnderTest(getter);
+            // assert
+            expect(act).to.throw(expectedError);
+        });
+    });
+    describe('getAppAsync', () => {
+        it('returns result from the getter', async () => {
+            // arrange
+            const expected = new ApplicationStub();
+            const getter: ApplicationGetter = () => expected;
+            const sut = new SystemUnderTest(getter);
+            // act
+            const actual = await Promise.all( [
+                sut.getAppAsync(),
+                sut.getAppAsync(),
+                sut.getAppAsync(),
+                sut.getAppAsync(),
+            ]);
+            // assert
+            expect(actual.every((value) => value === expected));
+        });
+        it('only executes getter once', async () => {
+            // arrange
+            let totalExecution = 0;
+            const expected = new ApplicationStub();
+            const getter: ApplicationGetter = () => {
+                totalExecution++;
+                return expected;
+            };
+            const sut = new SystemUnderTest(getter);
+            // act
+            await Promise.all( [
+                sut.getAppAsync(),
+                sut.getAppAsync(),
+                sut.getAppAsync(),
+                sut.getAppAsync(),
+            ]);
+            // assert
+            expect(totalExecution).to.equal(1);
+        });
+    });
+});
+
+class SystemUnderTest extends ApplicationFactory {
+    public constructor(costlyGetter: ApplicationGetter) {
+        super(costlyGetter);
+    }
+}