feat: add CompositeAsyncHandler to support multiple handlers

test/unit/util/AsyncHandler.test.ts

@@ -0,0 +1,30 @@
+import { AsyncHandler } from '../../../src/util/AsyncHandler';
+import { StaticAsyncHandler } from '../../util/StaticAsyncHandler';
+
+describe('An AsyncHandler', (): void => {
+  it('calls canHandle and handle when handleSafe is called.', async (): Promise<void> => {
+    const handlerTrue: AsyncHandler<any, any> = new StaticAsyncHandler(true, null);
+    const canHandleFn = jest.fn(async (input: any): Promise<void> => input);
+    const handleFn = jest.fn(async (input: any): Promise<any> => input);
+
+    handlerTrue.canHandle = canHandleFn;
+    handlerTrue.handle = handleFn;
+    await expect(handlerTrue.handleSafe('test')).resolves.toBe('test');
+    expect(canHandleFn).toHaveBeenCalledTimes(1);
+    expect(handleFn).toHaveBeenCalledTimes(1);
+  });
+
+  it('does not call handle when canHandle errors during a handleSafe call.', async (): Promise<void> => {
+    const handlerFalse: AsyncHandler<any, any> = new StaticAsyncHandler(false, null);
+    const canHandleFn = jest.fn(async (): Promise<void> => {
+      throw new Error('test');
+    });
+    const handleFn = jest.fn(async (input: any): Promise<any> => input);
+
+    handlerFalse.canHandle = canHandleFn;
+    handlerFalse.handle = handleFn;
+    await expect(handlerFalse.handleSafe('test')).rejects.toThrow(Error);
+    expect(canHandleFn).toHaveBeenCalledTimes(1);
+    expect(handleFn).toHaveBeenCalledTimes(0);
+  });
+});

test/unit/util/CompositeAsyncHandler.test.ts

@@ -0,0 +1,76 @@
+import { AsyncHandler } from '../../../src/util/AsyncHandler';
+import { CompositeAsyncHandler } from '../../../src/util/CompositeAsyncHandler';
+import { StaticAsyncHandler } from '../../util/StaticAsyncHandler';
+
+describe('A CompositeAsyncHandler', (): void => {
+  describe('with no handlers', (): void => {
+    it('can never handle data.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([]);
+
+      await expect(handler.canHandle(null)).rejects.toThrow(Error);
+    });
+
+    it('errors if its handle function is called.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([]);
+
+      await expect(handler.handle(null)).rejects.toThrow(Error);
+    });
+  });
+
+  describe('with multiple handlers', (): void => {
+    let handlerTrue: AsyncHandler<any, any>;
+    let handlerFalse: AsyncHandler<any, any>;
+    let canHandleFn: jest.Mock<Promise<void>, [any]>;
+    let handleFn: jest.Mock<Promise<void>, [any]>;
+
+    beforeEach(async (): Promise<void> => {
+      handlerTrue = new StaticAsyncHandler(true, null);
+      handlerFalse = new StaticAsyncHandler(false, null);
+
+      canHandleFn = jest.fn(async (input: any): Promise<any> => input);
+      handleFn = jest.fn(async (input: any): Promise<any> => input);
+      handlerTrue.canHandle = canHandleFn;
+      handlerTrue.handle = handleFn;
+    });
+
+    it('can handle data if a handler supports it.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([ handlerFalse, handlerTrue ]);
+
+      await expect(handler.canHandle(null)).resolves.toBeUndefined();
+    });
+
+    it('can not handle data if no handler supports it.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([ handlerFalse, handlerFalse ]);
+
+      await expect(handler.canHandle(null)).rejects.toThrow('[Not supported., Not supported.]');
+    });
+
+    it('handles data if a handler supports it.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([ handlerFalse, handlerTrue ]);
+
+      await expect(handler.handle('test')).resolves.toEqual('test');
+      expect(canHandleFn).toHaveBeenCalledTimes(1);
+      expect(handleFn).toHaveBeenCalledTimes(1);
+    });
+
+    it('errors if the handle function is called but no handler supports the data.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([ handlerFalse, handlerFalse ]);
+
+      await expect(handler.handle('test')).rejects.toThrow('All handlers failed.');
+    });
+
+    it('only calls the canHandle function once of its handlers when handleSafe is called.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([ handlerFalse, handlerTrue ]);
+
+      await expect(handler.handleSafe('test')).resolves.toEqual('test');
+      expect(canHandleFn).toHaveBeenCalledTimes(1);
+      expect(handleFn).toHaveBeenCalledTimes(1);
+    });
+
+    it('throws the same error as canHandle when calling handleSafe if no handler supports the data.', async (): Promise<void> => {
+      const handler = new CompositeAsyncHandler([ handlerFalse, handlerFalse ]);
+
+      await expect(handler.handleSafe(null)).rejects.toThrow('[Not supported., Not supported.]');
+    });
+  });
+});