fix: Let ParallelHandler only run able handlers

Author: joachimvh

src/HandlerUtil.ts

@@ -52,3 +52,18 @@ Promise<AsyncHandler<TIn, TOut>[]> {
 
   throw new AggregateError(errors, 'No handler can handle the input');
 }
+
+/**
+ * Returns the value of the `handle` call if the handle can handle input.
+ *
+ * @param handler - Handler to run the input.
+ * @param input - Input to handle.
+ */
+export async function handleIfAble<TIn, TOut>(handler: AsyncHandler<TIn, TOut>, input: TIn): Promise<TOut | void> {
+  try {
+    await handler.canHandle(input);
+  } catch {
+    return;
+  }
+  return handler.handle(input);
+}

src/ParallelHandler.ts

@@ -1,7 +1,9 @@
 import { AsyncHandler } from './AsyncHandler';
+import { handleIfAble } from './HandlerUtil';
 
 /**
  * A composite handler that executes handlers in parallel.
+ * Handlers that can not handler the input will be ignored.
  * The `canHandle` check of this handler will always succeed.
  */
 export class ParallelHandler<TIn = void> extends AsyncHandler<TIn> {
@@ -14,7 +16,7 @@ export class ParallelHandler<TIn = void> extends AsyncHandler<TIn> {
 
   public async handle(input: TIn): Promise<void> {
     await Promise.all(
-      this.handlers.map(async(handler): Promise<unknown> => handler.handleSafe(input)),
+      this.handlers.map(async(handler): Promise<unknown> => handleIfAble(handler, input)),
     );
   }
 }

src/SequenceHandler.ts

@@ -16,13 +16,11 @@ export class SequenceHandler<TIn = void, TOut = void> extends AsyncHandler<TIn,
   public async handle(input: TIn): Promise<TOut | undefined> {
     let result: TOut | undefined;
     for (const handler of this.handlers) {
-      let supported: boolean;
+      let supported = false;
       try {
         await handler.canHandle(input);
         supported = true;
-      } catch {
-        supported = false;
-      }
+      } catch {}
       if (supported) {
         result = await handler.handle(input);
       }

test/unit/HandlerUtil.test.ts

@@ -1,5 +1,5 @@
 import type { AsyncHandler } from '../../src/AsyncHandler';
-import { filterHandlers, findHandler } from '../../src/HandlerUtil';
+import { filterHandlers, findHandler, handleIfAble } from '../../src/HandlerUtil';
 import { getError } from '../TestUtil';
 
 describe('HandlerUtil', (): void => {
@@ -9,6 +9,7 @@ describe('HandlerUtil', (): void => {
   beforeEach(async(): Promise<void> => {
     handlerTrue = {
       canHandle: jest.fn(),
+      handle: jest.fn().mockResolvedValue(true),
     } satisfies Partial<AsyncHandler<any, any>> as any;
     handlerFalse = {
       canHandle: jest.fn().mockRejectedValue(new Error('test')),
@@ -45,4 +46,14 @@ describe('HandlerUtil', (): void => {
       expect(error.errors[1].message).toBe('test');
     });
   });
+
+  describe('#handleIfAble', (): void => {
+    it('returns undefined if the handler cannot handle the input.', async(): Promise<void> => {
+      await expect(handleIfAble(handlerFalse, 'a')).resolves.toBeUndefined();
+    });
+
+    it('returns the value if the handler can handle the input.', async(): Promise<void> => {
+      await expect(handleIfAble(handlerTrue, 'a')).resolves.toBe(true);
+    });
+  });
 });

test/unit/ParallelHandler.test.ts

@@ -8,33 +8,38 @@ describe('ParallelHandler', (): void => {
   beforeEach(async(): Promise<void> => {
     handlers = [
       {
-        handleSafe: jest.fn().mockResolvedValue('0'),
+        canHandle: jest.fn(),
+        handle: jest.fn().mockResolvedValue('0'),
       } satisfies Partial<AsyncHandler<string, string>> as any,
       {
-        handleSafe: jest.fn().mockResolvedValue('1'),
+        canHandle: jest.fn(),
+        handle: jest.fn().mockResolvedValue('1'),
       } satisfies Partial<AsyncHandler<string, string>> as any,
       {
-        handleSafe: jest.fn().mockResolvedValue('2'),
+        canHandle: jest.fn(),
+        handle: jest.fn().mockResolvedValue('2'),
       } satisfies Partial<AsyncHandler<string, string>> as any,
     ];
 
     parallel = new ParallelHandler(handlers);
   });
 
   it('can handle all requests.', async(): Promise<void> => {
-    handlers[0].handleSafe.mockRejectedValueOnce(new Error('error'));
+    handlers[0].canHandle.mockRejectedValueOnce(new Error('error'));
+    handlers[1].handle.mockRejectedValueOnce(new Error('error'));
     await expect(parallel.canHandle('input')).resolves.toBeUndefined();
   });
 
   it('runs all handlers that can handle the input.', async(): Promise<void> => {
+    handlers[0].canHandle.mockRejectedValueOnce(new Error('error'));
     await expect(parallel.handle('abc')).resolves.toBeUndefined();
 
-    expect(handlers[0].handleSafe).toHaveBeenCalledTimes(1);
-    expect(handlers[1].handleSafe).toHaveBeenCalledTimes(1);
-    expect(handlers[2].handleSafe).toHaveBeenCalledTimes(1);
+    expect(handlers[0].canHandle).toHaveBeenLastCalledWith('abc');
+    expect(handlers[1].canHandle).toHaveBeenLastCalledWith('abc');
+    expect(handlers[2].canHandle).toHaveBeenLastCalledWith('abc');
 
-    expect(handlers[0].handleSafe).toHaveBeenCalledWith('abc');
-    expect(handlers[1].handleSafe).toHaveBeenCalledWith('abc');
-    expect(handlers[2].handleSafe).toHaveBeenCalledWith('abc');
+    expect(handlers[0].handle).toHaveBeenCalledTimes(0);
+    expect(handlers[1].handle).toHaveBeenLastCalledWith('abc');
+    expect(handlers[2].handle).toHaveBeenLastCalledWith('abc');
   });
 });