ts-mockito spy为何不委托方法调用？求助排查

我有一个名为MyPresenter的简单类，其doOperation()方法会调用传入的View接口实现类的方法，相关代码如下：

export interface View {
  displayMessage: (message: string, duration: number) => void;
}

export class MyPresenter {
  private view: View;

  public constructor(view: View) {
    this.view = view;
  }

  public doOperation(): void {
    this.view.displayMessage("The message", 0);
  }
}

import { View } from "./MyPresenter";

export class ViewImpl implements View {
  public displayMessage(message: string, duration: number) {
    console.log(`The message is: ${message}, with duration ${duration}`);
  }
}

我编写了测试用例，创建spy对象并调用其doOperation()方法，尝试验证spy是否调用了view的displayMessage方法，但测试失败。测试代码如下：

import { MyPresenter, View } from "../../src/presenter/MyPresenter";
import { anything, instance, mock, spy, when, verify } from "ts-mockito";

describe("MyPresenter", () => {
  let presenterSpy: MyPresenter;
  let mockView: View;

  beforeEach(() => {
    mockView = mock<View>();
    const mockViewInstance = instance(mockView);

//    presenterSpy = new MyPresenter(mockViewInstance);
    presenterSpy = spy(new MyPresenter(mockViewInstance));
  });

  it("do operation", () => {
    presenterSpy.doOperation();
    verify(mockView.displayMessage(anything(), anything())).once();
  });
});

测试失败信息如下：

FAIL  test/presenter/Test.test.ts
● MyPresenter › do operation

Expected "displayMessage(anything(), anything())" to be called 1 time(s). But has been called 0 time(s).

  16 |   it("do operation", () => {
  17 |     presenterSpy.doOperation();
> 18 |     verify(mockView.displayMessage(anything(), anything())).once();
     |                                                             ^
  19 |   });
  20 | });
  21 |

  at MethodStubVerificator.Object.<anonymous>.MethodStubVerificator.times (../node_modules/ts-mockito/src/MethodStubVerificator.ts:35:19)
  at MethodStubVerificator.Object.<anonymous>.MethodStubVerificator.once (../node_modules/ts-mockito/src/MethodStubVerificator.ts:20:14)
  at Object.<anonymous> (test/presenter/Test.test.ts:18:61)

若将spy替换为普通实例，测试则通过，显然是spy未委托方法调用导致的问题。注：本示例为简化场景，实际需用spy模拟其他依赖的工厂方法。

以下是我的package.json和tsconfig.json配置：

{
  "name": "ts-mockito-spy-example",
  "private": true,
  "version": "0.0.0",
  "scripts": {
    "dev": "vite",
    "build": "tsc && vite build",
    "preview": "vite preview",
    "test": "jest"
  },
  "devDependencies": {
    "typescript": "^5.0.2",
    "vite": "^4.4.5",
    "@babel/core": "^7.23.3",
    "@babel/preset-env": "^7.23.3",
    "@babel/preset-typescript": "^7.23.3",
    "@types/jest": "^29.5.8",
    "babel-jest": "^29.7.0",
    "jest": "^29.7.0",
    "ts-jest": "^29.1.1",
    "ts-mockito": "^2.6.1"
  }
}

{
  "compilerOptions": {
    "target": "ES2020",
    "useDefineForClassFields": true,
    "module": "ESNext",
    "lib": ["ES2020", "DOM", "DOM.Iterable"],
    "skipLibCheck": false,
    "esModuleInterop": true,

    /* Bundler mode */
    "moduleResolution": "bundler",
    "allowImportingTsExtensions": true,
    "resolveJsonModule": true,
    "isolatedModules": true,
    "noEmit": true,

    /* Linting */
    "strict": true,
    "noFallthroughCasesInSwitch": true
  },
  "include": ["src"]
}

请问为何ts-mockito的spy不委托方法调用？

核心原因 #

ts-mockito的spy()函数默认不会自动将所有方法调用委托到原始实例，尤其是当类方法涉及私有字段（比如你的MyPresenter里的private view）时，spy的代理机制无法正确拦截并转发调用逻辑。你调用presenterSpy.doOperation()时，实际并没有执行原始实例里的代码，自然也就不会触发mockView.displayMessage。

另外，ts-mockito的spy设计初衷更偏向于捕获已有mock对象的调用，而非包裹真实实例保留原始行为，旧版本（比如你用的2.6.1）对类实例的代理支持存在局限性。

解决方案 #

方案1：用instance()包装spy对象

修改测试中的spy创建逻辑，把spy后的对象通过instance()转换，让代理能正确转发调用：

beforeEach(() => {
  mockView = mock<View>();
  const mockViewInstance = instance(mockView);

  const presenterInstance = new MyPresenter(mockViewInstance);
  const presenterSpyObj = spy(presenterInstance);
  presenterSpy = instance(presenterSpyObj); // 关键步骤
});

方案2：显式存根需要保留行为的方法

如果只需要特定方法保留原始逻辑，可以用when()手动指定委托：

beforeEach(() => {
  mockView = mock<View>();
  const mockViewInstance = instance(mockView);

  presenterSpy = spy(new MyPresenter(mockViewInstance));
  // 显式让spy的doOperation调用原始逻辑
  when(presenterSpy.doOperation()).thenCall(() => {
    presenterSpy['view'].displayMessage("The message", 0);
  });
});

方案3：升级ts-mockito版本

你当前使用的2.6.1版本较旧，3.x+版本对类实例spy的代理逻辑做了优化，大概率能解决这个问题。执行以下命令升级：

npm install ts-mockito@latest --save-dev

额外建议 #

如果实际场景需要模拟工厂方法，更推荐用mock()创建工厂的mock对象，再通过when()指定工厂方法的返回值，这种方式更符合测试隔离原则，也能避开spy代理的坑。

内容的提问来源于stack exchange，提问作者Jww