Vue 3源码深度解析之：`Vue`组件的`mixins`和`extends`：它们的工作原理与`Composition API`的对比。

Alright, gather ’round everyone! Let’s dive into the fascinating world of Vue 3 components and explore two classic features: mixins and extends. We’ll dissect how they work, compare them to the newer Composition API, and see when you might still want to reach for these old friends.

I. A Blast from the Past: Introducing mixins and extends

Back in the Vue 2 days (and even still in Vue 3 for legacy reasons), mixins and extends were the go-to solutions for code reuse and component inheritance. Think of them as recipes or blueprints you could plug into your components to add extra functionality.

• mixins: A mixin is essentially a bag of component options (data, methods, computed properties, lifecycle hooks) that you can merge into multiple components. Think of it like adding a sprinkle of common functionality to several different cakes.

• extends: extends is a way to create a base component and then build upon it. It’s like inheriting from a parent class in object-oriented programming. You’re creating a new component that has all the properties of the base component, with the ability to override or add new ones.

II. mixins in Action: Sharing is Caring (Sometimes)

Let’s say you have a bunch of components that all need to fetch data from an API. You could write the same data fetching logic in each component, but that’s repetitive and prone to errors. Enter the mixin!

// apiMixin.js
export const apiMixin = {
  data() {
    return {
      isLoading: false,
      apiData: null,
      errorMessage: null
    };
  },
  methods: {
    async fetchData(url) {
      this.isLoading = true;
      this.errorMessage = null;
      try {
        const response = await fetch(url);
        this.apiData = await response.json();
      } catch (error) {
        this.errorMessage = error.message;
      } finally {
        this.isLoading = false;
      }
    }
  }
};

Now, in your components:

<template>
  <div>
    <p v-if="isLoading">Loading...</p>
    <p v-if="errorMessage">Error: {{ errorMessage }}</p>
    <div v-if="apiData">
      <!-- Display your data here -->
      <p>Data: {{ apiData }}</p>
    </div>
  </div>
</template>

<script>
import { apiMixin } from './apiMixin.js';

export default {
  mixins: [apiMixin],
  mounted() {
    this.fetchData('https://jsonplaceholder.typicode.com/todos/1');
  }
};
</script>

Boom! You’ve now injected isLoading, apiData, errorMessage, and fetchData into your component. Easy peasy, right?

The Fine Print: Potential Pitfalls of mixins

While mixins seem great on the surface, they can lead to some problems, especially in larger projects:

• Naming Collisions: If two mixins (or a mixin and the component itself) define the same data property or method, Vue will merge them, but the component’s definition wins. This can lead to unexpected behavior and debugging headaches. Imagine two mixins both defining a title property – which one gets used?

• Implicit Dependencies: It can be hard to know where a particular property or method is coming from. Is it defined in the component itself? Or is it being injected by a mixin? This makes code harder to understand and maintain. You’re essentially inheriting behavior without explicit declaration.

• The "Mixin Soup": When a component uses many mixins, it becomes difficult to reason about the component’s overall behavior. You end up with a "soup" of properties and methods, making it hard to trace the origin of any particular piece of functionality.

III. extends: Building Upon Foundations

extends provides a way to inherit the properties and methods of another component, creating a base component that can be extended in different ways.

// BaseButton.js
export const BaseButton = {
  props: {
    label: {
      type: String,
      required: true
    },
    disabled: {
      type: Boolean,
      default: false
    }
  },
  template: `
    <button :disabled="disabled">
      {{ label }}
    </button>
  `
};

Now, let’s create a primary button that extends this base button:

<template>
  <BaseButton :label="label" :disabled="disabled" class="primary-button" />
</template>

<script>
import { BaseButton } from './BaseButton.js';

export default {
  components: {
    BaseButton
  },
  extends: BaseButton,
  props: {
    //Override the type of the label property to number
    label: {
      type: Number,
      required: true
    }
  },
  mounted() {
    console.log("Label is a number: ", typeof this.label);
  }
};
</script>

<style scoped>
.primary-button {
  background-color: blue;
  color: white;
  padding: 10px 20px;
  border: none;
  cursor: pointer;
}
</style>

In this example, PrimaryButton inherits all the properties and methods of BaseButton, and we can add custom styling.

extends: A Bit More Structured, But Still…

extends offers a slightly more structured approach than mixins, but it still shares some of the same drawbacks:

• Limited Flexibility: You can only extend one component. Multiple inheritance isn’t supported.

• Tight Coupling: Extending a component creates a strong dependency between the parent and child. Changes to the parent can have unintended consequences for the child components.

• Namespace Clashes (Again!): While less common than with mixins, namespace collisions can still occur if you’re not careful about property names.

IV. The Rise of the Composition API: A More Explicit and Flexible Approach

Vue 3 introduced the Composition API, which provides a more explicit and flexible way to reuse code and share logic between components. Instead of injecting properties and methods through mixins or extends, you create reusable functions (called "composables") that encapsulate the logic.

Let’s rewrite our API fetching example using the Composition API:

// useApi.js
import { ref, reactive, onMounted } from 'vue';

export function useApi(url) {
  const isLoading = ref(false);
  const apiData = reactive({});
  const errorMessage = ref(null);

  const fetchData = async () => {
    isLoading.value = true;
    errorMessage.value = null;
    try {
      const response = await fetch(url);
      const data = await response.json();
      Object.assign(apiData, data); // Assign the data to the reactive object
    } catch (error) {
      errorMessage.value = error.message;
    } finally {
      isLoading.value = false;
    }
  };

  onMounted(fetchData);

  return {
    isLoading,
    apiData,
    errorMessage,
    fetchData
  };
}

Now, in your component:

<template>
  <div>
    <p v-if="isLoading">Loading...</p>
    <p v-if="errorMessage">Error: {{ errorMessage }}</p>
    <div v-if="Object.keys(apiData).length > 0">
      <!-- Display your data here -->
      <p>Data: {{ apiData }}</p>
    </div>
  </div>
</template>

<script>
import { useApi } from './useApi.js';

export default {
  setup() {
    const { isLoading, apiData, errorMessage, fetchData } = useApi('https://jsonplaceholder.typicode.com/todos/1');

    return {
      isLoading,
      apiData,
      errorMessage,
      fetchData
    };
  }
};
</script>

Composition API: Why It’s the New Sheriff in Town

The Composition API addresses many of the shortcomings of mixins and extends:

• Explicit Dependencies: It’s crystal clear where each piece of logic is coming from. You explicitly import and destructure the values you need from the composable function.

• No Naming Collisions: You have complete control over the names of the variables and functions you expose from your composables.

• Improved Code Organization: Composables promote a more modular and organized codebase. You can group related logic into reusable functions, making your code easier to understand and maintain.

• Testability: Composables are just plain JavaScript functions, making them easy to test in isolation.

• Flexibility: You can compose multiple composables together to create complex logic without creating a tangled web of dependencies.

V. mixins vs. extends vs. Composition API: A Head-to-Head Comparison

Let’s break down the key differences in a table:

Feature	mixins	extends	Composition API
Code Reuse	Yes	Yes	Yes
Inheritance	No (merging, not true inheritance)	Yes (single inheritance)	No (composition)
Explicit Deps	No (implicit injection)	No (implicit inheritance)	Yes (explicit imports)
Naming Conflicts	High risk	Moderate risk	Low risk (explicit control)
Testability	Difficult	Difficult	Easy
Flexibility	Moderate	Low	High
Code Clarity	Low	Moderate	High
Maintainability	Low	Moderate	High

VI. When to Use mixins and extends (If Ever)

Given the advantages of the Composition API, you might be wondering if there’s ever a reason to use mixins or extends in Vue 3. Here are a few scenarios where they might still be useful:

• Legacy Codebases: If you’re working on a large Vue 2 project, it might not be feasible to refactor everything to use the Composition API. In that case, mixins and extends can still be a useful way to share code.

• Simple, Self-Contained Logic: For very small, self-contained pieces of logic that don’t have any dependencies, mixins might be a quick and easy solution. However, even in these cases, it’s generally better to use the Composition API for consistency and maintainability.

• Framework/Library Interoperability: Some third-party libraries or frameworks might still rely on the Options API and benefit from mixins.

VII. Migrating from mixins and extends to Composition API

The process involves extracting the reusable logic from your mixins and extends into composable functions. This typically involves:

1. Identifying Reusable Logic: Pinpoint the specific data properties, methods, and lifecycle hooks that are being shared by your mixins or extends.
2. Creating Composables: Create new JavaScript functions (composables) that encapsulate this logic. Use ref, reactive, and lifecycle hooks from Vue to manage the state and behavior within the composable.
3. Importing and Using Composables: Import the composables into your components and use the setup function to access the returned values.

Example: Migrating a mixin to a Composable

Let’s revisit our apiMixin example:

Original mixin:

// apiMixin.js
export const apiMixin = {
  data() {
    return {
      isLoading: false,
      apiData: null,
      errorMessage: null
    };
  },
  methods: {
    async fetchData(url) {
      this.isLoading = true;
      this.errorMessage = null;
      try {
        const response = await fetch(url);
        this.apiData = await response.json();
      } catch (error) {
        this.errorMessage = error.message;
      } finally {
        this.isLoading = false;
      }
    }
  }
};

Migrated Composable:

// useApi.js
import { ref, reactive, onMounted } from 'vue';

export function useApi(url) {
  const isLoading = ref(false);
  const apiData = reactive({});
  const errorMessage = ref(null);

  const fetchData = async () => {
    isLoading.value = true;
    errorMessage.value = null;
    try {
      const response = await fetch(url);
      const data = await response.json();
      Object.assign(apiData, data); // Assign the data to the reactive object
    } catch (error) {
      errorMessage.value = error.message;
    } finally {
      isLoading.value = false;
    }
  };

  onMounted(fetchData);

  return {
    isLoading,
    apiData,
    errorMessage,
    fetchData
  };
}

The composable is functionally equivalent to the mixin, but it provides a more explicit and maintainable way to share the API fetching logic.

VIII. Conclusion: Embrace the Future (and the Composition API)

While mixins and extends served their purpose in the past, the Composition API offers a more powerful, flexible, and maintainable approach to code reuse in Vue 3. By embracing the Composition API, you can write cleaner, more organized, and easier-to-understand code. So, while it’s good to know the history and understand how mixins and extends work, focus on leveraging the Composition API for your future Vue projects. You’ll thank yourself later!