Polymer：Behavior能否引入Mixin？还是需全转为Mixin保障依赖？

Great question—this is a common pain point when working with these code reuse patterns, especially when implicit dependencies lead to fragile, hard-to-debug code. Let’s break down your options clearly:

1. You Can’t Directly "Import" a Mixin Into a Behavior #

Most frameworks and libraries treat behaviors and mixins as distinct mechanisms (behaviors are often runtime-attached configuration objects, while mixins are compile-time class extensions). There’s no built-in way to force a mixin to be applied to an element just because a behavior is used. Your original approach relies entirely on the element author remembering to include both, which is error-prone and easy to break.

2. The Most Reliable Fix: Unify on Mixins With Explicit Dependencies #

If you want to guarantee that dependencies are always present, converting all your behavior logic into mixins is the way to go. Mixins can explicitly extend other mixins, creating a chain that ensures all required functionality is included automatically:

// Define your core mixin with shared functionality
const CoreMixin = superclass => class extends superclass {
  coreUtilityMethod() {
    // Your original mixin logic here
  }
};

// Create a mixin that replaces your behavior, with explicit dependency on CoreMixin
const BehaviorReplacementMixin = superclass => class extends CoreMixin(superclass) {
  connectedCallback() {
    // You can safely call coreUtilityMethod() here—it’s guaranteed to exist
    this.coreUtilityMethod();
    // Your original behavior logic here
  }
};

When you apply BehaviorReplacementMixin to an element, CoreMixin is automatically included. No more crossing your fingers that someone remembers to add both!

3. If You Must Keep Behaviors: Add Defensive Runtime Checks #

If you can’t fully switch to mixins, you can add validation in your behavior to confirm the required mixin is present. This won’t auto-include the mixin, but it will catch missing dependencies early with clear, actionable errors:

const MyBehavior = {
  attached() {
    // Check for a method or property unique to your mixin
    if (typeof this.coreUtilityMethod !== 'function') {
      throw new Error('MyBehavior requires CoreMixin to be applied to the element');
    }
    // Now you can safely use the mixin’s functionality without silent failures
    this.coreUtilityMethod();
  }
};

This turns your implicit assumption into an explicit contract, making it obvious when someone forgets to include the necessary mixin.

Summary #

• To guarantee dependencies are always present: unify your code into mixins with explicit nested dependencies. This is the cleanest, most maintainable approach long-term.
• If behaviors are non-negotiable: add defensive checks to validate mixin presence at runtime, so missing dependencies don’t cause mysterious bugs.

内容的提问来源于stack exchange，提问作者Maarten