Implementation Principles of Provide and Inject in Vue3

Description
Provide and Inject are APIs in Vue3 that enable cross-component data transfer. They allow ancestor components to provide data, which descendant components can inject and use regardless of their nesting depth, avoiding the need to pass props through every intermediate level.

Detailed Implementation Principles

1. Data Structure Design of Component Instances
Each component instance internally maintains two key properties:

• provides: Stores data provided by the current component
• parent: Points to the parent component instance

interface ComponentInternalInstance {
  provides: Record<string, any>
  parent: ComponentInternalInstance | null
  // ... other properties
}

2. provides Processing During Component Instance Initialization
When creating a component instance, it inherits the parent component's provides object:

function createComponentInstance() {
  const instance = {
    provides: parent ? Object.create(parent.provides) : Object.create(null),
    parent,
    // ... other initializations
  }
  return instance
}

Here, Object.create(parent.provides) is used to achieve prototype chain inheritance, allowing the current component's provides object to access parent-provided data via the prototype chain.

3. Implementation Principle of provide
The provide function stores data in the current component instance's provides object:

function provide(key, value) {
  const currentInstance = getCurrentInstance()
  
  if (currentInstance) {
    let provides = currentInstance.provides
    
    // If the current component's provides is the same as the parent's, it's the first call to provide
    // A new provides object needs to be created to avoid polluting the parent
    if (currentInstance.parent?.provides === provides) {
      provides = currentInstance.provides = Object.create(provides)
    }
    
    provides[key] = value
  }
}

Key point: The first call to provide creates a new provides object, ensuring modifications don't affect the parent.

4. Implementation Principle of inject
The inject function searches up the component chain for the provided data:

function inject(key, defaultValue) {
  const currentInstance = getCurrentInstance()
  
  if (currentInstance) {
    const provides = currentInstance.parent?.provides
    
    if (provides && key in provides) {
      return provides[key]
    } else if (arguments.length > 1) {
      return defaultValue
    } else {
      // Warning in development environment if not found
      warn(`Injection key "${key}" not found`)
    }
  }
}

The search process essentially leverages JavaScript's prototype chain lookup:

• Search in the current component's provides
• If not found, search in parent provides via prototype chain
• Continue upward until the root component

5. Handling of Reactive Data
When providing reactive data, the injected component maintains reactivity:

// Provide reactive data
const count = ref(0)
provide('count', count)

// Injected data remains reactive
const injectedCount = inject('count')

This works because provide/inject only passes references; the reactivity mechanism of the reactive data itself remains intact.

6. Support for Symbol Keys
To avoid naming conflicts, Vue3 recommends using Symbol as keys for provided data:

const MyKey = Symbol()

provide(MyKey, 'some value')
const value = inject(MyKey)

7. Default Value Handling
inject supports default values, which are used when no corresponding provided value is found:

const value = inject('non-existent-key', 'default value')

Summary
The core implementation of provide/inject utilizes JavaScript's prototype chain mechanism, establishing a data provision chain through the component tree hierarchy. This design ensures cross-level data transfer while maintaining a clear component tree structure, making it a vital component communication mechanism in Vue3.