core/packages/runtime-core/src/apiInject.ts

import { isFunction } from '@vue/shared'
import { currentInstance } from './component'
import { currentRenderingInstance } from './componentRenderContext'
import { currentApp } from './apiCreateApp'
import { warn } from './warning'

interface InjectionConstraint<T> {}

export type InjectionKey<T> = symbol & InjectionConstraint<T>

export function provide<T, K = InjectionKey<T> | string | number>(
  key: K,
  value: K extends InjectionKey<infer V> ? V : T,
): void {
  if (!currentInstance) {
    if (__DEV__) {
      warn(`provide() can only be used inside setup().`)
    }
  } else {
    let provides = currentInstance.provides
    // by default an instance inherits its parent's provides object
    // but when it needs to provide values of its own, it creates its
    // own provides object using parent provides object as prototype.
    // this way in `inject` we can simply look up injections from direct
    // parent and let the prototype chain do the work.
    const parentProvides =
      currentInstance.parent && currentInstance.parent.provides
    if (parentProvides === provides) {
      provides = currentInstance.provides = Object.create(parentProvides)
    }
    // TS doesn't allow symbol as index type
    provides[key as string] = value
  }
}

export function inject<T>(key: InjectionKey<T> | string): T | undefined
export function inject<T>(
  key: InjectionKey<T> | string,
  defaultValue: T,
  treatDefaultAsFactory?: false,
): T
export function inject<T>(
  key: InjectionKey<T> | string,
  defaultValue: T | (() => T),
  treatDefaultAsFactory: true,
): T
export function inject(
  key: InjectionKey<any> | string,
  defaultValue?: unknown,
  treatDefaultAsFactory = false,
) {
  // fallback to `currentRenderingInstance` so that this can be called in
  // a functional component
  const instance = currentInstance || currentRenderingInstance

  // also support looking up from app-level provides w/ `app.runWithContext()`
  if (instance || currentApp) {
    // #2400
    // to support `app.use` plugins,
    // fallback to appContext's `provides` if the instance is at root
    // #11488, in a nested createApp, prioritize using the provides from currentApp
    const provides = currentApp
      ? currentApp._context.provides
      : instance
        ? instance.parent == null
          ? instance.vnode.appContext && instance.vnode.appContext.provides
          : instance.parent.provides
        : undefined

    if (provides && (key as string | symbol) in provides) {
      // TS doesn't allow symbol as index type
      return provides[key as string]
    } else if (arguments.length > 1) {
      return treatDefaultAsFactory && isFunction(defaultValue)
        ? defaultValue.call(instance && instance.proxy)
        : defaultValue
    } else if (__DEV__) {
      warn(`injection "${String(key)}" not found.`)
    }
  } else if (__DEV__) {
    warn(`inject() can only be used inside setup() or functional components.`)
  }
}

/**
 * Returns true if `inject()` can be used without warning about being called in the wrong place (e.g. outside of
 * setup()). This is used by libraries that want to use `inject()` internally without triggering a warning to the end
 * user. One example is `useRoute()` in `vue-router`.
 */
export function hasInjectionContext(): boolean {
  return !!(currentInstance || currentRenderingInstance || currentApp)
}