元归约器

A meta reducer is a higher order reducer that allows you to take action on the global state rather than a state slice. In NGXS, we don't have this concept but you can accomplish this with plugins.

An example of a meta reducer might be to clear the entire state when a user logs out. An example implementation would be:

import { getActionTypeFromInstance } from '@ngxs/store';

export function logoutPlugin(state, action, next) {
  // Use the get action type helper to determine the type
  if (getActionTypeFromInstance(action) === Logout.type) {
    // if we are a logout type, lets erase all the state
    state = {};
  }

  // return the next function with the empty state
  return next(state, action);
}

Then we import that like:

import { NgModule } from '@angular/core';
import { NGXS_PLUGINS } from '@ngxs/store';

@NgModule({
  imports: [NgxsModule.forRoot([])],
  providers: [
    {
      provide: NGXS_PLUGINS,
      useValue: logoutPlugin,
      multi: true
    }
  ]
})
export class AppModule {}

Now when we dispatch the logout action it will use our new plugin and erase the state.