Life-cycle
States can implement life-cycle events.
If a state implements the NgxsOnChanges interface, its ngxsOnChanges method respond when (re)sets state. The states' ngxsOnChanges methods are invoked in a topological sorted order going from parent to child. The first parameter is the NgxsSimpleChange object of current and previous state.
export interface ZooStateModel {
animals: string[];
}
@State<ZooStateModel>({
name: 'zoo',
defaults: {
animals: []
}
})
@Injectable()
export class ZooState implements NgxsOnChanges {
ngxsOnChanges(change: NgxsSimpleChange) {
console.log('prev state', change.previousValue);
console.log('next state', change.currentValue);
}
}
If a state implements the
NgxsOnInit
interface, its ngxsOnInit
method will be invoked after all the states from the state's module definition have been initialized and pushed into the state stream. The states' ngxsOnInit
methods are invoked in a topological sorted order going from parent to child. The first parameter is the StateContext
where you can get the current state and dispatch actions as usual.export interface ZooStateModel {
animals: string[];
}
@State<ZooStateModel>({
name: 'zoo',
defaults: {
animals: []
}
})
@Injectable()
export class ZooState implements NgxsOnInit {
ngxsOnInit(ctx: StateContext<ZooStateModel>) {
console.log('State initialized, now getting animals');
ctx.dispatch(new GetAnimals());
}
}
If a state implements the
NgxsAfterBootstrap
interface, its ngxsAfterBootstrap
method will be invoked after the root view and all its children have been rendered, because Angular invokes functions, retrieved from the injector by APP_BOOTSTRAP_LISTENER
token, only after creating and attaching ComponentRef
of the root component to the tree of views.export interface ZooStateModel {
animals: string[];
}
@State<ZooStateModel>({
name: 'zoo',
defaults: {
animals: []
}
})
@Injectable()
export class ZooState implements NgxsAfterBootstrap {
ngxsAfterBootstrap(ctx: StateContext<ZooStateModel>) {
console.log('The application has been fully rendered');
ctx.dispatch(new GetAnimals());
}
}
After creating the state by calling its constructor, NGXS calls the lifecycle hook methods in the following sequence at specific moments:
Hook | Purpose and Timing |
---|---|
ngxsOnChanges() | Called before ngxsOnInit() and whenever state changes. |
ngxsOnInit() | Called once, after the first ngxsOnChanges() and before the APP_INITIALIZER token is resolved. |
ngxsAfterBootstrap() | Called once, after the root view and all its children have been rendered. |
If you have feature modules they need to be imported after the root module:
// feature.module.ts
@NgModule({
imports: [NgxsModule.forFeature([FeatureState])]
})
export class FeatureModule {}
// app.module.ts
@NgModule({
imports: [NgxsModule.forRoot([]), FeatureModule]
})
export class AppModule {}
The
APP_INITIALIZER
is just a token that references Promise factories. If you've ever used the APP_INITIALIZER
token, then you are already familiar with its syntax:export function appInitializerFactory() {
return () => Promise.resolve();
}
@NgModule({
providers: [
{
provide: APP_INITIALIZER,
useFactory: appInitializerFactory,
multi: true
}
]
})
export class AppModule {}
This token is injected into
ApplicationInitStatus
class. What does Angular do under the hood? It gets an instance of this class and invokes the runInitializers
method:const initStatus = moduleRef.injector.get(ApplicationInitStatus);
initStatus.runInitializers();
All that it does inside this method is looping via the
APP_INITIALIZER
array (as it's a multi
token) and invoking those factories, that you've provided in useFactory
properties:for (let i = 0; i < this.appInits.length; i++) {
const initResult = this.appInits[i]();
if (isPromise(initResult)) {
asyncInitPromises.push(initResult);
}
}
Then
asyncInitPromises
are provided into Promise.all
. That's all the magic. That's why the bootstrapModule
returns a Promise
:platformBrowser()
.bootstrapModule(AppModule)
.then(() => {
console.log('Hey there!');
});
Everything that we examined earlier is very important, because from this comes the fact that
APP_INITIALIZER
is resolved after NGXS states are initialized. They are initialized by the NgxsModule
that is imported into the AppModule
. The ngxsOnInit
method on states is also invoked before the APP_INITIALIZER
token is resolved. Given the following code:@Injectable({ providedIn: 'root' })
export class ConfigService {
private version: string | null = null;
constructor(private http: HttpClient) {}
loadVersion(): Observable<string> {
return this.http.get<string>('/api/version').pipe(
tap(version => {
this.version = version;
})
);
}
getVersion(): never | string {
if (this.version === null) {
throw new Error('"version" is not available yet!');
}
return this.version;
}
}
@State<string | null>({
name: 'version',
defaults: null
})
@Injectable()
export class VersionState implements NgxsOnInit {
constructor(private configService: ConfigService) {}
ngxsOnInit(ctx: StateContext<string | null>) {
ctx.setState(this.configService.getVersion());
}
}
export function appInitializerFactory(configService: ConfigService) {
return () => configService.loadVersion().toPromise();
}
@NgModule({
imports: [NgxsModule.forRoot([VersionState])],
providers: [
{
provide: APP_INITIALIZER,
useFactory: appInitializerFactory,
multi: true,
deps: [ConfigService]
}
]
})
export class AppModule {}
The above example is used only for demonstration purposes! This code will throw an error because the
getVersion
method is invoked before the version
property is set. Why? Because the ngxsOnInit
methods on states are invoked before the APP_INITIALIZER
is invoked!There are different solutions. Let's look at the simplest. The first solution would be to use the
ngxsAfterBootstrap
method:@State<string | null>({
name: 'version',
defaults: null
})
@Injectable()
export class VersionState implements NgxsAfterBootstrap {
constructor(private configService: ConfigService) {}
ngxsAfterBootstrap(ctx: StateContext<string | null>) {
ctx.setState(this.configService.getVersion());
}
}
The second solution would be dispatching some
SetVersion
action right after the version is fetched:export class SetVersion {
static readonly type = '[Version] Set version';
constructor(public version: string) {}
}
@State<string | null>({
name: 'version',
defaults: null
})
@Injectable()
export class VersionState {
@Action(SetVersion)
setVersion(ctx: StateContext<string | null>, action: SetVersion): void {
ctx.setState(action.version);
}
}
@Injectable({ providedIn: 'root' })
export class ConfigService {
constructor(private http: HttpClient, private store: Store) {}
loadVersion() {
return this.http.get<string>('/api/version').pipe(
tap(version => {
this.store.dispatch(new SetVersion(version));
})
);
}
}
In conclusion, do not try to access any data in state constructors or
ngxsOnInit
methods that is fetched during the APP_INITIALIZER
stage.Last modified 1yr ago