/**
 * @license
 * Copyright Google Inc. All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

import {Observable, Observer, Subscription, merge} from 'rxjs';
import {share} from 'rxjs/operators';

import {ApplicationInitStatus} from './application_init';
import {APP_BOOTSTRAP_LISTENER, PLATFORM_INITIALIZER} from './application_tokens';
import {getCompilerFacade} from './compiler/compiler_facade';
import {Console} from './console';
import {Injectable, InjectionToken, Injector, StaticProvider} from './di';
import {INJECTOR_SCOPE} from './di/scope';
import {ErrorHandler} from './error_handler';
import {DEFAULT_LOCALE_ID} from './i18n/localization';
import {LOCALE_ID} from './i18n/tokens';
import {Type} from './interface/type';
import {ivyEnabled} from './ivy_switch';
import {COMPILER_OPTIONS, CompilerFactory, CompilerOptions} from './linker/compiler';
import {ComponentFactory, ComponentRef} from './linker/component_factory';
import {ComponentFactoryBoundToModule, ComponentFactoryResolver} from './linker/component_factory_resolver';
import {InternalNgModuleRef, NgModuleFactory, NgModuleRef} from './linker/ng_module_factory';
import {InternalViewRef, ViewRef} from './linker/view_ref';
import {isComponentResourceResolutionQueueEmpty, resolveComponentResources} from './metadata/resource_loading';
import {WtfScopeFn, wtfCreateScope, wtfLeave} from './profile/profile';
import {assertNgModuleType} from './render3/assert';
import {ComponentFactory as R3ComponentFactory} from './render3/component_ref';
import {setLocaleId} from './render3/i18n';
import {NgModuleFactory as R3NgModuleFactory} from './render3/ng_module_ref';
import {publishDefaultGlobalUtils as _publishDefaultGlobalUtils} from './render3/util/global_utils';
import {Testability, TestabilityRegistry} from './testability/testability';
import {isDevMode} from './util/is_dev_mode';
import {isPromise} from './util/lang';
import {scheduleMicroTask} from './util/microtask';
import {stringify} from './util/stringify';
import {NgZone, NoopNgZone} from './zone/ng_zone';

let _platform: PlatformRef;

let compileNgModuleFactory:
    <M>(injector: Injector, options: CompilerOptions, moduleType: Type<M>) =>
        Promise<NgModuleFactory<M>> = compileNgModuleFactory__PRE_R3__;

function compileNgModuleFactory__PRE_R3__<M>(
    injector: Injector, options: CompilerOptions,
    moduleType: Type<M>): Promise<NgModuleFactory<M>> {
  const compilerFactory: CompilerFactory = injector.get(CompilerFactory);
  const compiler = compilerFactory.createCompiler([options]);
  return compiler.compileModuleAsync(moduleType);
}

export function compileNgModuleFactory__POST_R3__<M>(
    injector: Injector, options: CompilerOptions,
    moduleType: Type<M>): Promise<NgModuleFactory<M>> {
  ngDevMode && assertNgModuleType(moduleType);
  const moduleFactory = new R3NgModuleFactory(moduleType);

  if (isComponentResourceResolutionQueueEmpty()) {
    return Promise.resolve(moduleFactory);
  }

  const compilerOptions = injector.get(COMPILER_OPTIONS, []).concat(options);
  const compilerProviders = _mergeArrays(compilerOptions.map(o => o.providers !));

  // In case there are no compiler providers, we just return the module factory as
  // there won't be any resource loader. This can happen with Ivy, because AOT compiled
  // modules can be still passed through "bootstrapModule". In that case we shouldn't
  // unnecessarily require the JIT compiler.
  if (compilerProviders.length === 0) {
    return Promise.resolve(moduleFactory);
  }

  const compiler = getCompilerFacade();
  const compilerInjector = Injector.create({providers: compilerProviders});
  const resourceLoader = compilerInjector.get(compiler.ResourceLoader);
  // The resource loader can also return a string while the "resolveComponentResources"
  // always expects a promise. Therefore we need to wrap the returned value in a promise.
  return resolveComponentResources(url => Promise.resolve(resourceLoader.get(url)))
      .then(() => moduleFactory);
}

// the `window.ng` global utilities are only available in non-VE versions of
// Angular. The function switch below will make sure that the code is not
// included into Angular when PRE mode is active.
export function publishDefaultGlobalUtils__PRE_R3__() {}
export function publishDefaultGlobalUtils__POST_R3__() {
  ngDevMode && _publishDefaultGlobalUtils();
}

let publishDefaultGlobalUtils: () => any = publishDefaultGlobalUtils__PRE_R3__;

let isBoundToModule: <C>(cf: ComponentFactory<C>) => boolean = isBoundToModule__PRE_R3__;

export function isBoundToModule__PRE_R3__<C>(cf: ComponentFactory<C>): boolean {
  return cf instanceof ComponentFactoryBoundToModule;
}

export function isBoundToModule__POST_R3__<C>(cf: ComponentFactory<C>): boolean {
  return (cf as R3ComponentFactory<C>).isBoundToModule;
}

export const ALLOW_MULTIPLE_PLATFORMS = new InjectionToken<boolean>('AllowMultipleToken');



/**
 * A token for third-party components that can register themselves with NgProbe.
 *
 * @publicApi
 */
export class NgProbeToken {
  constructor(public name: string, public token: any) {}
}

/**
 * Creates a platform.
 * Platforms have to be eagerly created via this function.
 *
 * @publicApi
 */
export function createPlatform(injector: Injector): PlatformRef {
  if (_platform && !_platform.destroyed &&
      !_platform.injector.get(ALLOW_MULTIPLE_PLATFORMS, false)) {
    throw new Error(
        'There can be only one platform. Destroy the previous one to create a new one.');
  }
  publishDefaultGlobalUtils();
  _platform = injector.get(PlatformRef);
  const inits = injector.get(PLATFORM_INITIALIZER, null);
  if (inits) inits.forEach((init: any) => init());
  return _platform;
}

/**
 * Creates a factory for a platform
 *
 * @publicApi
 */
export function createPlatformFactory(
    parentPlatformFactory: ((extraProviders?: StaticProvider[]) => PlatformRef) | null,
    name: string, providers: StaticProvider[] = []): (extraProviders?: StaticProvider[]) =>
    PlatformRef {
  const desc = `Platform: ${name}`;
  const marker = new InjectionToken(desc);
  return (extraProviders: StaticProvider[] = []) => {
    let platform = getPlatform();
    if (!platform || platform.injector.get(ALLOW_MULTIPLE_PLATFORMS, false)) {
      if (parentPlatformFactory) {
        parentPlatformFactory(
            providers.concat(extraProviders).concat({provide: marker, useValue: true}));
      } else {
        const injectedProviders: StaticProvider[] =
            providers.concat(extraProviders).concat({provide: marker, useValue: true}, {
              provide: INJECTOR_SCOPE,
              useValue: 'platform'
            });
        createPlatform(Injector.create({providers: injectedProviders, name: desc}));
      }
    }
    return assertPlatform(marker);
  };
}

/**
 * Checks that there currently is a platform which contains the given token as a provider.
 *
 * @publicApi
 */
export function assertPlatform(requiredToken: any): PlatformRef {
  const platform = getPlatform();

  if (!platform) {
    throw new Error('No platform exists!');
  }

  if (!platform.injector.get(requiredToken, null)) {
    throw new Error(
        'A platform with a different configuration has been created. Please destroy it first.');
  }

  return platform;
}

/**
 * Destroy the existing platform.
 *
 * @publicApi
 */
export function destroyPlatform(): void {
  if (_platform && !_platform.destroyed) {
    _platform.destroy();
  }
}

/**
 * Returns the current platform.
 *
 * @publicApi
 */
export function getPlatform(): PlatformRef|null {
  return _platform && !_platform.destroyed ? _platform : null;
}

/**
 * Provides additional options to the bootstraping process.
 *
 *
 */
export interface BootstrapOptions {
  /**
   * Optionally specify which `NgZone` should be used.
   *
   * - Provide your own `NgZone` instance.
   * - `zone.js` - Use default `NgZone` which requires `Zone.js`.
   * - `noop` - Use `NoopNgZone` which does nothing.
   */
  ngZone?: NgZone|'zone.js'|'noop';
}

/**
 * The Angular platform is the entry point for Angular on a web page. Each page
 * has exactly one platform, and services (such as reflection) which are common
 * to every Angular application running on the page are bound in its scope.
 *
 * A page's platform is initialized implicitly when a platform is created via a platform factory
 * (e.g. {@link platformBrowser}), or explicitly by calling the {@link createPlatform} function.
 *
 * @publicApi
 */
@Injectable()
export class PlatformRef {
  private _modules: NgModuleRef<any>[] = [];
  private _destroyListeners: Function[] = [];
  private _destroyed: boolean = false;

  /** @internal */
  constructor(private _injector: Injector) {}

  /**
   * Creates an instance of an `@NgModule` for the given platform
   * for offline compilation.
   *
   * @usageNotes
   * ### Simple Example
   *
   * ```typescript
   * my_module.ts:
   *
   * @NgModule({
   *   imports: [BrowserModule]
   * })
   * class MyModule {}
   *
   * main.ts:
   * import {MyModuleNgFactory} from './my_module.ngfactory';
   * import {platformBrowser} from '@angular/platform-browser';
   *
   * let moduleRef = platformBrowser().bootstrapModuleFactory(MyModuleNgFactory);
   * ```
   */
  bootstrapModuleFactory<M>(moduleFactory: NgModuleFactory<M>, options?: BootstrapOptions):
      Promise<NgModuleRef<M>> {
    // Note: We need to create the NgZone _before_ we instantiate the module,
    // as instantiating the module creates some providers eagerly.
    // So we create a mini parent injector that just contains the new NgZone and
    // pass that as parent to the NgModuleFactory.
    const ngZoneOption = options ? options.ngZone : undefined;
    const ngZone = getNgZone(ngZoneOption);
    const providers: StaticProvider[] = [{provide: NgZone, useValue: ngZone}];
    // Attention: Don't use ApplicationRef.run here,
    // as we want to be sure that all possible constructor calls are inside `ngZone.run`!
    return ngZone.run(() => {
      const ngZoneInjector = Injector.create(
          {providers: providers, parent: this.injector, name: moduleFactory.moduleType.name});
      const moduleRef = <InternalNgModuleRef<M>>moduleFactory.create(ngZoneInjector);
      const exceptionHandler: ErrorHandler|null = moduleRef.injector.get(ErrorHandler, null);
      if (!exceptionHandler) {
        throw new Error('No ErrorHandler. Is platform module (BrowserModule) included?');
      }
      // If the `LOCALE_ID` provider is defined at bootstrap we set the value for runtime i18n (ivy)
      if (ivyEnabled) {
        const localeId = moduleRef.injector.get(LOCALE_ID, DEFAULT_LOCALE_ID);
        setLocaleId(localeId || DEFAULT_LOCALE_ID);
      }
      moduleRef.onDestroy(() => remove(this._modules, moduleRef));
      ngZone !.runOutsideAngular(
          () => ngZone !.onError.subscribe(
              {next: (error: any) => { exceptionHandler.handleError(error); }}));
      return _callAndReportToErrorHandler(exceptionHandler, ngZone !, () => {
        const initStatus: ApplicationInitStatus = moduleRef.injector.get(ApplicationInitStatus);
        initStatus.runInitializers();
        return initStatus.donePromise.then(() => {
          this._moduleDoBootstrap(moduleRef);
          return moduleRef;
        });
      });
    });
  }

  /**
   * Creates an instance of an `@NgModule` for a given platform using the given runtime compiler.
   *
   * @usageNotes
   * ### Simple Example
   *
   * ```typescript
   * @NgModule({
   *   imports: [BrowserModule]
   * })
   * class MyModule {}
   *
   * let moduleRef = platformBrowser().bootstrapModule(MyModule);
   * ```
   *
   */
  bootstrapModule<M>(
      moduleType: Type<M>, compilerOptions: (CompilerOptions&BootstrapOptions)|
      Array<CompilerOptions&BootstrapOptions> = []): Promise<NgModuleRef<M>> {
    const options = optionsReducer({}, compilerOptions);
    return compileNgModuleFactory(this.injector, options, moduleType)
        .then(moduleFactory => this.bootstrapModuleFactory(moduleFactory, options));
  }

  private _moduleDoBootstrap(moduleRef: InternalNgModuleRef<any>): void {
    const appRef = moduleRef.injector.get(ApplicationRef) as ApplicationRef;
    if (moduleRef._bootstrapComponents.length > 0) {
      moduleRef._bootstrapComponents.forEach(f => appRef.bootstrap(f));
    } else if (moduleRef.instance.ngDoBootstrap) {
      moduleRef.instance.ngDoBootstrap(appRef);
    } else {
      throw new Error(
          `The module ${stringify(moduleRef.instance.constructor)} was bootstrapped, but it does not declare "@NgModule.bootstrap" components nor a "ngDoBootstrap" method. ` +
          `Please define one of these.`);
    }
    this._modules.push(moduleRef);
  }

  /**
   * Register a listener to be called when the platform is disposed.
   */
  onDestroy(callback: () => void): void { this._destroyListeners.push(callback); }

  /**
   * Retrieve the platform {@link Injector}, which is the parent injector for
   * every Angular application on the page and provides singleton providers.
   */
  get injector(): Injector { return this._injector; }

  /**
   * Destroy the Angular platform and all Angular applications on the page.
   */
  destroy() {
    if (this._destroyed) {
      throw new Error('The platform has already been destroyed!');
    }
    this._modules.slice().forEach(module => module.destroy());
    this._destroyListeners.forEach(listener => listener());
    this._destroyed = true;
  }

  get destroyed() { return this._destroyed; }
}

function getNgZone(ngZoneOption?: NgZone | 'zone.js' | 'noop'): NgZone {
  let ngZone: NgZone;

  if (ngZoneOption === 'noop') {
    ngZone = new NoopNgZone();
  } else {
    ngZone = (ngZoneOption === 'zone.js' ? undefined : ngZoneOption) ||
        new NgZone({enableLongStackTrace: isDevMode()});
  }
  return ngZone;
}

function _callAndReportToErrorHandler(
    errorHandler: ErrorHandler, ngZone: NgZone, callback: () => any): any {
  try {
    const result = callback();
    if (isPromise(result)) {
      return result.catch((e: any) => {
        ngZone.runOutsideAngular(() => errorHandler.handleError(e));
        // rethrow as the exception handler might not do it
        throw e;
      });
    }

    return result;
  } catch (e) {
    ngZone.runOutsideAngular(() => errorHandler.handleError(e));
    // rethrow as the exception handler might not do it
    throw e;
  }
}

function optionsReducer<T extends Object>(dst: any, objs: T | T[]): T {
  if (Array.isArray(objs)) {
    dst = objs.reduce(optionsReducer, dst);
  } else {
    dst = {...dst, ...(objs as any)};
  }
  return dst;
}

/**
 * A reference to an Angular application running on a page.
 *
 * @usageNotes
 *
 * {@a is-stable-examples}
 * ### isStable examples and caveats
 *
 * Note two important points about `isStable`, demonstrated in the examples below:
 * - the application will never be stable if you start any kind
 * of recurrent asynchronous task when the application starts
 * (for example for a polling process, started with a `setInterval`, a `setTimeout`
 * or using RxJS operators like `interval`);
 * - the `isStable` Observable runs outside of the Angular zone.
 *
 * Let's imagine that you start a recurrent task
 * (here incrementing a counter, using RxJS `interval`),
 * and at the same time subscribe to `isStable`.
 *
 * ```
 * constructor(appRef: ApplicationRef) {
 *   appRef.isStable.pipe(
 *      filter(stable => stable)
 *   ).subscribe(() => console.log('App is stable now');
 *   interval(1000).subscribe(counter => console.log(counter));
 * }
 * ```
 * In this example, `isStable` will never emit `true`,
 * and the trace "App is stable now" will never get logged.
 *
 * If you want to execute something when the app is stable,
 * you have to wait for the application to be stable
 * before starting your polling process.
 *
 * ```
 * constructor(appRef: ApplicationRef) {
 *   appRef.isStable.pipe(
 *     first(stable => stable),
 *     tap(stable => console.log('App is stable now')),
 *     switchMap(() => interval(1000))
 *   ).subscribe(counter => console.log(counter));
 * }
 * ```
 * In this example, the trace "App is stable now" will be logged
 * and then the counter starts incrementing every second.
 *
 * Note also that this Observable runs outside of the Angular zone,
 * which means that the code in the subscription
 * to this Observable will not trigger the change detection.
 *
 * Let's imagine that instead of logging the counter value,
 * you update a field of your component
 * and display it in its template.
 *
 * ```
 * constructor(appRef: ApplicationRef) {
 *   appRef.isStable.pipe(
 *     first(stable => stable),
 *     switchMap(() => interval(1000))
 *   ).subscribe(counter => this.value = counter);
 * }
 * ```
 * As the `isStable` Observable runs outside the zone,
 * the `value` field will be updated properly,
 * but the template will not be refreshed!
 *
 * You'll have to manually trigger the change detection to update the template.
 *
 * ```
 * constructor(appRef: ApplicationRef, cd: ChangeDetectorRef) {
 *   appRef.isStable.pipe(
 *     first(stable => stable),
 *     switchMap(() => interval(1000))
 *   ).subscribe(counter => {
 *     this.value = counter;
 *     cd.detectChanges();
 *   });
 * }
 * ```
 *
 * Or make the subscription callback run inside the zone.
 *
 * ```
 * constructor(appRef: ApplicationRef, zone: NgZone) {
 *   appRef.isStable.pipe(
 *     first(stable => stable),
 *     switchMap(() => interval(1000))
 *   ).subscribe(counter => zone.run(() => this.value = counter));
 * }
 * ```
 *
 * @publicApi
 */
@Injectable()
export class ApplicationRef {
  /** @internal */
  static _tickScope: WtfScopeFn = wtfCreateScope('ApplicationRef#tick()');
  private _bootstrapListeners: ((compRef: ComponentRef<any>) => void)[] = [];
  private _views: InternalViewRef[] = [];
  private _runningTick: boolean = false;
  private _enforceNoNewChanges: boolean = false;
  private _stable = true;

  /**
   * Get a list of component types registered to this application.
   * This list is populated even before the component is created.
   */
  public readonly componentTypes: Type<any>[] = [];

  /**
   * Get a list of components registered to this application.
   */
  public readonly components: ComponentRef<any>[] = [];

  /**
   * Returns an Observable that indicates when the application is stable or unstable.
   *
   * @see  [Usage notes](#is-stable-examples) for examples and caveats when using this API.
   */
  // TODO(issue/24571): remove '!'.
  public readonly isStable !: Observable<boolean>;

  /** @internal */
  constructor(
      private _zone: NgZone, private _console: Console, private _injector: Injector,
      private _exceptionHandler: ErrorHandler,
      private _componentFactoryResolver: ComponentFactoryResolver,
      private _initStatus: ApplicationInitStatus) {
    this._enforceNoNewChanges = isDevMode();

    this._zone.onMicrotaskEmpty.subscribe(
        {next: () => { this._zone.run(() => { this.tick(); }); }});

    const isCurrentlyStable = new Observable<boolean>((observer: Observer<boolean>) => {
      this._stable = this._zone.isStable && !this._zone.hasPendingMacrotasks &&
          !this._zone.hasPendingMicrotasks;
      this._zone.runOutsideAngular(() => {
        observer.next(this._stable);
        observer.complete();
      });
    });

    const isStable = new Observable<boolean>((observer: Observer<boolean>) => {
      // Create the subscription to onStable outside the Angular Zone so that
      // the callback is run outside the Angular Zone.
      let stableSub: Subscription;
      this._zone.runOutsideAngular(() => {
        stableSub = this._zone.onStable.subscribe(() => {
          NgZone.assertNotInAngularZone();

          // Check whether there are no pending macro/micro tasks in the next tick
          // to allow for NgZone to update the state.
          scheduleMicroTask(() => {
            if (!this._stable && !this._zone.hasPendingMacrotasks &&
                !this._zone.hasPendingMicrotasks) {
              this._stable = true;
              observer.next(true);
            }
          });
        });
      });

      const unstableSub: Subscription = this._zone.onUnstable.subscribe(() => {
        NgZone.assertInAngularZone();
        if (this._stable) {
          this._stable = false;
          this._zone.runOutsideAngular(() => { observer.next(false); });
        }
      });

      return () => {
        stableSub.unsubscribe();
        unstableSub.unsubscribe();
      };
    });

    (this as{isStable: Observable<boolean>}).isStable =
        merge(isCurrentlyStable, isStable.pipe(share()));
  }

  /**
   * Bootstrap a new component at the root level of the application.
   *
   * @usageNotes
   * ### Bootstrap process
   *
   * When bootstrapping a new root component into an application, Angular mounts the
   * specified application component onto DOM elements identified by the componentType's
   * selector and kicks off automatic change detection to finish initializing the component.
   *
   * Optionally, a component can be mounted onto a DOM element that does not match the
   * componentType's selector.
   *
   * ### Example
   * {@example core/ts/platform/platform.ts region='longform'}
   */
  bootstrap<C>(componentOrFactory: ComponentFactory<C>|Type<C>, rootSelectorOrNode?: string|any):
      ComponentRef<C> {
    if (!this._initStatus.done) {
      throw new Error(
          'Cannot bootstrap as there are still asynchronous initializers running. Bootstrap components in the `ngDoBootstrap` method of the root module.');
    }
    let componentFactory: ComponentFactory<C>;
    if (componentOrFactory instanceof ComponentFactory) {
      componentFactory = componentOrFactory;
    } else {
      componentFactory =
          this._componentFactoryResolver.resolveComponentFactory(componentOrFactory) !;
    }
    this.componentTypes.push(componentFactory.componentType);

    // Create a factory associated with the current module if it's not bound to some other
    const ngModule =
        isBoundToModule(componentFactory) ? undefined : this._injector.get(NgModuleRef);
    const selectorOrNode = rootSelectorOrNode || componentFactory.selector;
    const compRef = componentFactory.create(Injector.NULL, [], selectorOrNode, ngModule);

    compRef.onDestroy(() => { this._unloadComponent(compRef); });
    const testability = compRef.injector.get(Testability, null);
    if (testability) {
      compRef.injector.get(TestabilityRegistry)
          .registerApplication(compRef.location.nativeElement, testability);
    }

    this._loadComponent(compRef);
    if (isDevMode()) {
      this._console.log(
          `Angular is running in the development mode. Call enableProdMode() to enable the production mode.`);
    }
    return compRef;
  }

  /**
   * Invoke this method to explicitly process change detection and its side-effects.
   *
   * In development mode, `tick()` also performs a second change detection cycle to ensure that no
   * further changes are detected. If additional changes are picked up during this second cycle,
   * bindings in the app have side-effects that cannot be resolved in a single change detection
   * pass.
   * In this case, Angular throws an error, since an Angular application can only have one change
   * detection pass during which all change detection must complete.
   */
  tick(): void {
    if (this._runningTick) {
      throw new Error('ApplicationRef.tick is called recursively');
    }

    const scope = ApplicationRef._tickScope();
    try {
      this._runningTick = true;
      for (let view of this._views) {
        view.detectChanges();
      }
      if (this._enforceNoNewChanges) {
        for (let view of this._views) {
          view.checkNoChanges();
        }
      }
    } catch (e) {
      // Attention: Don't rethrow as it could cancel subscriptions to Observables!
      this._zone.runOutsideAngular(() => this._exceptionHandler.handleError(e));
    } finally {
      this._runningTick = false;
      wtfLeave(scope);
    }
  }

  /**
   * Attaches a view so that it will be dirty checked.
   * The view will be automatically detached when it is destroyed.
   * This will throw if the view is already attached to a ViewContainer.
   */
  attachView(viewRef: ViewRef): void {
    const view = (viewRef as InternalViewRef);
    this._views.push(view);
    view.attachToAppRef(this);
  }

  /**
   * Detaches a view from dirty checking again.
   */
  detachView(viewRef: ViewRef): void {
    const view = (viewRef as InternalViewRef);
    remove(this._views, view);
    view.detachFromAppRef();
  }

  private _loadComponent(componentRef: ComponentRef<any>): void {
    this.attachView(componentRef.hostView);
    this.tick();
    this.components.push(componentRef);
    // Get the listeners lazily to prevent DI cycles.
    const listeners =
        this._injector.get(APP_BOOTSTRAP_LISTENER, []).concat(this._bootstrapListeners);
    listeners.forEach((listener) => listener(componentRef));
  }

  private _unloadComponent(componentRef: ComponentRef<any>): void {
    this.detachView(componentRef.hostView);
    remove(this.components, componentRef);
  }

  /** @internal */
  ngOnDestroy() {
    // TODO(alxhub): Dispose of the NgZone.
    this._views.slice().forEach((view) => view.destroy());
  }

  /**
   * Returns the number of attached views.
   */
  get viewCount() { return this._views.length; }
}

function remove<T>(list: T[], el: T): void {
  const index = list.indexOf(el);
  if (index > -1) {
    list.splice(index, 1);
  }
}

function _mergeArrays(parts: any[][]): any[] {
  const result: any[] = [];
  parts.forEach((part) => part && result.push(...part));
  return result;
}