在Angular中路由改变前，如何评估条件？

import { Injectable } from '@angular/core';
import { CanActivate, ActivatedRouteSnapshot, RouterStateSnapshot, UrlTree } from '@angular/router';
import { Observable } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class MyAuthGuard implements CanActivate {
  canActivate(
    next: ActivatedRouteSnapshot,
    state: RouterStateSnapshot): Observable<boolean | UrlTree> | Promise<boolean | UrlTree> | boolean | UrlTree {
    // 在这里评估条件，返回一个布尔值或可观察对象
    // 如果条件满足，返回true，允许路由的改变
    // 如果条件不满足，返回false或一个UrlTree对象，阻止路由的改变
    return true;
  }
}

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';
import { MyAuthGuard } from './my-auth.guard';

const routes: Routes = [
  {
    path: 'example',
    canActivate: [MyAuthGuard],
    loadChildren: () => import('./example/example.module').then(m => m.ExampleModule)
  },
  // 其他路由配置...
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }