How to Lazy Load Components in Angular 4 in Three Steps

// angular按模块开发，跟模块，特性模块，核心模块(整个应用只在跟模块只加载一次)，惰性加载模块等

只是有一个问题，即在惰性加载模块里（selector方式的模板方式，即

'app-title' is not a known element:
1. If 'app-title' is an Angular component,then verify that it is part of this module.
2. If 'app-title' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. ("
    [ERROR ->]<app-title ></app-title>

网上有人遇到同样的问题，https://github.com/angular/angular/issues/12809，貌似没有解决

https://angular.cn/guide/ngmodule

// 另一个模块惰性加载的例子

https://angularfirebase.com/lessons/how-to-lazy-load-components-in-angular-4-in-three-steps/

Step 1 - Create a new App with Routing

初始化一个带routing的新project

ng new lazyDemo --routing

Now add a link to the lazy url in the app component.

     
     

      
      <button routerLink="/lazy/load-me">
      
      </button>
     
     
     
     

      
      <router-outlet>
      
      </router-outlet>
     
     

Step 2 - Generate the “Lazy” Module

Let’s create a module that will be lazy loaded,along with a couple components. The --flat flag prevents a directory from being created,then we can easily add components to the module via the Angular CLI.
ng g module lazy --flat
ng g component lazy-parent --module lazy
ng g component lazy-child --module lazy

Step 3 - Point the App Router to the Lazy Module

Lazy loading is a technique in Angular that allows you to load JavaScript components asynchronously when a specific route is activated. This can add some initial performance during the initial load,especially if you have many components with complex routing. There are some goodpostsabout lazy loading in angular,but I wanted to simplify it further. This lesson will show you how to enable lazy loading in 3 easy steps with a brand new app.

Step 1 - Create a new App with Routing

Our app will load theAppComponentby default at the root URL,then when the user navigates tolazy/load-me,the lazy module will be loaded asynchronously.

     
     

      
      ng new lazyDemo --routing
     
     

Now add a link to the lazy url in the app component.

     
     

      
      </router-outlet>
     
     

Step 2 - Generate the “Lazy” Module

Let’s create a module that will be lazy loaded,along with a couple components. The--flatflag prevents a directory from being created,then we can easily add components to the module via the Angular CLI.

     
     

      
      ng g module lazy --flat
     
     
     
     

      
      ng g component lazy-parent --module lazy
     
     
     
     

      
      ng g component lazy-child --module lazy
     
     

Inside the lazy module we need to import theRouterModule. Two things to notice here:

1. The route path to the component ispath: 'load-me',even though it will actually be activated onlazy/load-me. This is a child route,which you will see come together in step 3
2. TheRouterModulecallsforChild(routes).

     
     

      
      import { NgModule } 
      
      from 
      
      '@angular/core';
     
     
     
     

      
      import { CommonModule } 
      
      '@angular/common';
     
     
     
     

      
      import { LazyParentComponent } 
      
      './lazy-parent/lazy-parent.component';
     
     
     
     

      
      import { LazyChildComponent } 
      
      './lazy-child/lazy-child.component';
     
     
     
     

     
     

      
      import { Routes,RouterModule } 
      
      '@angular/router';
     
     
     
     

     
     

      
      const routes: Routes = [
     
     
     
     

      
          { path: 
      
      'load-me',component: LazyParentComponent }
     
     
     
     

      
      ];
     
     
     
     

     
     

      
      @NgModule({
     
     
     
     

      
        imports: [
     
     
     
     

      
          CommonModule,
     
     

      
          RouterModule.forChild(routes)
     
     
     
     

      
        ],
     
     

      
        declarations: [
     
     
     
     

      
          LazyParentComponent,
     
     

      
          LazyChildComponent
     
     
     
     

      
        ]
     
     
     
     

      
      })
     
     
     
     

      
      export 
      
      class LazyModule { }
     
     

And here’s what theLazyParentComponentlooks like. Just looping over the child component a few times.

     
     

      
      <div *ngFor="let name of ['Foo','Bar','Baz']">
     
     
     
     
  
      
      <p>Hi,my name is {{name}}. I'm a lazy child component.
      
      </p>
     
     
     
     

     
     
  
      
      <lazy-child>
      
      </lazy-child>
     
     
     
     

     
     

      
      </div>
     
     

Your Angular CLI config might append anappprefix to the component selector name. If so,make sure remove the prefix and for the last section to work properly. For example…

@Component({ selector: 'lazy-child' })

Step 3 - Point the App Router to the Lazy Module

The final step is to point the lazy route to the lazy module from the app router. We can do this with theloadChildrenproperty with the path to the module file,then reference the module itself with a hash #. This tells angular to only loadLazyModulewhen the lazy url is activated.

     
     

      
      'lazy',loadChildren: 
      
      './lazy.module#LazyModule'}
     
     
     
     

      
      ];
     
     
     
     

     
     

      
      @NgModule({
     
     
     
     

      
        imports: [RouterModule.forRoot(routes)],
     
     

      
        exports: [RouterModule]
     
     
     
     

      
      })
     
     
     
     

      
      class AppRoutingModule { }
     
     

Verify Lazy Loading is Working

Let’s make sure Lazy Loading is working. In chrome,open developer tools and click the network tab. When you navigate to the lazy url,you should see a0.chunk.jsfile rendered. In this demo,you can see it took 2ms to load.

The lazy loaded module is the last chunk.js file in the chrome network logs

When you take a look inside this file,you should see a bunch ofwebpackJavaScript code with related to the lazy module and its components.

Looking inside,we see that is a bunch of JavaScript webpack code

That’s it for lazy loading components in Angular. Good luck!

（编辑：李大同）

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