This article mainly explains how to dynamically create components in Angular (Note: Components used in templates can be called statically created components). Let's take a look at this article together
If you have used AngularJS (the first generation Angular framework) to program before, you may have used the $compile service to generate HTML, And connect to the data model to obtain the two-way binding function:
const template = '<span>generated on the fly: {{name}}</span>'
const linkFn = $compile(template);
const dataModel = $scope.$new();
dataModel.name = 'dynamic';
// link data model to a template
linkFn(dataModel);Instructions in AngularJS can modify the DOM, but there is no way to know what has been modified. The problem with this approach is the same as in a dynamic environment, which makes it difficult to optimize performance. Dynamic templates are certainly not the main culprit of AngularJS's slow performance, but they are also one of the important reasons.
After looking at Angular’s internal code for a while, I found that this newly designed framework attaches great importance to performance. You will often find these sentences in the Angular source code (note: not translated for clear understanding):
Attention: Adding fields to this is performance sensitive! Note: We use one type for all nodes so that loops that loop over all nodes of a ViewDefinition stay monomorphic! For performance reasons, we want to check and update the list every five seconds.
So, Angular designers decided to sacrifice flexibility to gain huge performance improvements, such as the introduction of JIT and AOT Compiler, static templates (static templates), directive/module factory (ComponentFactory ), factory resolver (ComponentFactoryResolver). These concepts may be unfamiliar and even hostile to the AngularJS community, but don’t worry, if you’ve only heard of these concepts before and now want to know what they are, continue reading this article and it will enlighten you.
Note: In fact, JIT/AOT Compiler refers to the same Compiler, but this Compiler is used in the building time stage or the running time stage.As for factory, Angular Compiler compiles the component you wrote such as a.component.ts into a.component.ngfactory.js. That is, Compiler uses @Component decorator as the raw material to compile the component/instruction class you wrote into another component. A view factory class.
Back to the JIT/AOT Compiler just now, if a.component.ngfactory.js is generated in the build phase, it is the AOT Compiler. This Compiler will not be packaged into the dependency package; if it is generated in the run phase generated, then the Compiler needs to be packaged into a dependency package and downloaded locally by the user. At runtime, the Compiler will compile the component/instruction class to generate the corresponding view factory class, and that's it. Below we will see what the code of these *.ngfactory.js files looks like.
As for the factory resolver, it is even simpler. It is an object through which you can get the compiled factory objects.
Each component in Angular is created by a component factory, and the component factory is written by the compiler based on what you write@Component Compiled and generated metadata in the decorator. If you are still a little confused after reading a lot of decorator articles on the Internet, you can refer to this Medium article I wrote Implementing custom component decorator.
Angular uses the concept of View internally, or the entire framework is a view tree. Each view is composed of a large number of different types of nodes: element nodes, text nodes, etc. (Note: You can view Translation Angular DOM Update Mechanism). Each node has its own specialized role, so that the processing of each node only takes a small amount of time, and each node has services such as ViewContainerRef and TemplateRef for use. You can use ViewChild/ViewChildren and ContentChild/ContentChildren to do DOM query for these nodes.
Note: To put it simply, the Angular program is a view tree. Each view is composed of multiple nodes. Each node provides a template operation API to developers. Using, these nodes can be obtained through the DOM Query API.
Each node contains a large amount of information, and for performance reasons, it takes effect once the node is created, and no subsequent changes are allowed (note: the created node will be cached). The node generation process is that the compiler collects the component information you write (note: mainly the template information in the component you write), and encapsulates it in the form of a component factory.
Suppose you write the following component:
@Component({
selector: 'a-comp',
template: '<span>A Component</span>'
})
class AComponent {}The compiler generates component factory code similar to the following based on the information you wrote. The code only contains the important parts (Note: The entire code below can be understood is View, where elementDef2 and jit_textDef3 can be understood as node):
function View_AComponent_0(l) {
return jit_viewDef1(0,[
elementDef2(0,null,null,1,'span',...),
jit_textDef3(null,['My name is ',...])
]The above code basically describes the component The structure of a view and is used to instantiate a component. Among them, the first node elementDef2 is the element node definition, and the second node jit_textDef3 is the text node definition. You can see that each node has enough parameter information to instantiate, and this parameter information is generated by the compiler parsing all dependencies, and the specific values of these dependencies are provided by the framework at runtime.
从上文知道，如果你能够访问到组件工厂，就可以使用它实例化出对应的组件对象，并使用 ViewContainerRef API 把该组件/视图插入 DOM 中。如果你对 ViewContainerRef 感兴趣，可以查看 译 探索 Angular 使用 ViewContainerRef 操作 DOM。应该如何使用这个 API 呢（注：下面代码展示如何使用 ViewContainerRef API 往视图树上插入一个视图）：
export class SampleComponent implements AfterViewInit {
@ViewChild("vc", {read: ViewContainerRef}) vc: ViewContainerRef;
ngAfterViewInit() {
this.vc.createComponent(componentFactory);
}
}好的，从上面代码可知道只要拿到组件工厂，一切问题就解决了。现在，问题是如何拿到 ComponentFactory 组件工厂对象，继续看。
尽管 AngularJS 也有模块，但它缺少指令所需要的真正的命名空间，并且会有潜在的命名冲突，还没法在单独的模块里封装指令。然而，很幸运，Angular 吸取了教训，为各种声明式类型，如指令、组件和管道，提供了合适的命名空间（注：即 Angular 提供的 Module，使用装饰器函数 @NgModule 装饰一个类就能得到一个 Module）。
就像 AngularJS 那样，Angular 中的组件是被封装在模块中。组件自己并不能独立存在，如果你想要使用另一个模块的一个组件，你必须导入这个模块：
@NgModule({
// imports CommonModule with declared directives like
// ngIf, ngFor, ngClass etc.
imports: [CommonModule],
...
})
export class SomeModule {}同样道理，如果一个模块想要提供一些组件给别的模块使用，就必须导出这些组件，可以查看 exports 属性。比如，可以查看 CommonModule 源码的做法（注：查看 L24-L25）：
const COMMON_DIRECTIVES: Provider[] = [
NgClass,
NgComponentOutlet,
NgForOf,
NgIf,
...
];
@NgModule({
declarations: [COMMON_DIRECTIVES, ...],
exports: [COMMON_DIRECTIVES, ...],
...
})
export class CommonModule {
}所以每一个组件都是绑定在一个模块里，并且不能在不同模块里申明同一个组件，如果你这么做了，Angular 会抛出错误：
Type X is part of the declarations of 2 modules: ...
当 Angular 编译程序时，编译器会把在模块中 entryComponents 属性注册的组件，或模板里使用的组件编译为组件工厂（注：在所有静态模板中使用的组件如 <a-comp></a-comp>，即静态组件；在 entryComponents 定义的组件，即动态组件，动态组件的一个最佳示例如 Angular Material Dialog 组件，可以在 entryComponents 中注册 DialogContentComp 组件动态加载对话框内容）。你可以在 Sources 标签里看到编译后的组件工厂文件：
从上文中我们知道，如果我们能拿到组件工厂，就可以使用组件工厂创建对应的组件对象，并插入到视图里。实际上，每一个模块都为所有组件提供了一个获取组件工厂的服务 ComponentFactoryResolver。所以，如果你在模块中定义了一个 BComponent 组件并想要拿到它的组件工厂，你可以在这个组件内注入这个服务并使用它：
export class AppComponent {
constructor(private resolver: ComponentFactoryResolver) {
// now the `factory` contains a reference to the BComponent factory
const factory = this.resolver.resolveComponentFactory(BComponent);
}这是在两个组件 AppComponent 和 BComponent 都定义在一个模块里才行，或者导入其他模块时该模块已经有组件 BComponent 对应的组件工厂。
但是如果组件在其他模块定义，并且这个模块是按需加载，这样的话是不是完蛋了呢？实际上我们照样可以拿到某个组件的组件工厂，方法同路由使用 loadChildren 配置项按需加载模块很类似。
有两种方式可以在运行时加载模块。第一种方式 是使用 SystemJsNgModuleLoader 模块加载器，如果你使用 SystemJS 加载器的话，路由在加载子路由模块时也是用的 SystemJsNgModuleLoader 作为模块加载器。SystemJsNgModuleLoader 模块加载器有一个 load 方法来把模块加载到浏览器里，同时编译该模块和在该模块中申明的所有组件。load 方法需要传入文件路径参数，并加上导出模块的名称，返回值是 NgModuleFactory：
loader.load('path/to/file#exportName')注：NgModuleFactory 源码是在packages/core/linker文件夹内，该文件夹里的代码主要是粘合剂代码，主要都是一些接口类供Core模块使用，具体实现在其他文件夹内。
如果没有指定具体的导出模块名称，加载器会使用默认关键字 default 导出的模块名。还需注意的是，想要使用 SystemJsNgModuleLoader 还需像这样去注册它：
providers: [
{
provide: NgModuleFactoryLoader,
useClass: SystemJsNgModuleLoader
}
]你当然可以在 provide 里使用任何标识（token），不过路由模块使用 NgModuleFactoryLoader 标识，所以最好也使用相同 token。（注：NgModuleFactoryLoader 注册可查看源码 L68，使用可查看 L78）
模块加载并获取组件工厂的完整代码如下：
@Component({
providers: [
{
provide: NgModuleFactoryLoader,
useClass: SystemJsNgModuleLoader
}
]
})
export class ModuleLoaderComponent {
constructor(private _injector: Injector,
private loader: NgModuleFactoryLoader) {
}
ngAfterViewInit() {
this.loader.load('app/t.module#TModule').then((factory) => {
const module = factory.create(this._injector);
const r = module.componentFactoryResolver;
const cmpFactory = r.resolveComponentFactory(AComponent);
// create a component and attach it to the view
const componentRef = cmpFactory.create(this._injector);
this.container.insert(componentRef.hostView);
})
}
}但是在使用 SystemJsNgModuleLoader 时还有个问题，上面代码的 load() 函数内部（注：参见 L70）其实是使用了编译器的 compileModuleAsync 方法，该方法只会为在 entryComponents 中注册的或者在组件模板中使用的组件，去创建组件工厂。但是如果你就是不想要把组件注册在 entryComponents 属性里，是不是就完蛋了呢？仍然有解决方案 —— 使用 compileModuleAndAllComponentsAsync 方法自己去加载模块。该方法会为模块里所有组件生成组件工厂，并返回 ModuleWithComponentFactories 对象：
class ModuleWithComponentFactories<t> {
componentFactories: ComponentFactory<any>[];
ngModuleFactory: NgModuleFactory<t>;</t></any></t>下面代码完整展示如何使用该方法加载模块并获取所有组件的组件工厂（注：这是上面说的 第二种方式）：
ngAfterViewInit() {
System.import('app/t.module').then((module) => {
_compiler.compileModuleAndAllComponentsAsync(module.TModule)
.then((compiled) => {
const m = compiled.ngModuleFactory.create(this._injector);
const factory = compiled.componentFactories[0];
const cmp = factory.create(this._injector, [], null, m);
})
})
}然而，记住，这个方法使用了编译器的私有 API，下面是源码中的 文档说明：
One intentional omission from this list is@angular/compiler, which is currently considered a low level api and is subject to internal changes. These changes will not affect any applications or libraries using the higher-level apis (the command line interface or JIT compilation via@angular/platform-browser-dynamic). Only very specific use-cases require direct access to the compiler API (mostly tooling integration for IDEs, linters, etc). If you are working on this kind of integration, please reach out to us first.
从上文中我们知道如何通过模块中的组件工厂来动态创建组件，其中模块是在运行时之前定义的，并且模块是可以提前或延迟加载的。但是，也可以不需要提前定义模块，可以像 AngularJS 的方式在运行时创建模块和组件。
首先看看上文中的 AngularJS 的代码是如何做的：
const template = '<span>generated on the fly: {{name}}</span>'
const linkFn = $compile(template);
const dataModel = $scope.$new();
dataModel.name = 'dynamic'
// link data model to a template
linkFn(dataModel);从上面代码可以总结动态创建视图的一般流程如下：
定义组件类及其属性，并使用装饰器装饰组件类
定义模块类，在模块类中申明组件类，并使用装饰器装饰模块类
编译模块和模块中所有组件，拿到所有组件工厂
模块类也仅仅是带有模块装饰器的普通类，组件类也同样如此，而由于装饰器也仅仅是简单地函数而已，在运行时可用，所以只要我们需要，就可以使用这些装饰器如 @NgModule()/@Component() 去装饰任何类。下面代码完整展示如何动态创建组件：
@ViewChild('vc', {read: ViewContainerRef}) vc: ViewContainerRef;
constructor(private _compiler: Compiler,
private _injector: Injector,
private _m: NgModuleRef<any>) {
}
ngAfterViewInit() {
const template = '<span>generated on the fly: {{name}}</span>';
const tmpCmp = Component({template: template})(class {
});
const tmpModule = NgModule({declarations: [tmpCmp]})(class {
});
this._compiler.compileModuleAndAllComponentsAsync(tmpModule)
.then((factories) => {
const f = factories.componentFactories[0];
const cmpRef = this.vc.createComponent(tmpCmp);
cmpRef.instance.name = 'dynamic';
})
}</any>为了更好的调试信息，你可以使用任何类来替换上面代码中的匿名类。
上文中说到的编译器说的是 Just-In-Time(JIT) 编译器，你可能听说过 Ahead-of-Time(AOT) 编译器，实际上 Angular 只有一个编译器，它们仅仅是根据编译器使用在不同阶段，而采用的不同叫法。如果编译器是被下载到浏览器里，在运行时使用就叫 JIT 编译器；如果是在编译阶段去使用，而不需要下载到浏览器里，在编译时使用就叫 AOT 编译器。使用 AOT 方法是被 Angular 官方推荐的，并且官方文档上有详细的 原因解释 —— 渲染速度更快并且代码包更小。（想看更多就到PHP中文网AngularJS开发手册中学习）
如果你使用 AOT 的话，意味着运行时不存在编译器，那上面的不需要编译的示例仍然有效，仍然可以使用 ComponentFactoryResolver 来做，但是动态编译需要编译器，就没法运行了。但是，如果非得要使用动态编译，那就得把编译器作为开发依赖一起打包，然后代码被下载到浏览器里，这样做需要点安装步骤，不过也没啥特别的，看看代码：
import { JitCompilerFactory } from '@angular/compiler';
export function createJitCompiler() {
return new JitCompilerFactory([{
useDebug: false,
useJit: true
}]).createCompiler();
}
import { AppComponent } from './app.component';
@NgModule({
providers: [{provide: Compiler, useFactory: createJitCompiler}],
...
})
export class AppModule {
}上面代码中，我们使用 @angular/compiler 的 JitCompilerFactory 类来实例化出一个编译器工厂，然后通过标识 Compiler 来注册编译器工厂实例。以上就是所需要修改的全部代码，就这么点东西需要修改添加，很简单不是么。
如果你使用动态加载组件方式，最后需要注意的是，当父组件销毁时，该动态加载组件需要被销毁：
ngOnDestroy() {
if(this.cmpRef) {
this.cmpRef.destroy();
}
}上面代码将会从视图容器里移除该动态加载组件视图并销毁它。
对于所有动态加载的组件，Angular 会像对静态加载组件一样也执行变更检测，这意味着 ngDoCheck 也同样会被调用（注：可查看 Medium 这篇文章 If you think ngDoCheck means your component is being checked — read this article）。然而，就算动态加载组件申明了 @Input 输入绑定，但是如果父组件输入绑定属性发生改变，该动态加载组件的 ngOnChanges 不会被触发。这是因为这个检查输入变化的 ngOnChanges 函数，只是在编译阶段由编译器编译后重新生成，该函数是组件工厂的一部分，编译时是根据模板信息编译生成的。因为动态加载组件没有在模板中被使用，所以该函数不会由编译器编译生成。
本文的所有示例代码存放在 Github。
注：本文主要讲了组件b-comp如何动态加载组件a-comp，如果两个在同一个module，直接调用 ComponentFactoryResolver 等 API 就行；如果不在同一个module，就使用 SystemJsNgModuleLoader 模块加载器就行。
好了，本篇文章到这就结束了（想看更多就到PHP中文网AngularJS使用手册中学习），有问题的可以在下方留言提问
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