什么是 HMR？

​ HMR 是hot module replacement 的简称，直译：热模块替换，如果不开启HMR模式，angular项目在模块更改的时候会从根节点开始刷新，开启HMR模式以后，只会刷新有修改的地方，开发效率在某种意义上可以提高

配置步骤

配置前提：你已经创建了一个angular6项目，安装hmr依赖

npm install --save-dev @angularclass/hmr

1）在 src/environments 目录下添加 environment.hmr.ts 配置文件，文件内容为:

export const environment = {
production: false,
hmr: true
};

2）然后分别在 environment.prod.ts 和 environment.ts 添加 hmr:false 配置项

3）配置 src/tsconfig.app.json 文件 ，内容为：

"compilerOptions": {
...
"types": ["node"]
}

4）配置 src/hmr.ts 文件内容如下 ：

import { NgModuleRef, ApplicationRef } from "@angular/core";
import { createNewHosts } from "@angularclass/hmr";export const hmrBootstrap = (
module: any,
bootstrap: () => Promise<NgModuleRef<any>>
) => {
let ngModule: NgModuleRef<any>;
module.hot.accept();
bootstrap().then(mod => (ngModule = mod));
module.hot.dispose(() => {
const appRef: ApplicationRef = ngModule.injector.get(ApplicationRef);
const elements = appRef.components.map(c => c.location.nativeElement);
const makeVisible = createNewHosts(elements);
ngModule.destroy();
makeVisible();
});
};

5）更新 src/main.ts 文件内容如下

import { enableProdMode } from "@angular/core";
import { platformBrowserDynamic } from "@angular/platform-browser-dynamic";import { AppModule } from "./app/app.module";
import { environment } from "./environments/environment";import { hmrBootstrap } from "./hmr";if (environment.production) {
enableProdMode();
}const bootstrap = () => platformBrowserDynamic().bootstrapModule(AppModule);if (environment.hmr) {
if (module["hot"]) {
hmrBootstrap(module, bootstrap);
} else {
console.error("HMR is not enabled for webpack-dev-server!");
console.log("Are you using the --hmr flag for ng serve?");
}
} else {
bootstrap().catch(err => console.log(err));
}

6）配置 angular.json 文件 ：

"build": {
"builder": "@angular-devkit/build-angular:browser",
"options": {
"outputPath": "dist/${project-name}",
"index": "src/index.html",
"main": "src/main.ts",
"polyfills": "src/polyfills.ts",
"tsConfig": "src/tsconfig.app.json",
"assets": ["src/favicon.ico", "src/assets"],
"styles": ["src/styles.css"],
"scripts": []
},
"configurations": {"hmr": {
"fileReplacements": [
{
"replace": "src/environments/environment.ts",
"with": "src/environments/environment.hmr.ts"
}
]
},"production": {
"fileReplacements": [
{
"replace": "src/environments/environment.ts",
"with": "src/environments/environment.prod.ts"
}
],
"optimization": true,
"outputHashing": "all",
"sourceMap": false,
"extractCss": true,
"namedChunks": false,
"aot": true,
"extractLicenses": true,
"vendorChunk": false,
"buildOptimizer": true
}
}
},
...
"serve": {
"builder": "@angular-devkit/build-angular:dev-server",
"options": {
"browserTarget": "${project-name}:build"
},
"configurations": {
"production": {
"browserTarget": "${project-name}:build:production"
},"hmr": {
"browserTarget": "${project-name}:build:hmr",
"hmr": true
}}
},

7）启动方式:两种

第一种：

ng serve --configuration hmr

第二种:

ng run ${project-name}:serve:hmr

8)为方便一般添加在package.json的scripts里:

"scripts": {
...
"hmr": "ng serve --configuration hmr --open" 
}