Angular CLI builders

A number of Angular CLI commands run a complex process on your code, such as building, testing, or serving your application. The commands use an internal tool called Architect to run CLI builders, which invoke another tool (bundler, test runner, server) to accomplish the desired task. Custom builders can perform an entirely new task, or to change which third-party tool is used by an existing command.

This document explains how CLI builders integrate with the workspace configuration file, and shows how you can create your own builder.

CLI builders

The internal Architect tool delegates work to handler functions called builders. A builder handler function receives two arguments:

The separation of concerns here is the same as with schematics, which are used for other CLI commands that touch your code (such as ng generate).

• The options object is provided by the CLI user's options and configuration, while the context object is provided by the CLI Builder API automatically.
• In addition to the contextual information, the context object also provides access to a scheduling method, context.scheduleTarget(). The scheduler executes the builder handler function with a given target configuration.

The builder handler function can be synchronous (return a value), asynchronous (return a Promise), or watch and return multiple values (return an Observable). The return values must always be of type BuilderOutput. This object contains a Boolean success field and an optional error field that can contain an error message.

Angular provides some builders that are used by the CLI for commands such as ng build and ng test. Default target configurations for these and other built-in CLI builders can be found and configured in the "architect" section of the workspace configuration file, angular.json. Also, extend and customize Angular by creating your own builders, which you can run directly using the ng run CLI command.

Builder project structure

A builder resides in a "project" folder that is similar in structure to an Angular workspace, with global configuration files at the top level, and more specific configuration in a source folder with the code files that define the behavior. For example, your myBuilder folder could contain the following files.

Builders can be published to npm, see Publishing your Library.

Creating a builder

As an example, create a builder that copies a file to a new location. To create a builder, use the createBuilder() CLI Builder function, and return a Promise<BuilderOutput> object.

      
import {BuilderContext, BuilderOutput, createBuilder} from '@angular-devkit/architect';
import {JsonObject} from '@angular-devkit/core';
import {promises as fs} from 'fs';
interface Options extends JsonObject {
  source: string;
  destination: string;
}
export default createBuilder(copyFileBuilder);
async function copyFileBuilder(options: Options, context: BuilderContext): Promise<BuilderOutput> {
  context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: (err as Error).message,
    };
  }
  context.reportStatus('Done.');
  return {success: true};
}

    

Now let's add some logic to it. The following code retrieves the source and destination file paths from user options and copies the file from the source to the destination (using the Promise version of the built-in NodeJS copyFile() function). If the copy operation fails, it returns an error with a message about the underlying problem.

      
import {BuilderContext, BuilderOutput, createBuilder} from '@angular-devkit/architect';
import {JsonObject} from '@angular-devkit/core';
import {promises as fs} from 'fs';
interface Options extends JsonObject {
  source: string;
  destination: string;
}
export default createBuilder(copyFileBuilder);
async function copyFileBuilder(options: Options, context: BuilderContext): Promise<BuilderOutput> {
  context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: (err as Error).message,
    };
  }
  context.reportStatus('Done.');
  return {success: true};
}

    

Handling output

By default, copyFile() does not print anything to the process standard output or error. If an error occurs, it might be difficult to understand exactly what the builder was trying to do when the problem occurred. Add some additional context by logging additional information using the Logger API. This also lets the builder itself be executed in a separate process, even if the standard output and error are deactivated.

You can retrieve a Logger instance from the context.

      
import {BuilderContext, BuilderOutput, createBuilder} from '@angular-devkit/architect';
import {JsonObject} from '@angular-devkit/core';
import {promises as fs} from 'fs';
interface Options extends JsonObject {
  source: string;
  destination: string;
}
export default createBuilder(copyFileBuilder);
async function copyFileBuilder(options: Options, context: BuilderContext): Promise<BuilderOutput> {
  context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: (err as Error).message,
    };
  }
  context.reportStatus('Done.');
  return {success: true};
}

    

Progress and status reporting

The CLI Builder API includes progress and status reporting tools, which can provide hints for certain functions and interfaces.

To report progress, use the context.reportProgress() method, which takes a current value, optional total, and status string as arguments. The total can be any number. For example, if you know how many files you have to process, the total could be the number of files, and current should be the number processed so far. The status string is unmodified unless you pass in a new string value.

In our example, the copy operation either finishes or is still executing, so there's no need for a progress report, but you can report status so that a parent builder that called our builder would know what's going on. Use the context.reportStatus() method to generate a status string of any length.

Pass an empty string to remove the status.

      
import {BuilderContext, BuilderOutput, createBuilder} from '@angular-devkit/architect';
import {JsonObject} from '@angular-devkit/core';
import {promises as fs} from 'fs';
interface Options extends JsonObject {
  source: string;
  destination: string;
}
export default createBuilder(copyFileBuilder);
async function copyFileBuilder(options: Options, context: BuilderContext): Promise<BuilderOutput> {
  context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: (err as Error).message,
    };
  }
  context.reportStatus('Done.');
  return {success: true};
}

    

Builder input

You can invoke a builder indirectly through a CLI command such as ng build, or directly with the Angular CLI ng run command. In either case, you must provide required inputs, but can let other inputs default to values that are pre-configured for a specific target, specified by a configuration, or set on the command line.

Input validation

You define builder inputs in a JSON schema associated with that builder. Similar to schematics, the Architect tool collects the resolved input values into an options object, and validates their types against the schema before passing them to the builder function.

For our example builder, options should be a JsonObject with two keys: a source and a destination, each of which are a string.

You can provide the following schema for type validation of these values.

      
{
  "$schema": "http://json-schema.org/schema",
  "type": "object",
  "properties": {
    "source": {
      "type": "string"
    },
    "destination": {
      "type": "string"
    }
  }
}
    

To link our builder implementation with its schema and name, you need to create a builder definition file, which you can point to in package.json.

Create a file named builders.json that looks like this:

      
{
  "builders": {
    "copy": {
      "implementation": "./dist/my-builder.js",
      "schema": "./src/schema.json",
      "description": "Copies a file."
    }
  }
}
    

In the package.json file, add a builders key that tells the Architect tool where to find our builder definition file.

      
{
  "name": "@example/copy-file",
  "version": "1.0.0",
  "description": "Builder for copying files",
  "builders": "builders.json",
  "dependencies": {
    "@angular-devkit/architect": "~0.1200.0",
    "@angular-devkit/core": "^12.0.0"
  }
}
    

The official name of our builder is now @example/copy-file:copy. The first part of this is the package name and the second part is the builder name as specified in the builders.json file.

These values are accessed on options.source and options.destination.

      
import {BuilderContext, BuilderOutput, createBuilder} from '@angular-devkit/architect';
import {JsonObject} from '@angular-devkit/core';
import {promises as fs} from 'fs';
interface Options extends JsonObject {
  source: string;
  destination: string;
}
export default createBuilder(copyFileBuilder);
async function copyFileBuilder(options: Options, context: BuilderContext): Promise<BuilderOutput> {
  context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: (err as Error).message,
    };
  }
  context.reportStatus('Done.');
  return {success: true};
}

    

Target configuration

A builder must have a defined target that associates it with a specific input configuration and project.

Targets are defined in the angular.json CLI configuration file. A target specifies the builder to use, its default options configuration, and named alternative configurations. Architect in the Angular CLI uses the target definition to resolve input options for a given run.

The angular.json file has a section for each project, and the "architect" section of each project configures targets for builders used by CLI commands such as 'build', 'test', and 'serve'. By default, for example, the ng build command runs the builder @angular-devkit/build-angular:browser to perform the build task, and passes in default option values as specified for the build target in angular.json.

      
…
"myApp": {
  …
  "architect": {
    "build": {
      "builder": "@angular-devkit/build-angular:browser",
      "options": {
        "outputPath": "dist/myApp",
        "index": "src/index.html",
        …
      },
      "configurations": {
        "production": {
          "fileReplacements": [
            {
              "replace": "src/environments/environment.ts",
              "with": "src/environments/environment.prod.ts"
            }
          ],
          "optimization": true,
          "outputHashing": "all",
          …
        }
      }
    },
    …
  }
}
…
    

The command passes the builder the set of default options specified in the "options" section. If you pass the --configuration=production flag, it uses the override values specified in the production configuration. Specify further option overrides individually on the command line.

Target strings

The generic ng run CLI command takes as its first argument a target string of the following form.

      
project:target[:configuration]
    

If your builder calls another builder, it might need to read a passed target string. Parse this string into an object by using the targetFromTargetString() utility function from @angular-devkit/architect.

Schedule and run

Architect runs builders asynchronously. To invoke a builder, you schedule a task to be run when all configuration resolution is complete.

The builder function is not executed until the scheduler returns a BuilderRun control object. The CLI typically schedules tasks by calling the context.scheduleTarget() function, and then resolves input options using the target definition in the angular.json file.

Architect resolves input options for a given target by taking the default options object, then overwriting values from the configuration, then further overwriting values from the overrides object passed to context.scheduleTarget(). For the Angular CLI, the overrides object is built from command line arguments.

Architect validates the resulting options values against the schema of the builder. If inputs are valid, Architect creates the context and executes the builder.

For more information see Workspace Configuration.

Only the context.scheduleTarget() method resolves the configuration and overrides through the angular.json file.

Default architect configuration

Let's create a simple angular.json file that puts target configurations into context.

You can publish the builder to npm (see Publishing your Library), and install it using the following command:

      
npm install @example/copy-file
    

If you create a new project with ng new builder-test, the generated angular.json file looks something like this, with only default builder configurations.

      
{
  "projects": {
    "builder-test": {
      "architect": {
        "build": {
          "builder": "@angular-devkit/build-angular:browser",
          "options": {
            // more options...
            "outputPath": "dist/builder-test",
            "index": "src/index.html",
            "main": "src/main.ts",
            "polyfills": "src/polyfills.ts",
            "tsConfig": "src/tsconfig.app.json"
          },
          "configurations": {
            "production": {
              // more options...
              "optimization": true,
              "aot": true,
              "buildOptimizer": true
            }
          }
        }
      }
    }
  }
}
    

Adding a target

Add a new target that will run our builder to copy a file. This target tells the builder to copy the package.json file.

• We will add a new target section to the architect object for our project
• The target named copy-package uses our builder, which you published to @example/copy-file.
• The options object provides default values for the two inputs that you defined.
  • source - The existing file you are copying.
  • destination - The path you want to copy to.

      
{
  "projects": {
    "builder-test": {
      "architect": {
        "copy-package": {
          "builder": "@example/copy-file:copy",
          "options": {
            "source": "package.json",
            "destination": "package-copy.json"
          }
        },
        // Existing targets...
      }
    }
  }
}
    

Running the builder

To run our builder with the new target's default configuration, use the following CLI command.

      
ng run builder-test:copy-package
    

This copies the package.json file to package-copy.json.

Use command-line arguments to override the configured defaults. For example, to run with a different destination value, use the following CLI command.

      
ng run builder-test:copy-package --destination=package-other.json
    

This copies the file to package-other.json instead of package-copy.json. Because you did not override the source option, it will still copy from the default package.json file.

Testing a builder

Use integration testing for your builder, so that you can use the Architect scheduler to create a context, as in this example. In the builder source directory, create a new test file my-builder.spec.ts. The test creates new instances of JsonSchemaRegistry (for schema validation), TestingArchitectHost (an in-memory implementation of ArchitectHost), and Architect.

Here's an example of a test that runs the copy file builder. The test uses the builder to copy the package.json file and validates that the copied file's contents are the same as the source.

      
import {Architect} from '@angular-devkit/architect';
import {TestingArchitectHost} from '@angular-devkit/architect/testing';
import {schema} from '@angular-devkit/core';
import {promises as fs} from 'fs';
import {join} from 'path';
describe('Copy File Builder', () => {
  let architect: Architect;
  let architectHost: TestingArchitectHost;
  beforeEach(async () => {
    const registry = new schema.CoreSchemaRegistry();
    registry.addPostTransform(schema.transforms.addUndefinedDefaults);
    // TestingArchitectHost() takes workspace and current directories.
    // Since we don't use those, both are the same in this case.
    architectHost = new TestingArchitectHost(__dirname, __dirname);
    architect = new Architect(architectHost, registry);
    // This will either take a Node package name, or a path to the directory
    // for the package.json file.
    await architectHost.addBuilderFromPackage(join(__dirname, '..'));
  });
  it('can copy files', async () => {
    // A "run" can have multiple outputs, and contains progress information.
    const run = await architect.scheduleBuilder('@example/copy-file:copy', {
      source: 'package.json',
      destination: 'package-copy.json',
    });
    // The "result" member (of type BuilderOutput) is the next output.
    const output = await run.result;
    // Stop the builder from running. This stops Architect from keeping
    // the builder-associated states in memory, since builders keep waiting
    // to be scheduled.
    await run.stop();
    // Expect that the copied file is the same as its source.
    const sourceContent = await fs.readFile('package.json', 'utf8');
    const destinationContent = await fs.readFile('package-copy.json', 'utf8');
    expect(destinationContent).toBe(sourceContent);
  });
});

    

Watch mode

Most builders to run once and return. However, this behavior is not entirely compatible with a builder that watches for changes (like a devserver, for example). Architect can support watch mode, but there are some things to look out for.

• To be used with watch mode, a builder handler function should return an Observable. Architect subscribes to the Observable until it completes and might reuse it if the builder is scheduled again with the same arguments.

• The builder should always emit a BuilderOutput object after each execution. Once it's been executed, it can enter a watch mode, to be triggered by an external event. If an event triggers it to restart, the builder should execute the context.reportRunning() function to tell Architect that it is running again. This prevents Architect from stopping the builder if another run is scheduled.

When your builder calls BuilderRun.stop() to exit watch mode, Architect unsubscribes from the builder's Observable and calls the builder's teardown logic to clean up. This behavior also allows for long-running builds to be stopped and cleaned up.

In general, if your builder is watching an external event, you should separate your run into three phases.

Summary

The CLI Builder API provides a means of changing the behavior of the Angular CLI by using builders to execute custom logic.

• Builders can be synchronous or asynchronous, execute once or watch for external events, and can schedule other builders or targets.
• Builders have option defaults specified in the angular.json configuration file, which can be overwritten by an alternate configuration for the target, and further overwritten by command line flags
• The Angular team recommends that you use integration tests to test Architect builders. Use unit tests to validate the logic that the builder executes.
• If your builder returns an Observable, it should clean up the builder in the teardown logic of that Observable.