feature: add abort controller (#216)

Co-authored-by: jacobparis <jacobparis@users.noreply.github.com>
Co-authored-by: Nate Moore <nate@natemoo.re>
Co-authored-by: Nate Moore <natemoo-re@users.noreply.github.com>

Author: jacobparis

.changeset/lucky-maps-beam.md

@@ -0,0 +1,38 @@
+---
+"@clack/core": minor
+"@clack/prompts": minor
+---
+
+Adds a new `signal` option to support programmatic prompt cancellation with an [abort controller](https://kettanaito.com/blog/dont-sleep-on-abort-controller).
+
+One example use case is automatically cancelling a prompt after a timeout.
+
+```ts
+const shouldContinue = await confirm({
+  message: 'This message will self destruct in 5 seconds',
+  signal: AbortSignal.timeout(5000),
+});
+```
+
+Another use case is racing a long running task with a manual prompt.
+
+```ts
+const abortController = new AbortController()
+
+const projectType = await Promise.race([
+  detectProjectType({
+    signal: abortController.signal
+  }),
+  select({
+    message: 'Pick a project type.',
+    options: [
+      { value: 'ts', label: 'TypeScript' },
+      { value: 'js', label: 'JavaScript' },
+      { value: 'coffee', label: 'CoffeeScript', hint: 'oh no'},
+    ],
+     signal: abortController.signal,
+  })
+])
+
+abortController.abort()
+```

packages/core/src/prompts/prompt.ts

@@ -17,13 +17,15 @@ export interface PromptOptions<Self extends Prompt> {
 	input?: Readable;
 	output?: Writable;
 	debug?: boolean;
+	signal?: AbortSignal;
 }
 
 export default class Prompt {
 	protected input: Readable;
 	protected output: Writable;
+	private _abortSignal?: AbortSignal;
 
-	private rl!: ReadLine;
+	private rl: ReadLine | undefined;
 	private opts: Omit<PromptOptions<Prompt>, 'render' | 'input' | 'output'>;
 	private _render: (context: Omit<Prompt, 'prompt'>) => string | undefined;
 	private _track = false;
@@ -36,14 +38,15 @@ export default class Prompt {
 	public value: any;
 
 	constructor(options: PromptOptions<Prompt>, trackValue = true) {
-		const { input = stdin, output = stdout, render, ...opts } = options;
+		const { input = stdin, output = stdout, render, signal, ...opts } = options;
 
 		this.opts = opts;
 		this.onKeypress = this.onKeypress.bind(this);
 		this.close = this.close.bind(this);
 		this.render = this.render.bind(this);
 		this._render = render.bind(this);
 		this._track = trackValue;
+		this._abortSignal = signal;
 
 		this.input = input;
 		this.output = output;
@@ -111,11 +114,25 @@ export default class Prompt {
 
 	public prompt() {
 		return new Promise<string | symbol>((resolve, reject) => {
+			if (this._abortSignal) {
+				if (this._abortSignal.aborted) {
+					this.state = 'cancel';
+
+					this.close();
+					return resolve(CANCEL_SYMBOL);
+				}
+
+				this._abortSignal.addEventListener('abort', () => {
+					this.state = 'cancel';
+					this.close();
+				}, { once: true });
+			}
+
 			const sink = new WriteStream(0);
 			sink._write = (chunk, encoding, done) => {
 				if (this._track) {
-					this.value = this.rl.line.replace(/\t/g, '');
-					this._cursor = this.rl.cursor;
+					this.value = this.rl?.line.replace(/\t/g, '');
+					this._cursor = this.rl?.cursor ?? 0;
 					this.emit('value', this.value);
 				}
 				done();
@@ -171,7 +188,7 @@ export default class Prompt {
 		}
 		if (char === '\t' && this.opts.placeholder) {
 			if (!this.value) {
-				this.rl.write(this.opts.placeholder);
+				this.rl?.write(this.opts.placeholder);
 				this.emit('value', this.opts.placeholder);
 			}
 		}
@@ -185,7 +202,7 @@ export default class Prompt {
 				if (problem) {
 					this.error = problem;
 					this.state = 'error';
-					this.rl.write(this.value);
+					this.rl?.write(this.value);
 				}
 			}
 			if (this.state !== 'error') {
@@ -210,7 +227,8 @@ export default class Prompt {
 		this.input.removeListener('keypress', this.onKeypress);
 		this.output.write('\n');
 		setRawMode(this.input, false);
-		this.rl.close();
+		this.rl?.close();
+		this.rl = undefined;
 		this.emit(`${this.state}`, this.value);
 		this.unsubscribe();
 	}

packages/core/test/prompts/prompt.test.ts

@@ -229,4 +229,33 @@ describe('Prompt', () => {
 			expect(eventSpy).toBeCalledWith(key);
 		}
 	});
+
+	test('aborts on abort signal', () => {
+		const abortController = new AbortController();
+
+		const instance = new Prompt({
+			input,
+			output,
+			render: () => 'foo',
+			signal: abortController.signal,
+		});
+
+		instance.prompt();
+
+		expect(instance.state).to.equal('active');
+
+		abortController.abort();
+
+		expect(instance.state).to.equal('cancel');
+	});
+
+	test('returns immediately if signal is already aborted', () => {
+		const abortController = new AbortController();
+		abortController.abort();
+
+		const instance = new Prompt({ input, output, render: () => 'foo', signal: abortController.signal });
+		instance.prompt();
+
+		expect(instance.state).to.equal('cancel');
+	});
 });