add enabled function to a task - fixes #42

example.js

@@ -49,8 +49,8 @@ const tasks = new Listr([
 	},
 	{
 		title: 'Install dependencies with npm',
-		skip: ctx => ctx.yarn !== false && 'Already installed with Yarn',
-		task: () => delay(300)
+		enabled: ctx => ctx.yarn === false,
+		task: () => delay(3000)
 	},
 	{
 		title: 'Run tests',

index.js

@@ -3,7 +3,13 @@ const Task = require('./lib/task');
 const TaskWrapper = require('./lib/task-wrapper');
 const renderer = require('./lib/renderer');
 
-const runTask = (task, context) => new TaskWrapper(task).run(context);
+const runTask = (task, context) => {
+	if (!task.isEnabled()) {
+		return Promise.resolve();
+	}
+
+	return new TaskWrapper(task).run(context);
+};
 
 class Listr {
 
@@ -31,6 +37,12 @@ class Listr {
 		this.add(tasks || []);
 	}
 
+	_checkAll(context) {
+		for (const task of this._tasks) {
+			task.check(context);
+		}
+	}
+
 	get tasks() {
 		return this._tasks;
 	}
@@ -62,11 +74,17 @@ class Listr {
 
 		context = context || Object.create(null);
 
+		this._checkAll(context);
+
 		let tasks;
 		if (this._options.concurrent === true) {
 			tasks = Promise.all(this._tasks.map(task => runTask(task, context)));
 		} else {
-			tasks = this._tasks.reduce((promise, task) => promise.then(() => runTask(task, context)), Promise.resolve());
+			tasks = this._tasks.reduce((promise, task) => promise.then(() => {
+				this._checkAll(context);
+
+				return runTask(task, context);
+			}), Promise.resolve());
 		}
 
 		return tasks

lib/task.js

@@ -30,10 +30,16 @@ class Task extends Subject {
 			throw new TypeError(`Expected property \`skip\` of type \`function\` but got \`${typeof task.skip}\``);
 		}
 
+		if (task.enabled && typeof task.enabled !== 'function') {
+			throw new TypeError(`Expected property \`enabled\` of type \`function\` but got \`${typeof task.enabled}\``);
+		}
+
 		this._listr = listr;
 		this._options = options || {};
 		this._subtasks = [];
 		this._output = undefined;
+		this._enabledFn = task.enabled;
+		this._isEnabled = true;
 
 		this.title = task.title;
 		this.skip = task.skip || defaultSkipFn;
@@ -60,6 +66,22 @@ class Task extends Subject {
 		return state.toString(this._state);
 	}
 
+	check(ctx) {
+		// Check if a task is enabled or disabled
+		if (this._state === undefined && this._enabledFn) {
+			const isEnabled = this._enabledFn(ctx);
+
+			if (this._isEnabled !== isEnabled) {
+				this._isEnabled = isEnabled;
+
+				this.next({
+					type: 'ENABLED',
+					data: isEnabled
+				});
+			}
+		}
+	}
+
 	hasSubtasks() {
 		return this._subtasks.length > 0;
 	}
@@ -76,6 +98,10 @@ class Task extends Subject {
 		return this._state === state.COMPLETED;
 	}
 
+	isEnabled() {
+		return this._isEnabled;
+	}
+
 	hasFailed() {
 		return this._state === state.FAILED;
 	}

readme.md

@@ -47,11 +47,16 @@ const tasks = new Listr([
 		title: 'Install package dependencies with Yarn',
 		task: (ctx, task) => execa('yarn')
 			.catch(() => {
-				task.title = 'Install package dependencies with npm (Yarn not available)';
+				ctx.yarn === false;
 
-				return execa('npm', ['install']);
+				task.skip('Yarn not available, install it via `npm install -g yarn`');
 			})
 	},
+	{
+		title: 'Install package dependencies with npm',
+		enabled: ctx => ctx.yarn === false,
+		task: () => execa('npm', ['install'])
+	},
 	{
 		title: 'Run tests',
 		task: () => execa('npm', ['test'])
@@ -179,6 +184,33 @@ const tasks = new Listr([
 
 > Tip: You can still skip a task while already executing the `task` function with the [task object](#task-object).
 
+## Enabling tasks
+
+By default, every task is enabled which means that every task will be executed. However, it's also possible to provide an `enabled` function that returns wheter the task should be executed or not.
+
+```js
+const tasks = new Listr([
+	{
+		title: 'Install package dependencies with Yarn',
+		task: (ctx, task) => execa('yarn')
+			.catch(() => {
+				ctx.yarn === false;
+
+				task.skip('Yarn not available, install it via `npm install -g yarn`');
+			})
+	},
+	{
+		title: 'Install package dependencies with npm',
+		enabled: ctx => ctx.yarn === false,
+		task: () => execa('npm', ['install'])
+	}
+]);
+```
+
+In the above example, we try to run `yarn` first, if that fails we will fall back to `npm`. However, at first only the Yarn task will be visible. Because we set the `yarn` flag of the [context](https://github.com/SamVerschueren/listr#context) object to `false`, the second task will automatically be enabled and will be executed.
+
+> Note: This does not work in combination with [concurrent](https://github.com/SamVerschueren/listr#concurrent) tasks.
+
 
 ## Context
 
@@ -277,6 +309,7 @@ Every task is an observable. The task emits three different events and every eve
 2. The task outputted data (`DATA`).
 3. The task returns a subtask list (`SUBTASKS`).
 4. The task's title changed (`TITLE`).
+5. The task became enabled or disabled (`ENABLED`).
 
 This allows you to flexibly build up your UI. Let's render every task that starts executing.
 

test/enabled.js

@@ -0,0 +1,60 @@
+import {serial as test} from 'ava';
+import Listr from '../';
+import SimpleRenderer from './fixtures/simple-renderer';
+import {testOutput} from './fixtures/utils';
+
+test('do not run disabled tasks', async t => {
+	const list = new Listr([
+		{
+			title: 'foo',
+			task: () => Promise.resolve()
+		},
+		{
+			title: 'bar',
+			enabled: ctx => ctx.run === true,
+			task: () => Promise.resolve()
+		}
+	], {
+		renderer: SimpleRenderer
+	});
+
+	testOutput(t, [
+		'foo [started]',
+		'foo [completed]',
+		'done'
+	]);
+
+	await list.run();
+});
+
+test('run enabled task', async t => {
+	const list = new Listr([
+		{
+			title: 'foo',
+			task: (ctx, task) => Promise.reject(new Error('Something went wrong'))
+				.catch(() => {
+					task.skip('It failed');
+
+					ctx.failed = true;
+				})
+		},
+		{
+			title: 'bar',
+			enabled: ctx => ctx.failed === true,
+			task: () => Promise.resolve()
+		}
+	], {
+		renderer: SimpleRenderer
+	});
+
+	testOutput(t, [
+		'foo [started]',
+		'foo [skipped]',
+		'> It failed',
+		'bar [started]',
+		'bar [completed]',
+		'done'
+	]);
+
+	await list.run();
+});