Add an example for asyncProperty

Fixes #239

example/async-property/FakeApi.ts

@@ -0,0 +1,26 @@
+export class FakeApi {
+  private static Users = ['toto', 'titi', 'tata', 'tutu'];
+
+  // Simulate a db query to fetch all users
+  // Can be replaced by any other asynchronous code
+  // This calls is supposed to fail whenever the query get back exactly zero result
+  static searchUsers(query: string): Promise<string[]> {
+    const answer = FakeApi.Users.filter(name => name.indexOf(query) !== -1);
+    return new Promise((resolve, reject) => {
+      if (answer.length === 0) {
+        setTimeout(() => reject('Unhandled exception'), 0);
+        return;
+      }
+      setTimeout(() => resolve(answer), 0);
+    });
+  }
+
+  // Simulate a call to a rest api
+  static findUser(username: string): Promise<{ id: number } | null> {
+    const id = FakeApi.Users.indexOf(username);
+    const answer = id !== -1 ? { id } : null;
+    return new Promise((resolve, reject) => {
+      setTimeout(() => resolve(answer), 0);
+    });
+  }
+}

example/async-property/test.ts

@@ -0,0 +1,21 @@
+import * as fc from '../../lib/fast-check';
+import { FakeApi } from './FakeApi';
+
+describe('Asynchronous example', () => {
+  it('Search users asynchronously (fails)', async () => {
+    await fc.assert(
+      fc.asyncProperty(fc.string(), async query => {
+        const users = await FakeApi.searchUsers(query);
+        return Array.isArray(users) && users.every(name => name.indexOf(query) !== -1);
+      })
+    );
+  });
+  it('Get back user id from label (success)', async () => {
+    await fc.assert(
+      fc.asyncProperty(fc.constantFrom('toto', 'titi'), async username => {
+        const user = await FakeApi.findUser(username);
+        return user != null && typeof user.id === 'number' && user.id >= 0;
+      })
+    );
+  });
+});