fix(docs-infra): convert hard-coded comparing-observables examples …

…into a proper mini-app

Previously, the examples in the `comparing-observables` guide were hard-coded.
This made it impossible to test them and verify they are correct.

This commit fixes this by converting them into a proper mini-app. In a
subsequent commit, tests will be added to verify that the source code
works as expected (and guard against regressions).

Fixes #31024

.github/CODEOWNERS

@@ -867,6 +867,7 @@ testing/**                                                      @angular/fw-test
 /aio/content/guide/observables.md                               @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/examples/observables/**                            @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/guide/comparing-observables.md                     @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
+/aio/content/examples/comparing-observables/**                  @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/guide/observables-in-angular.md                    @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/examples/observables-in-angular/**                 @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes
 /aio/content/guide/practical-observable-usage.md                @angular/fw-docs-observables @angular/framework-global-approvers @angular/framework-global-approvers-for-docs-only-changes

aio/content/examples/comparing-observables/src/observables.ts

@@ -0,0 +1,40 @@
+import { map } from 'rxjs/operators';
+import { Observable } from 'rxjs';
+
+// #docregion observable
+
+// declare a publishing operation
+const observable = new Observable<number>(observer => {
+  // Subscriber fn...
+});
+
+// initiate execution
+observable.subscribe(() => {
+  // observer handles notifications
+});
+
+// #enddocregion observable
+
+// #docregion unsubscribe
+
+const subscription = observable.subscribe(() => {
+  // observer handles notifications
+});
+
+subscription.unsubscribe();
+
+// #enddocregion unsubscribe
+
+// #docregion error
+
+observable.subscribe(() => {
+  throw Error('my error');
+});
+
+// #enddocregion error
+
+// #docregion chain
+
+observable.pipe(map(v => 2 * v));
+
+// #enddocregion chain

aio/content/examples/comparing-observables/src/promises.ts

@@ -0,0 +1,25 @@
+// #docregion promise
+// initiate execution
+const promise = new Promise<number>((resolve, reject) => {
+  // Executer fn...
+});
+
+promise.then(value => {
+  // handle result here
+});
+
+// #enddocregion promise
+
+// #docregion chain
+
+promise.then(v => 2 * v);
+
+// #enddocregion chain
+
+// #docregion error
+
+promise.then(() => {
+  throw Error('my error');
+});
+
+// #enddocregion error

aio/content/guide/comparing-observables.md

@@ -21,68 +21,67 @@ Observables are often compared to promises. Here are some key differences:
 
 * Observables are not executed until a consumer subscribes. The `subscribe()` executes the defined behavior once, and it can be called again. Each subscription has its own computation. Resubscription causes recomputation of values.
 
-<code-example hideCopy>
-// declare a publishing operation
-new Observable((observer) => { subscriber_fn });
-// initiate execution
-observable.subscribe(() => {
-  // observer handles notifications
-});
-</code-example>
+  <code-example 
+    path="comparing-observables/src/observables.ts" 
+    header="src/observables.ts (observable)" 
+    region="observable">
+  </code-example>
 
 * Promises execute immediately, and just once. The computation of the result is initiated when the promise is created. There is no way to restart work. All `then` clauses (subscriptions) share the same computation.
 
-<code-example hideCopy>
-// initiate execution
-new Promise((resolve, reject) => { executer_fn });
-// handle return value
-promise.then((value) => {
-  // handle result here
-});
-</code-example>
+  <code-example 
+    path="comparing-observables/src/promises.ts" 
+    header="src/promises.ts (promise)"
+    region="promise">
+  </code-example>
 
 ### Chaining
 
 * Observables differentiate between transformation function such as a map and subscription. Only subscription activates the subscriber function to start computing the values.
 
-
-<code-example hideCopy>observable.pipe(map((v) => 2*v));</code-example>
-
+  <code-example 
+    path="comparing-observables/src/observables.ts" 
+    header="src/observables.ts (chain)" 
+    region="chain">
+  </code-example>
 
 * Promises do not differentiate between the last `.then` clauses (equivalent to subscription) and intermediate `.then` clauses (equivalent to map).
 
-
-<code-example hideCopy>promise.then((v) => 2*v);</code-example>
-
+  <code-example 
+    path="comparing-observables/src/promises.ts"
+    header="src/promises.ts (chain)" 
+    region="chain">
+  </code-example>
 
 ### Cancellation
 
 * Observable subscriptions are cancellable. Unsubscribing removes the listener from receiving further values, and notifies the subscriber function to cancel work.
 
-<code-example hideCopy>
-const sub = obs.subscribe(...);
-sub.unsubscribe();
-</code-example>
+  <code-example 
+    path="comparing-observables/src/observables.ts" 
+    header="src/observables.ts (unsubcribe)" 
+    region="unsubscribe">
+  </code-example>
 
 * Promises are not cancellable.
 
 ### Error handling
 
 * Observable execution errors are delivered to the subscriber's error handler, and the subscriber automatically unsubscribes from the observable.
 
-<code-example hideCopy>
-obs.subscribe(() => {
-  throw Error('my error');
-});
-</code-example>
+  <code-example 
+    path="comparing-observables/src/observables.ts" 
+    header="src/observables.ts (error)"
+    region="error">
+  </code-example>
 
 * Promises push errors to the child promises.
 
-<code-example hideCopy>
-promise.then(() => {
-  throw Error('my error');
-});
-</code-example>
+  <code-example 
+    path="comparing-observables/src/promises.ts" 
+    header="src/promises.ts (error)"
+    region="error">
+  </code-example>
 
 ### Cheat sheet
 
@@ -100,14 +99,16 @@ The following code snippets illustrate how the same kind of operation is defined
     <tr>
       <td>Creation</td>
       <td>
-        <pre>new Observable((observer) => {
-    observer.next(123);
-  });</pre>
+        <pre>
+new Observable((observer) => {
+  observer.next(123);
+});</pre>
       </td>
       <td>
-        <pre>new Promise((resolve, reject) => {
-    resolve(123);
-  });</pre>
+        <pre>
+new Promise((resolve, reject) => {
+  resolve(123);
+});</pre>
       </td>
     </tr>
     <tr>
@@ -118,14 +119,16 @@ The following code snippets illustrate how the same kind of operation is defined
     <tr>
       <td>Subscribe</td>
       <td>
-        <pre>sub = obs.subscribe((value) => {
-    console.log(value)
-  });</pre>
+        <pre>
+sub = obs.subscribe((value) => {
+  console.log(value)
+});</pre>
       </td>
       <td>
-        <pre>promise.then((value) => {
-    console.log(value);
-  });</pre>
+        <pre>
+promise.then((value) => {
+  console.log(value);
+});</pre>
       </td>
     </tr>
     <tr>
@@ -165,7 +168,6 @@ subscription.unsubscribe();</pre>
 <pre>function handler(e) {
   console.log(‘Clicked’, e);
 }
-
 // Setup & begin listening
 button.addEventListener(‘click’, handler);
 // Stop listening