LibWeb/WebAudio: Implement basic startRendering

Adds passing WPT. Does not handle actually rendering audio yet.

Author: Prospero23

Libraries/LibWeb/WebAudio/OfflineAudioContext.cpp

@@ -1,12 +1,17 @@
 /*
  * Copyright (c) 2024, Shannon Booth <shannon@serenityos.org>
+ * Copyright (c) 2025, Ben Eidson <b.e.eidson@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #include <LibWeb/Bindings/Intrinsics.h>
+#include <LibWeb/DOM/Event.h>
 #include <LibWeb/HTML/EventNames.h>
+#include <LibWeb/HTML/Navigable.h>
+#include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
 #include <LibWeb/HTML/Window.h>
+#include <LibWeb/WebAudio/AudioBuffer.h>
 #include <LibWeb/WebAudio/AudioDestinationNode.h>
 #include <LibWeb/WebAudio/OfflineAudioContext.h>
 
@@ -23,7 +28,7 @@ WebIDL::ExceptionOr<GC::Ref<OfflineAudioContext>> OfflineAudioContext::construct
     TRY(verify_audio_options_inside_nominal_range(realm, context_options.number_of_channels, context_options.length, context_options.sample_rate));
 
     // Let c be a new OfflineAudioContext object. Initialize c as follows:
-    auto c = realm.create<OfflineAudioContext>(realm, context_options.length, context_options.sample_rate);
+    auto c = realm.create<OfflineAudioContext>(realm, context_options.number_of_channels, context_options.length, context_options.sample_rate);
 
     // 1. Set the [[control thread state]] for c to "suspended".
     c->set_control_state(Bindings::AudioContextState::Suspended);
@@ -58,7 +63,86 @@ OfflineAudioContext::~OfflineAudioContext() = default;
 // https://webaudio.github.io/web-audio-api/#dom-offlineaudiocontext-startrendering
 WebIDL::ExceptionOr<GC::Ref<WebIDL::Promise>> OfflineAudioContext::start_rendering()
 {
-    return WebIDL::NotSupportedError::create(realm(), "FIXME: Implement OfflineAudioContext::start_rendering"_utf16);
+    auto& realm = this->realm();
+
+    // 1. If this’s relevant global object’s associated Document is not fully active then return a promise rejected with "InvalidStateError" DOMException.
+    auto& window = as<HTML::Window>(HTML::relevant_global_object(*this));
+    auto const& associated_document = window.associated_document();
+
+    if (!associated_document.is_fully_active()) {
+        auto error = WebIDL::InvalidStateError::create(realm, "Document is not fully active"_utf16);
+        return WebIDL::create_rejected_promise_from_exception(realm, error);
+    }
+
+    // AD-HOC: Not in spec explicitly, but this should account for detached iframes too. See /the-offlineaudiocontext-interface/startrendering-after-discard.html WPT.
+    auto navigable = window.navigable();
+    if (navigable && navigable->has_been_destroyed()) {
+        auto error = WebIDL::InvalidStateError::create(realm, "The iframe has been detached"_utf16);
+        return WebIDL::create_rejected_promise_from_exception(realm, error);
+    }
+
+    // 2. If the [[rendering started]] slot on the OfflineAudioContext is true, return a rejected promise with InvalidStateError, and abort these steps.
+    if (m_rendering_started) {
+        auto error = WebIDL::InvalidStateError::create(realm, "Rendering is already started"_utf16);
+        return WebIDL::create_rejected_promise_from_exception(realm, error);
+    }
+
+    // 3. Set the [[rendering started]] slot of the OfflineAudioContext to true.
+    m_rendering_started = true;
+
+    // 4. Let promise be a new promise.
+    auto promise = WebIDL::create_promise(realm);
+
+    // 5. Create a new AudioBuffer, with a number of channels, length and sample rate equal respectively to the
+    //    numberOfChannels, length and sampleRate values passed to this instance’s constructor in the contextOptions
+    //    parameter.
+    auto buffer_result = create_buffer(m_number_of_channels, length(), sample_rate());
+
+    // 6. If an exception was thrown during the preceding AudioBuffer constructor call, reject promise with this exception.
+    if (buffer_result.is_exception()) {
+        return WebIDL::create_rejected_promise_from_exception(realm, buffer_result.exception());
+    }
+
+    // Assign this buffer to an internal slot [[rendered buffer]] in the OfflineAudioContext.
+    m_rendered_buffer = buffer_result.release_value();
+
+    // 7. Otherwise, in the case that the buffer was successfully constructed, begin offline rendering.
+    begin_offline_rendering(promise);
+
+    // 8. Append promise to [[pending promises]].
+    m_pending_promises.append(promise);
+
+    // 9. Return promise.
+    return promise;
+}
+
+void OfflineAudioContext::begin_offline_rendering(GC::Ref<WebIDL::Promise> promise)
+{
+    auto& realm = this->realm();
+    // To begin offline rendering, the following steps MUST happen on a rendering thread that is created for the occasion.
+    // FIXME: 1: Given the current connections and scheduled changes, start rendering length sample-frames of audio into [[rendered buffer]]
+    // FIXME: 2: For every render quantum, check and suspend rendering if necessary.
+    // FIXME: 3: If a suspended context is resumed, continue to render the buffer.
+    // 4: Once the rendering is complete, queue a media element task to execute the following steps:
+    queue_a_media_element_task(GC::create_function(heap(), [&realm, promise, this]() {
+        HTML::TemporaryExecutionContext context(realm, HTML::TemporaryExecutionContext::CallbacksEnabled::Yes);
+
+        // 4.1 Resolve the promise created by startRendering() with [[rendered buffer]].
+        WebIDL::resolve_promise(realm, promise, this->m_rendered_buffer);
+
+        // AD-HOC: Remove resolved promise from [[pending promises]]
+        // https://github.com/WebAudio/web-audio-api/issues/2648
+        m_pending_promises.remove_all_matching([promise](GC::Ref<WebIDL::Promise> const& p) {
+            return p.ptr() == promise.ptr();
+        });
+
+        // 4.2: Queue a media element task to fire an event named complete at the OfflineAudioContext using OfflineAudioCompletionEvent
+        //      whose renderedBuffer property is set to [[rendered buffer]].
+        // FIXME: Need to implement OfflineAudioCompletionEvent.
+        queue_a_media_element_task(GC::create_function(heap(), [&realm, this]() {
+            this->dispatch_event(DOM::Event::create(realm, HTML::EventNames::complete));
+        }));
+    }));
 }
 
 WebIDL::ExceptionOr<GC::Ref<WebIDL::Promise>> OfflineAudioContext::resume()
@@ -91,9 +175,10 @@ void OfflineAudioContext::set_oncomplete(GC::Ptr<WebIDL::CallbackType> value)
     set_event_handler_attribute(HTML::EventNames::complete, value);
 }
 
-OfflineAudioContext::OfflineAudioContext(JS::Realm& realm, WebIDL::UnsignedLong length, float sample_rate)
+OfflineAudioContext::OfflineAudioContext(JS::Realm& realm, WebIDL::UnsignedLong number_of_channels, WebIDL::UnsignedLong length, float sample_rate)
     : BaseAudioContext(realm, sample_rate)
     , m_length(length)
+    , m_number_of_channels(number_of_channels)
 {
 }
 
@@ -106,6 +191,7 @@ void OfflineAudioContext::initialize(JS::Realm& realm)
 void OfflineAudioContext::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
+    visitor.visit(m_rendered_buffer);
 }
 
 }

Libraries/LibWeb/WebAudio/OfflineAudioContext.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2024, Shannon Booth <shannon@serenityos.org>
+ * Copyright (c) 2025, Ben Eidson <b.e.eidson@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -42,12 +43,18 @@ class OfflineAudioContext final : public BaseAudioContext {
     void set_oncomplete(GC::Ptr<WebIDL::CallbackType>);
 
 private:
-    OfflineAudioContext(JS::Realm&, WebIDL::UnsignedLong length, float sample_rate);
+    OfflineAudioContext(JS::Realm&, WebIDL::UnsignedLong number_of_channels, WebIDL::UnsignedLong length, float sample_rate);
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
     WebIDL::UnsignedLong m_length {};
+    WebIDL::UnsignedLong m_number_of_channels {};
+    bool m_rendering_started { false };
+
+    GC::Ptr<AudioBuffer> m_rendered_buffer;
+
+    void begin_offline_rendering(GC::Ref<WebIDL::Promise> promise);
 };
 
 }

Tests/LibWeb/Text/expected/WebAudio/OfflineAudioContext.txt

@@ -1 +1,2 @@
 128
+InvalidStateError: Rendering is already started

Tests/LibWeb/Text/expected/wpt-import/webaudio/the-audio-api/the-offlineaudiocontext-interface/startrendering-after-discard.txt

@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Pass
+Pass	startRendering()

Tests/LibWeb/Text/input/WebAudio/OfflineAudioContext.html

@@ -1,11 +1,22 @@
 <!DOCTYPE html>
 <script src="../include.js"></script>
 <script>
-
-    // Once the ctor‑offlineaudiocontext WPT test is updated to check
-    // renderQuantumSize and renderSizeHint, this test is not needed.
-    test(() => {
+    asyncTest(async done => {
+        // Once the ctor‑offlineaudiocontext WPT test is updated to check
+        // renderQuantumSize and renderSizeHint, this test is not needed.
         const audioContext = new OfflineAudioContext(1, 1, 44100)
         println(`${audioContext.renderQuantumSize}`);
+
+        // Second call must reject with InvalidStateError
+        await audioContext.startRendering();
+
+        try {
+            await audioContext.startRendering();
+            println('FAIL: started rendering on repeated call');
+        } catch (e) {
+            println(`${e}`);
+        }
+
+        done();
     });
 </script>

Tests/LibWeb/Text/input/wpt-import/webaudio/the-audio-api/the-offlineaudiocontext-interface/startrendering-after-discard.html

@@ -0,0 +1,24 @@
+<!doctype html>
+<title>Test for rejected promise from startRendering() on an
+  OfflineAudioContext in a discarded browsing context</title>
+<script src=../../../resources/testharness.js></script>
+<script src=../../../resources/testharnessreport.js></script>
+<body></body>
+<script>
+let context;
+let childDOMException;
+setup(() => {
+  const frame = document.createElement('iframe');
+  document.body.appendChild(frame);
+  context = new frame.contentWindow.OfflineAudioContext(
+    {length: 1, sampleRate: 48000});
+  childDOMException = frame.contentWindow.DOMException;
+  frame.remove();
+});
+
+promise_test((t) => promise_rejects_dom(
+  t, 'InvalidStateError', childDOMException, context.startRendering()),
+             'startRendering()');
+// decodeAudioData() is tested in
+// offlineaudiocontext-detached-execution-context.html
+</script>