Blink: Throttle progressively loaded images.

third_party/WebKit/Source/core/fetch/ImageResource.cpp

Index: third_party/WebKit/Source/core/fetch/ImageResource.cpp
diff --git a/third_party/WebKit/Source/core/fetch/ImageResource.cpp b/third_party/WebKit/Source/core/fetch/ImageResource.cpp
index d0267f99ccf05b41ce82815333f210a8c96abad3..d3061d29509cae2fec1e8ff9c6cc952ac5c0f12b 100644
--- a/third_party/WebKit/Source/core/fetch/ImageResource.cpp
+++ b/third_party/WebKit/Source/core/fetch/ImageResource.cpp
@@ -44,6 +44,12 @@
 #include <v8.h>
 
 namespace blink {
+namespace {
+// The amount of time to wait before informing the clients that the image has
+// been updated (in seconds). This effectively throttles invalidations that
+// result from new data arriving for this image.
+constexpr double kFlushDelaySeconds = 1.;
+}  // namespace
 
 ImageResource* ImageResource::fetch(FetchRequest& request,
                                     ResourceFetcher* fetcher) {
@@ -136,15 +142,17 @@ void ImageResource::addObserver(ImageResourceObserver* observer) {
   if (isCacheValidator())
     return;
 
-  // When the response is not multipart, if |data()| exists, |m_image| must be
-  // created. This is assured that |updateImage()| is called when |appendData()|
-  // is called.
+  // When the response is not multipart, if |data()| exists and image is loaded,
+  // |m_image| must be created. This is assured that |updateImage()| is called
+  // when the image finishes loading. However, while loading |appendData()| can
+  // throttle |updateImage()| calls.
   //
   // On the other hand, when the response is multipart, |updateImage()| is not
   // called in |appendData()|, which means |m_image| might not be created even
   // when |data()| exists. This is intentional since creating a |m_image| on
   // receiving data might destroy an existing image in a previous part.
-  DCHECK((m_multipartParser && isLoading()) || !data() || m_image);
+  DCHECK((m_multipartParser && isLoading()) || !data() || m_image ||
+         !isLoaded());
 
   if (m_image && !m_image->isNull()) {
     observer->imageChanged(this);
@@ -242,7 +250,25 @@ void ImageResource::appendData(const char* data, size_t length) {
     m_multipartParser->appendData(data, length);
   } else {
     Resource::appendData(data, length);
-    updateImage(false);
+
+    // If we have an animated image, then update as soon as we have more data.
+    if (m_image && m_image->maybeAnimated()) {
+      updateImage(false);
+      return;
+    }
+
+    // For other images, only update at |kFlushDelaySeconds| intervals. This
+    // throttles how frequently we update |m_image| and how frequently we
+    // inform the clients which causes an invalidation of this image. In other
+    // words, we only invalidate this image every |kFlushDelaySeconds| seconds
+    // while loading.
+    double now = WTF::currentTime();
+    if (!m_lastFlushTime) {
+      m_lastFlushTime = now;
+    } else if (m_lastFlushTime + kFlushDelaySeconds <= now) {
+      updateImage(false);
+      m_lastFlushTime = now;
+    }
   }
 }