Clipboard: Make ClipboardReader::NextRead a pure virtual function.

Also, add some comments...

Bug: 1145787
Change-Id: I64fc705e671f5534d523d2b9c1ec1d7f01b64c1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522575
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825097}

third_party/blink/renderer/modules/clipboard/clipboard_reader.cc

@@ -78,12 +78,13 @@ class ClipboardImageReader final : public ClipboardReader {
                                             std::move(png_data)));
   }
 
-  // An empty vector indicates that the encoding step failed.
-  void NextRead(Vector<uint8_t> png_data) {
+  void NextRead(Vector<uint8_t> utf8_bytes) override {
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
     Blob* blob = nullptr;
-    if (png_data.size())
-      blob = Blob::Create(png_data.data(), png_data.size(), kMimeTypeImagePng);
+    if (utf8_bytes.size()) {
+      blob =
+          Blob::Create(utf8_bytes.data(), utf8_bytes.size(), kMimeTypeImagePng);
+    }
 
     promise_->OnRead(blob);
   }
@@ -135,7 +136,7 @@ class ClipboardTextReader final : public ClipboardReader {
                                             std::move(utf8_bytes)));
   }
 
-  void NextRead(Vector<uint8_t> utf8_bytes) {
+  void NextRead(Vector<uint8_t> utf8_bytes) override {
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
     Blob* blob = nullptr;
     if (utf8_bytes.size()) {
@@ -208,7 +209,7 @@ class ClipboardHtmlReader final : public ClipboardReader {
                                             std::move(utf8_bytes)));
   }
 
-  void NextRead(Vector<uint8_t> utf8_bytes) {
+  void NextRead(Vector<uint8_t> utf8_bytes) override {
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
     Blob* blob = nullptr;
     if (utf8_bytes.size()) {
@@ -284,7 +285,7 @@ class ClipboardSvgReader final : public ClipboardReader {
                                             std::move(utf8_bytes)));
   }
 
-  void NextRead(Vector<uint8_t> utf8_bytes) {
+  void NextRead(Vector<uint8_t> utf8_bytes) override {
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
     Blob* blob = nullptr;
     if (utf8_bytes.size()) {
@@ -295,8 +296,10 @@ class ClipboardSvgReader final : public ClipboardReader {
   }
 };
 }  // anonymous namespace
+
 // ClipboardReader functions.
 
+// static
 ClipboardReader* ClipboardReader::Create(SystemClipboard* system_clipboard,
                                          const String& mime_type,
                                          ClipboardPromise* promise) {

third_party/blink/renderer/modules/clipboard/clipboard_reader.h

@@ -25,7 +25,8 @@ class ClipboardPromise;
 //
 // ClipboardReader takes as input a ClipboardPromise, from which it expects
 // a clipboard format, and to which it provides a Blob containing an encoded
-// SystemClipboard-originated clipboard payload.
+// SystemClipboard-originated clipboard payload. All System Clipboard
+// operations should be called from the main thread.
 //
 // Subclasses of ClipboardReader should be implemented for each supported
 // format. Subclasses should:
@@ -52,10 +53,13 @@ class ClipboardReader : public GarbageCollected<ClipboardReader> {
   // TaskRunner for interacting with the system clipboard.
   const scoped_refptr<base::SingleThreadTaskRunner> clipboard_task_runner_;
 
-  explicit ClipboardReader(SystemClipboard* system_clipboard,
-                           ClipboardPromise* promise);
+  ClipboardReader(SystemClipboard* system_clipboard, ClipboardPromise* promise);
+  // Send a Blob holding `utf8_bytes` to the owning ClipboardPromise.
+  // An empty `utf8_bytes` indicates that the encoding step failed.
+  virtual void NextRead(Vector<uint8_t> utf8_bytes) = 0;
 
   SystemClipboard* system_clipboard() { return system_clipboard_; }
+  // This ClipboardPromise owns this ClipboardReader.
   Member<ClipboardPromise> promise_;
 
   SEQUENCE_CHECKER(sequence_checker_);

third_party/blink/renderer/modules/clipboard/clipboard_writer.h

@@ -63,7 +63,7 @@ class ClipboardWriter : public GarbageCollected<ClipboardWriter>,
   virtual void StartWrite(
       DOMArrayBuffer* raw_data,
       scoped_refptr<base::SingleThreadTaskRunner> task_runner) = 0;
-  // This ClipboardPromise owns this.
+  // This ClipboardPromise owns this ClipboardWriter.
   Member<ClipboardPromise> promise_;
 
   // Ensure that System Clipboard operations occur on the main thread.