Prefetch diff for next file

If there is another file remaining in the change, fetch it in the
background while viewing the current file.

The /diff URL is cacheable by the browser for up to 1 week, allowing
the browser to skip loading the data when the next file is opened.

Change-Id: I236b11628bfacae622dc32f7b7621df7da157bf2

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/Header.java

@@ -64,6 +64,7 @@ class Header extends Composite {
   private final String path;
   private boolean hasPrev;
   private boolean hasNext;
+  private String nextPath;
 
   Header(KeyCommandSet keys, PatchSet.Id patchSetId, String path) {
     initWidget(uiBinder.createAndBindUi(this));
@@ -107,10 +108,13 @@ class Header extends Composite {
             break;
           }
         }
+        FileInfo nextInfo = index == files.length() - 1
+            ? null
+            : files.get(index + 1);
         setupNav(prev, '[', PatchUtil.C.previousFileHelp(),
             index == 0 ? null : files.get(index - 1));
-        setupNav(next, ']', PatchUtil.C.nextFileHelp(),
-            index == files.length() - 1 ? null : files.get(index + 1));
+        setupNav(next, ']', PatchUtil.C.nextFileHelp(), nextInfo);
+        nextPath = nextInfo != null ? nextInfo.path() : null;
       }
     });
 
@@ -194,4 +198,8 @@ class Header extends Composite {
   boolean hasNext() {
     return hasNext;
   }
+
+  String getNextPath() {
+    return nextPath;
+  }
 }

gerrit-gwtui/src/main/java/com/google/gerrit/client/diff/SideBySide2.java

@@ -257,6 +257,8 @@ public class SideBySide2 extends Screen {
     cmB.setOption("viewportMargin", 10);
     cmB.setCursor(LineCharacter.create(0));
     cmB.focus();
+
+    prefetchNextFile();
   }
 
   @Override
@@ -1340,4 +1342,28 @@ public class SideBySide2 extends Screen {
   CodeMirror getCmB() {
     return cmB;
   }
+
+  private void prefetchNextFile() {
+    String nextPath = header.getNextPath();
+    if (nextPath != null) {
+      DiffApi.diff(revision, nextPath)
+        .base(base)
+        .wholeFile()
+        .intraline(pref.isIntralineDifference())
+        .ignoreWhitespace(pref.getIgnoreWhitespace())
+        .get(new AsyncCallback<DiffInfo>() {
+          @Override
+          public void onSuccess(DiffInfo info) {
+            new ModeInjector()
+              .add(getContentType(info.meta_a()))
+              .add(getContentType(info.meta_b()))
+              .inject(CallbackGroup.<Void> emptyCallback());
+          }
+
+          @Override
+          public void onFailure(Throwable caught) {
+          }
+        });
+    }
+  }
 }