Disable send button when commit message is unchanged

dd2bd5c adds a check in the commit message edit service to
prevent a new patch set from being added when the given commit
message is the same as the existing one.  However it is still
possible for the user to send a commit edit request from the Web
UI with an unchanged commit message, which results in an error
dialog.

Add a new helper class that watches for changes on a text box
and notifies changes.  Use this in the commit message editor
dialog to enable the send button only when the text differs from
the existing commit message text.

Change-Id: I249beaa24e8a30bddd6f6a471053396a3fd2dafa

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/CommitMessageBlock.java

@@ -19,6 +19,7 @@ import com.google.gerrit.client.Gerrit;
 import com.google.gerrit.client.ui.ChangeLink;
 import com.google.gerrit.client.ui.CommentLinkProcessor;
 import com.google.gerrit.client.ui.CommentedActionDialog;
+import com.google.gerrit.client.ui.TextBoxChangeListener;
 import com.google.gerrit.common.PageLinks;
 import com.google.gerrit.common.data.ChangeDetail;
 import com.google.gerrit.reviewdb.client.Change;
@@ -72,13 +73,24 @@ public class CommitMessageBlock extends Composite {
   }
 
   private abstract class CommitMessageEditDialog extends CommentedActionDialog<ChangeDetail> {
+    private final String originalMessage;
     public CommitMessageEditDialog(final String title, final String heading,
         final String commitMessage, AsyncCallback<ChangeDetail> callback) {
       super(title, heading, callback);
+      originalMessage = commitMessage.trim();
       message.setCharacterWidth(72);
       message.setVisibleLines(20);
-      message.setText(commitMessage);
+      message.setText(originalMessage);
       message.addStyleName(Gerrit.RESOURCES.css().changeScreenDescription());
+      sendButton.setEnabled(false);
+
+      new TextBoxChangeListener(message) {
+        public void onTextChanged(String newText) {
+          // Trim the new text so we don't consider trailing
+          // newlines as changes
+          sendButton.setEnabled(!newText.trim().equals(originalMessage));
+        }
+      };
     }
 
     public String getMessageText() {

gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/TextBoxChangeListener.java

@@ -0,0 +1,96 @@
+// Copyright (C) 2012 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.client.ui;
+
+import com.google.gwt.core.client.Scheduler;
+import com.google.gwt.core.client.Scheduler.ScheduledCommand;
+import com.google.gwt.event.dom.client.FocusEvent;
+import com.google.gwt.event.dom.client.FocusHandler;
+import com.google.gwt.event.dom.client.KeyDownEvent;
+import com.google.gwt.event.dom.client.KeyDownHandler;
+import com.google.gwt.event.dom.client.KeyPressEvent;
+import com.google.gwt.event.dom.client.KeyPressHandler;
+import com.google.gwt.event.dom.client.MouseUpEvent;
+import com.google.gwt.event.dom.client.MouseUpHandler;
+import com.google.gwt.event.shared.GwtEvent;
+import com.google.gwt.user.client.ui.TextBoxBase;
+
+public abstract class TextBoxChangeListener implements KeyPressHandler, KeyDownHandler, MouseUpHandler {
+
+  private String oldText;
+
+  public TextBoxChangeListener(final TextBoxBase tb) {
+    oldText = tb.getText();
+
+    tb.addKeyPressHandler(this);
+
+    // Is there another way to capture middle button X11 pastes in browsers
+    // which do not yet support ONPASTE events (Firefox)?
+    tb.addMouseUpHandler(this);
+
+    // Resetting the "original text" on focus ensures that we are
+    // up to date with non-user updates of the text (calls to
+    // setText()...) and also up to date with user changes which
+    // occurred after enabling "widget".
+    tb.addFocusHandler(new FocusHandler() {
+        @Override
+        public void onFocus(FocusEvent event) {
+          oldText = tb.getText();
+        }
+      });
+
+    // CTRL-V Pastes in Chrome seem only detectable via BrowserEvents or
+    // KeyDownEvents, the latter is better.
+    tb.addKeyDownHandler(this);
+  }
+
+
+  @Override
+  public void onKeyPress(final KeyPressEvent e) {
+    on(e);
+  }
+
+  @Override
+  public void onKeyDown(final KeyDownEvent e) {
+    on(e);
+  }
+
+  @Override
+  public void onMouseUp(final MouseUpEvent e) {
+    on(e);
+  }
+
+  private void on(final GwtEvent<?> e) {
+    final TextBoxBase tb = (TextBoxBase) e.getSource();
+
+    if (!tb.getText().equals(oldText)) {
+      onTextChanged(tb.getText());
+      oldText = tb.getText();
+    } else {
+      // The text appears to not always get updated until the handlers complete.
+      Scheduler.get().scheduleDeferred(new ScheduledCommand() {
+        @Override
+        public void execute() {
+          if (!tb.getText().equals(oldText)) {
+            onTextChanged(tb.getText());
+            oldText = tb.getText();
+          }
+        }
+      });
+    }
+  }
+
+  public abstract void onTextChanged(String newText);
+}