Allow binding to CPPopUpButton's selectedTag.

AppKit/CPPopUpButton.j

@@ -49,6 +49,11 @@ CPPopUpButtonStatePullsDown = CPThemeState("pulls-down");
     return "popup-button";
 }
 
++ (CPSet)keyPathsForValuesAffectingSelectedTag
+{
+    return [CPSet setWithObject:CPValueBinding];
+}
+
 /*!
     Initializes the pop-up button to the specified size.
     @param aFrame the size for the button
@@ -233,6 +238,32 @@ CPPopUpButtonStatePullsDown = CPThemeState("pulls-down");
     return _selectedIndex;
 }
 
+/*!
+    @ignore
+*/
+- (int)selectedTag
+{
+    return [[self selectedItem] tag];
+}
+
+/*!
+    @ignore
+*/
+- (void)setSelectedTag:(int)aTag
+{
+    [self selectItemWithTag:aTag];
+}
+
+/*!
+    @ignore
+*/
+- (void)_reverseSetBinding
+{
+    var theBinding = [CPKeyValueBinding getBinding:@"selectedTag" forObject:self];
+    [theBinding reverseSetValueFor:@"selectedTag"];
+    [super _reverseSetBinding];
+}
+
 // Setting the Current Selection
 /*!
     Selects the specified menu item.
@@ -252,6 +283,8 @@ CPPopUpButtonStatePullsDown = CPThemeState("pulls-down");
     if (_selectedIndex == anIndex)
         return;
 
+    [self willChangeValueForKey:CPValueBinding];
+
     if (_selectedIndex >= 0 && ![self pullsDown])
         [[self selectedItem] setState:CPOffState];
 
@@ -261,6 +294,8 @@ CPPopUpButtonStatePullsDown = CPThemeState("pulls-down");
         [[self selectedItem] setState:CPOnState];
 
     [self synchronizeTitleAndSelectedItem];
+
+    [self didChangeValueForKey:CPValueBinding];
 }
 
 /*!

Tests/AppKit/CPPopUpButtonTest.j

@@ -20,4 +20,33 @@
     [self assert:["Option A", "Option B"] equals:[button itemTitles]];
 }
 
+- (void)testBindingSelectedTag
+{
+    var dict = [CPDictionary dictionary],
+        item = nil;
+
+    item = [[CPMenuItem alloc] initWithTitle:@"Mouse" action:nil keyEquivalent:nil];
+    [item setTag:20];
+    [button addItem:item];
+
+    item = [[CPMenuItem alloc] initWithTitle:@"Elephant" action:nil keyEquivalent:nil];
+    [item setTag:4990000];
+    [button addItem:item];
+
+    [dict setObject:20 forKey:@"weight"];
+
+    [button bind:@"selectedTag"
+        toObject:dict
+     withKeyPath:@"weight"
+         options:nil];
+
+    [self assert:20 equals:[button selectedTag]];
+
+    [button selectItemAtIndex:1];
+    // simulate selection by the user
+    [button sendAction:[button action] to:[button target]];
+
+    [self assert:4990000 equals:[dict objectForKey:@"weight"]];
+}
+
 @end