implement _setCurrentValueIsPlaceholder in CPTextField

CPKeyValueBinding will sent _setCurrentValueIsPlaceholder: to indicate to an object that current value is a placeholder and should be displayed as such. This commit implements the method for CPTextField and adds a unit test to test the behavior.

AppKit/CPTextField.j

@@ -86,6 +86,8 @@ CPTextFieldStatePlaceholder = CPThemeState("placeholder");
     CPColor                 _textFieldBackgroundColor;
 
     id                      _placeholderString;
+    id                      _originalPlaceholderString;
+    BOOL                    _currentValueIsPlaceholder;
 
     id                      _delegate;
 
@@ -780,6 +782,30 @@ CPTextFieldStatePlaceholder = CPThemeState("placeholder");
     return _placeholderString;
 }
 
+- (void)_setCurrentValueIsPlaceholder:(BOOL)isPlaceholder
+{
+    if (isPlaceholder)
+    {
+        // Save the original placeholder value so we can restore it later
+        // Only do this if the placeholder is not already overridden because the bindings logic might call this method 
+        // several times and we don't want the bindings placeholder to ever become the original placeholder
+        if (!_currentValueIsPlaceholder)
+            _originalPlaceholderString = [self placeholderString];
+
+        // Set the current string value as the current placeholder and clear the string value
+        [self setPlaceholderString:[self stringValue]];
+        [self setStringValue:@""];
+    }
+    else
+    {
+        // Restore the original placeholder, the actual textfield value is already correct
+        // because it was set using setValue:forKey:
+        [self setPlaceholderString:_originalPlaceholderString];
+    }
+
+    _currentValueIsPlaceholder = isPlaceholder;
+}
+
 /*!
     Size to fit has two behavior, depending on if the receiver is an editable text field or not.
 

Tests/AppKit/CPKeyValueBindingTest.j

@@ -190,6 +190,29 @@
     [self assert:'value' equals:testView.lastKey];
 }
 
+- (void)testTextField
+{
+    var textField = [[CPTextField alloc] initWithFrame:CGRectMakeZero()];
+    [textField setPlaceholderString:@"cheese"];
+
+
+    content = [
+        [BindingTester testerWithCheese:@"yellow"],
+        [BindingTester testerWithCheese:@"green"]
+    ];
+    arrayController = [[CPArrayController alloc] initWithContent:content];
+
+    [arrayController setSelectionIndexes:[CPIndexSet indexSetWithIndexesInRange:CPMakeRange(0, 2)]];
+
+    var options = [CPDictionary dictionaryWithJSObject:{CPMultipleValuesPlaceholderBindingOption:@"Multiple Values"}];
+    [textField bind:@"value" toObject:arrayController withKeyPath:@"selection.cheese" options:options];
+
+    [self assert:@"Multiple Values" equals:[textField placeholderString]];
+
+    [arrayController setSelectionIndex:0];
+    [self assert:@"cheese" equals:[textField placeholderString]];
+}
+
 - (void)observeValueForKeyPath:(CPString)aKeyPath ofObject:(id)anObject change:(CPDictionary)changes context:(id)aContext
 {
     CPLog(@"here: "+aKeyPath+" value: "+[anObject valueForKey:aKeyPath]);
@@ -202,6 +225,13 @@
     id cheese;
 }
 
++ (id)testerWithCheese:(id)aCheese
+{
+    var tester = [[self alloc] init];
+    [tester setCheese:aCheese];
+    return tester;
+}
+
 - (void)setCheese:(id)aCheese
 {
     cheese = aCheese;