Upstream Chrome on iOS source code [9/11].

ios/chrome/browser/ui/util/label_observer.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IOS_CHROME_BROWSER_UI_UTIL_LABEL_OBSERVER_H_
+#define IOS_CHROME_BROWSER_UI_UTIL_LABEL_OBSERVER_H_
+
+#import <UIKit/UIKit.h>
+
+// Class that observes changes to UILabel properties via KVO.  This allows
+// various classes that manage a UILabel's style via NSAttributedStrings to
+// reapply their styling in response to property changes that would invalidate
+// the attributed text.  This class also synchronizes notifications of style
+// invalidation so that property changes that occur as the result of a
+// LabelObserverAction do not trigger other actions.
+@interface LabelObserver : NSObject
+
+// Returns the LabelObserver for |label|, laziliy instantiating one if
+// necessary.  LabelObservers are associated with the label, and will be
+// deallocated upon |label|'s deallocation.
++ (instancetype)observerForLabel:(UILabel*)label;
+
+// LabelObservers should be created via |+observerForLabel:|.
+- (instancetype)init NS_UNAVAILABLE;
+
+// Block type that takes a label.  Blocks registered for a label will be called
+// when property values are updated.
+typedef void (^LabelObserverAction)(UILabel* label);
+
+// Registers |action| to be called when stylistic properties on the observed
+// label are changed.  Style changes include changes to the label's font,
+// textColor, textAlignment, lineBreakMode, shadowColor, or shadowOffset.
+- (void)addStyleChangedAction:(LabelObserverAction)action;
+
+// Registers |action| to be called when the observed label's layout has changed.
+// Layout changes include changes to the label's bounds, frame, or superview, as
+// well as changes to its center, which doesn't affect the label's layout
+// internally but does affect its position in its superview.
+- (void)addLayoutChangedAction:(LabelObserverAction)action;
+
+// Registers |action| to be called when the observed label's text has changed.
+// Text changes include changes to the label's text or attributedText.
+- (void)addTextChangedAction:(LabelObserverAction)action;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_UI_UTIL_LABEL_OBSERVER_H_