LabelObserver is no longer retained by the label

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.
+// necessary. LabelObservers are associated with label but must be kept alive by
+// the caller. |-startObserving| must be called before the |label| is observed.
 + (instancetype)observerForLabel:(UILabel*)label;

[marq (ping after 24h), 2017/04/19 15:25:19]

Is this method still needed? It seems like any caller would also have access to
a direct pointer to the observer already, so there should just be an
-initWithLabel: instead.

[gambard, 2017/04/19 15:48:18]

No, there is one use case in CRUILabel+AttributeUtils.h

 
 // LabelObservers should be created via |+observerForLabel:|.
 - (instancetype)init NS_UNAVAILABLE;
 
+// Starts observing the label. For each call to this function, |-stopObserving|
+// should be called before |label| is deallocated.
+- (void)startObserving;
+
+// Stops observing the label. The label stop being observed once the number of
+// call to this function match the number of call to |-startObserving|.
+- (void)stopObserving;
+
 // 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_