LabelObserver is no longer retained by the label

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

 // 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.
 
 #import "ios/chrome/browser/ui/util/label_observer.h"
 
 #import <objc/runtime.h>
 
 #import "base/ios/weak_nsobject.h"
 #import "base/mac/scoped_block.h"
@@ skipping 20 matching lines @@
   base::scoped_nsobject<NSMutableArray> _styleActions;
   base::scoped_nsobject<NSMutableArray> _layoutActions;
   base::scoped_nsobject<NSMutableArray> _textActions;
 }
 
 // Whether or not observer actions are currently being executed.  This is used
 // to prevent infinite loops caused by a LinkObserverAction updating a
 // property on |_label|.
 @property(nonatomic, assign, getter=isRespondingToKVO) BOOL respondingToKVO;
 
+// The number of times this observer has been asked to observe the label. When
+// reaching zero, the label stops being observed.
+@property(nonatomic, assign) NSInteger observingCount;
+
 // Initializes a LabelObserver that observes |label|.
 - (instancetype)initWithLabel:(UILabel*)label NS_DESIGNATED_INITIALIZER;
 
+// Adds |self| as observer for the |_label|.
+- (void)registerAsObserver;
+
+// Removes |self| as observer for the |_label|.
+- (void)removeObserver;
+
 // Performs all LabelObserverActions in |actions|.
 - (void)performActions:(NSArray*)actions;
 
 // Takes |_label|'s values for each key from |styleKeys| and uses them to
 // construct a uniformly attributed value to use for |_label|'s attributedText.
 - (void)resetLabelAttributes;
 
 @end
 
 // Properties of UILabel that, when changed, will cause the label's attributed
 // text to change.
 static NSSet* styleKeys;
 // Properties of UILabel that invalidate the layout of the label if they change.
 static NSSet* layoutKeys;
 // Properties of UILabel that may invalidate the text of the label if they
 // change.
 static NSSet* textKeys;
 
 @implementation LabelObserver
 
 @synthesize respondingToKVO = _respondingToKVO;
+@synthesize observingCount = _observingCount;
 
 + (void)initialize {
   if (self == [LabelObserver class]) {
     styleKeys = [[NSSet alloc] initWithArray:@[
       @"font", @"textColor", @"textAlignment", @"lineBreakMode", @"shadowColor",
       @"shadowOffset"
     ]];
     layoutKeys = [[NSSet alloc]
         initWithArray:@[ @"bounds", @"frame", @"superview", @"center" ]];
     textKeys = [[NSSet alloc] initWithArray:@[ @"text", @"attributedText" ]];
   }
 }
 
 - (instancetype)initWithLabel:(UILabel*)label {
   if ((self = [super init])) {
     DCHECK(label);
     _label.reset(label);
-    for (NSSet* keySet in @[ styleKeys, layoutKeys, textKeys ]) {
-      for (NSString* key in keySet) {
-        [_label addObserver:self
-                 forKeyPath:key
-                    options:NSKeyValueObservingOptionNew
-                    context:nullptr];
-      }
-    }
     [self resetLabelAttributes];
   }
   return self;
 }
 
-- (void)dealloc {
-  for (NSSet* keySet in @[ styleKeys, layoutKeys, textKeys ]) {
-    for (NSString* key in keySet) {
-      [_label removeObserver:self forKeyPath:key];
-    }
-  }
-  [super dealloc];
-}
 
 #pragma mark - Public interface
 
 + (instancetype)observerForLabel:(UILabel*)label {
   if (!label)
     return nil;
   id observer = objc_getAssociatedObject(label, kLabelObserverKey);
   if (!observer) {
     observer = [[LabelObserver alloc] initWithLabel:label];
     objc_setAssociatedObject(label, kLabelObserverKey, observer,
-                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
-    [observer release];
+                             OBJC_ASSOCIATION_ASSIGN);
+    [observer autorelease];
   }
   return observer;
 }
 
+- (void)startObserving {
+  if (self.observingCount++ == 0) {

[stkhapugin, 2017/04/19 16:39:40]

Please don't put two operations - increment and comparison - in one line. 

Time taken to read this:

if (self.observingCount == 0){
  self.observingCount ++;
  [self registerAsObserver];
}

is about 10 times less than decrypting self.observingCount++ == 0, and I'm not
even sure I've done it correctly. 

[pkl (ping after 24h if needed), 2017/04/19 16:48:03]

Readability aside, do the two implementations do the same thing?

[gambard, 2017/04/20 08:30:18]

No, the increment should be outside the if.
Done.

+    [self registerAsObserver];
+  }
+}
+
+- (void)stopObserving {
+  if (self.observingCount == 0)
+    return;
+
+  if (self.observingCount-- == 1) {
+    [self removeObserver];
+  }
+}
+
 - (void)addStyleChangedAction:(LabelObserverAction)action {
   DCHECK(action);
   if (!_styleActions)
     _styleActions.reset([[NSMutableArray alloc] init]);
   base::mac::ScopedBlock<LabelObserverAction> actionCopy([action copy]);
   [_styleActions addObject:actionCopy];
 }
 
 - (void)addLayoutChangedAction:(LabelObserverAction)action {
   DCHECK(action);
   if (!_layoutActions)
     _layoutActions.reset([[NSMutableArray alloc] init]);
   base::mac::ScopedBlock<LabelObserverAction> actionCopy([action copy]);
   [_layoutActions addObject:actionCopy];
 }
 
 - (void)addTextChangedAction:(LabelObserverAction)action {
   DCHECK(action);
   if (!_textActions)
     _textActions.reset([[NSMutableArray alloc] init]);
   base::mac::ScopedBlock<LabelObserverAction> actionCopy([action copy]);
   [_textActions addObject:actionCopy];
 }
 
 #pragma mark -
 
+- (void)registerAsObserver {
+  for (NSSet* keySet in @[ styleKeys, layoutKeys, textKeys ]) {
+    for (NSString* key in keySet) {
+      [_label addObserver:self
+               forKeyPath:key
+                  options:NSKeyValueObservingOptionNew
+                  context:nullptr];
+    }
+  }
+}
+
+- (void)removeObserver {
+  for (NSSet* keySet in @[ styleKeys, layoutKeys, textKeys ]) {
+    for (NSString* key in keySet) {
+      [_label removeObserver:self forKeyPath:key];
+    }
+  };
+}
+
 - (void)performActions:(NSArray*)actions {
   for (LabelObserverAction action in actions)
     action(_label);
 }
 
 - (void)resetLabelAttributes {
   if ([_label attributedText] || ![_label text])
     return;
   NSMutableDictionary* labelStyle =
       [NSMutableDictionary dictionaryWithCapacity:styleKeys.count];
@@ skipping 25 matching lines @@
       [self resetLabelAttributes];
     [self performActions:_textActions];
   } else {
     NOTREACHED() << "Unexpected label key <" << base::SysNSStringToUTF8(key)
                  << "> observed";
   }
   self.respondingToKVO = NO;
 }
 
 @end