[Author: grobbins]

Modernize GTMObjectSingleton: Use dispatch_once, make ARC compatible, remove non-allocation NSObject methods, 
remove dependence on GTMDevAssert, add link to Chris Hanson's discussion of singletons, add comment discouraging developers from using this header file.

R=thomasvl
APPROVED=thomasvl
DELTA=75  (24 added, 39 deleted, 12 changed)

1 files changed, 36 insertions, 51 deletions

diff --git a/Foundation/GTMObjectSingleton.h b/Foundation/GTMObjectSingleton.h
index fe1cc75..eeb9ec7 100644
--- a/Foundation/GTMObjectSingleton.h
+++ b/Foundation/GTMObjectSingleton.h
@@ -1,6 +1,6 @@
 //
 //  GTMObjectSingleton.h
-//  Macro to implement methods for a singleton
+//  Macro to implement a creation method for a singleton
 //
 //  Copyright 2005-2008 Google Inc.
 //
@@ -17,56 +17,41 @@
 //  the License.
 //
 
-#import "GTMDefines.h"
-
-/// This macro implements the various methods needed to make a safe singleton.
 //
-/// This Singleton pattern was taken from:
-/// http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/chapter_3_section_10.html
-///
-/// Sample usage:
-///
-/// GTMOBJECT_SINGLETON_BOILERPLATE(SomeUsefulManager, sharedSomeUsefulManager)
-/// (with no trailing semicolon)
-///
+// This file is still around for compatibility with apps relying on its macro,
+// but given how simple this is, there is not a compelling reason for any app to
+// use this macro.
+//
+//  For a reasonable discussion of Objective-C singletons, see
+//  http://eschatologist.net/blog/?p=178
+//
+// Sample usage:
+//
+// GTMOBJECT_SINGLETON_BOILERPLATE(SomeUsefulManager, sharedSomeUsefulManager)
+// (with no trailing semicolon)
+//
+
+#if NS_BLOCKS_AVAILABLE
+
+#define GTMOBJECT_SINGLETON_BOILERPLATE(_object_name_, _shared_obj_name_) \
++ (_object_name_ *)_shared_obj_name_ {     \
+  static _object_name_ *obj;               \
+  static dispatch_once_t onceToken;        \
+  dispatch_once(&onceToken, ^{             \
+     obj = [[self alloc] init];            \
+  });                                      \
+  return obj;                              \
+}
+
+#else
+
 #define GTMOBJECT_SINGLETON_BOILERPLATE(_object_name_, _shared_obj_name_) \
-static _object_name_ *z##_shared_obj_name_ = nil;  \
-+ (_object_name_ *)_shared_obj_name_ {             \
-  @synchronized(self) {                            \
-    if (z##_shared_obj_name_ == nil) {             \
-      /* Note that 'self' may not be the same as _object_name_ */                               \
-      /* first assignment done in allocWithZone but we must reassign in case init fails */      \
-      z##_shared_obj_name_ = [[self alloc] init];                                               \
-      _GTMDevAssert((z##_shared_obj_name_ != nil), @"didn't catch singleton allocation");       \
-    }                                              \
-  }                                                \
-  return z##_shared_obj_name_;                     \
-}                                                  \
-+ (id)allocWithZone:(NSZone *)zone {               \
-  @synchronized(self) {                            \
-    if (z##_shared_obj_name_ == nil) {             \
-      z##_shared_obj_name_ = [super allocWithZone:zone]; \
-      return z##_shared_obj_name_;                 \
-    }                                              \
-  }                                                \
-                                                   \
-  /* We can't return the shared instance, because it's been init'd */ \
-  _GTMDevAssert(NO, @"use the singleton API, not alloc+init");        \
-  return nil;                                      \
-}                                                  \
-- (id)retain {                                     \
-  return self;                                     \
-}                                                  \
-- (NSUInteger)retainCount {                        \
-  return NSUIntegerMax;                            \
-}                                                  \
-- (oneway void)release {                           \
-}                                                  \
-- (id)autorelease {                                \
-  return self;                                     \
-}                                                  \
-- (id)copyWithZone:(NSZone *)zone {                \
-  GTM_UNUSED(zone);                                \
-  return self;                                     \
-}                                                  \
++ (_object_name_ *)_shared_obj_name_ {     \
+  static _object_name_ *obj;               \
+  if (obj == nil) {                        \
+    obj = [[self alloc] init];             \
+  }                                        \
+  return obj;                              \
+}
 
+#endif  // NS_BLOCKS_AVAILABLE