我的单例访问方法通常是一些变种:
static MyClass *gInstance = NULL;
+ (MyClass *)instance
{
@synchronized(self)
{
if (gInstance == NULL)
gInstance = [[self alloc] init];
}
return(gInstance);
}
我能做些什么来改善这一点呢?
我的单例访问方法通常是一些变种:
static MyClass *gInstance = NULL;
+ (MyClass *)instance
{
@synchronized(self)
{
if (gInstance == NULL)
gInstance = [[self alloc] init];
}
return(gInstance);
}
我能做些什么来改善这一点呢?
当前回答
我只是想把这个放在这里,这样我就不会弄丢了。这个的优点是它可以在InterfaceBuilder中使用,这是一个巨大的优势。这是我问的另一个问题:
static Server *instance;
+ (Server *)instance { return instance; }
+ (id)hiddenAlloc
{
return [super alloc];
}
+ (id)alloc
{
return [[self instance] retain];
}
+ (void)initialize
{
static BOOL initialized = NO;
if(!initialized)
{
initialized = YES;
instance = [[Server hiddenAlloc] init];
}
}
- (id) init
{
if (instance)
return self;
self = [super init];
if (self != nil) {
// whatever
}
return self;
}
其他回答
我只是想把这个放在这里,这样我就不会弄丢了。这个的优点是它可以在InterfaceBuilder中使用,这是一个巨大的优势。这是我问的另一个问题:
static Server *instance;
+ (Server *)instance { return instance; }
+ (id)hiddenAlloc
{
return [super alloc];
}
+ (id)alloc
{
return [[self instance] retain];
}
+ (void)initialize
{
static BOOL initialized = NO;
if(!initialized)
{
initialized = YES;
instance = [[Server hiddenAlloc] init];
}
}
- (id) init
{
if (instance)
return self;
self = [super init];
if (self != nil) {
// whatever
}
return self;
}
我知道有很多关于这个“问题”的评论,但是我没有看到很多人建议使用宏来定义单例对象。这是一个非常常见的模式,一个宏极大地简化了单例。
下面是我根据我见过的几个Objc实现编写的宏。
Singeton。h
/**
@abstract Helps define the interface of a singleton.
@param TYPE The type of this singleton.
@param NAME The name of the singleton accessor. Must match the name used in the implementation.
@discussion
Typcially the NAME is something like 'sharedThing' where 'Thing' is the prefix-removed type name of the class.
*/
#define SingletonInterface(TYPE, NAME) \
+ (TYPE *)NAME;
/**
@abstract Helps define the implementation of a singleton.
@param TYPE The type of this singleton.
@param NAME The name of the singleton accessor. Must match the name used in the interface.
@discussion
Typcially the NAME is something like 'sharedThing' where 'Thing' is the prefix-removed type name of the class.
*/
#define SingletonImplementation(TYPE, NAME) \
static TYPE *__ ## NAME; \
\
\
+ (void)initialize \
{ \
static BOOL initialized = NO; \
if(!initialized) \
{ \
initialized = YES; \
__ ## NAME = [[TYPE alloc] init]; \
} \
} \
\
\
+ (TYPE *)NAME \
{ \
return __ ## NAME; \
}
使用示例:
我的经理
@interface MyManager
SingletonInterface(MyManager, sharedManager);
// ...
@end
我的经理
@implementation MyManager
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
SingletonImplementation(MyManager, sharedManager);
// ...
@end
为什么界面宏几乎是空的?头文件和代码文件之间的代码一致性;可维护性,以防您想添加更多的自动方法或更改它。
我使用initialize方法创建单例,就像这里(在撰写本文时)最流行的答案中使用的那样。
这不应该是线程安全的,避免第一次调用后昂贵的锁定吗?
+ (MySingleton*)sharedInstance
{
if (sharedInstance == nil) {
@synchronized(self) {
if (sharedInstance == nil) {
sharedInstance = [[MySingleton alloc] init];
}
}
}
return (MySingleton *)sharedInstance;
}
我的方法很简单:
static id instanceOfXXX = nil;
+ (id) sharedXXX
{
static volatile BOOL initialized = NO;
if (!initialized)
{
@synchronized([XXX class])
{
if (!initialized)
{
instanceOfXXX = [[XXX alloc] init];
initialized = YES;
}
}
}
return instanceOfXXX;
}
如果单例已经初始化,LOCK块将不会被输入。第二次检查if(!initialized)是为了确保当前线程获得LOCK时它还没有初始化。
我在sharedInstance上有一个有趣的变化,它是线程安全的,但在初始化后不锁定。我还不确定是否可以按照要求修改顶部的答案,但我将其提出以供进一步讨论:
// Volatile to make sure we are not foiled by CPU caches
static volatile ALBackendRequestManager *sharedInstance;
// There's no need to call this directly, as method swizzling in sharedInstance
// means this will get called after the singleton is initialized.
+ (MySingleton *)simpleSharedInstance
{
return (MySingleton *)sharedInstance;
}
+ (MySingleton*)sharedInstance
{
@synchronized(self)
{
if (sharedInstance == nil)
{
sharedInstance = [[MySingleton alloc] init];
// Replace expensive thread-safe method
// with the simpler one that just returns the allocated instance.
SEL origSel = @selector(sharedInstance);
SEL newSel = @selector(simpleSharedInstance);
Method origMethod = class_getClassMethod(self, origSel);
Method newMethod = class_getClassMethod(self, newSel);
method_exchangeImplementations(origMethod, newMethod);
}
}
return (MySingleton *)sharedInstance;
}