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
你不想在self上同步…因为self对象还不存在!最终锁定一个临时id值。你想要确保没有其他人可以运行类方法(sharedInstance, alloc, allocWithZone:等),所以你需要在类对象上同步:
@implementation MYSingleton
static MYSingleton * sharedInstance = nil;
+( id )sharedInstance {
@synchronized( [ MYSingleton class ] ) {
if( sharedInstance == nil )
sharedInstance = [ [ MYSingleton alloc ] init ];
}
return sharedInstance;
}
+( id )allocWithZone:( NSZone * )zone {
@synchronized( [ MYSingleton class ] ) {
if( sharedInstance == nil )
sharedInstance = [ super allocWithZone:zone ];
}
return sharedInstance;
}
-( id )init {
@synchronized( [ MYSingleton class ] ) {
self = [ super init ];
if( self != nil ) {
// Insert initialization code here
}
return self;
}
}
@end
其他回答
另一个选项是使用+(void)初始化方法。从文档中可以看到:
运行时将initialize发送给程序中的每个类一次,恰好在该类或从该类继承的任何类从程序中发送其第一条消息之前。(因此,如果类没有被使用,方法可能永远不会被调用。)运行时以线程安全的方式将初始化消息发送给类。超类在子类之前收到此消息。
所以你可以这样做:
static MySingleton *sharedSingleton;
+ (void)initialize
{
static BOOL initialized = NO;
if(!initialized)
{
initialized = YES;
sharedSingleton = [[MySingleton alloc] init];
}
}
这是我放在一起的一个宏:
http://github.com/cjhanson/Objective-C-Optimized-Singleton
它基于马特·加拉格尔的作品 但是将实现改为使用谷歌的Dave MacLachlan所描述的方法swizzling。
我欢迎评论/贡献。
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;
}
这也适用于非垃圾收集环境。
@interface MySingleton : NSObject {
}
+(MySingleton *)sharedManager;
@end
@implementation MySingleton
static MySingleton *sharedMySingleton = nil;
+(MySingleton*)sharedManager {
@synchronized(self) {
if (sharedMySingleton == nil) {
[[self alloc] init]; // assignment not done here
}
}
return sharedMySingleton;
}
+(id)allocWithZone:(NSZone *)zone {
@synchronized(self) {
if (sharedMySingleton == nil) {
sharedMySingleton = [super allocWithZone:zone];
return sharedMySingleton; // assignment and return on first allocation
}
}
return nil; //on subsequent allocation attempts return nil
}
-(void)dealloc {
[super dealloc];
}
-(id)copyWithZone:(NSZone *)zone {
return self;
}
-(id)retain {
return self;
}
-(unsigned)retainCount {
return UINT_MAX; //denotes an object that cannot be release
}
-(void)release {
//do nothing
}
-(id)autorelease {
return self;
}
-(id)init {
self = [super init];
sharedMySingleton = self;
//initialize here
return self;
}
@end
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;
}