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
static MyClass *sharedInst = nil; + (id)sharedInstance { @synchronize( self ) { if ( sharedInst == nil ) { /* sharedInst set up in init */ [[self alloc] init]; } } return sharedInst; } - (id)init { if ( sharedInst != nil ) { [NSException raise:NSInternalInconsistencyException format:@"[%@ %@] cannot be called; use +[%@ %@] instead"], NSStringFromClass([self class]), NSStringFromSelector(_cmd), NSStringFromClass([self class]), NSStringFromSelector(@selector(sharedInstance)"]; } else if ( self = [super init] ) { sharedInst = self; /* Whatever class specific here */ } return sharedInst; } /* These probably do nothing in a GC app. Keeps singleton as an actual singleton in a non CG app */ - (NSUInteger)retainCount { return NSUIntegerMax; } - (oneway void)release { } - (id)retain { return sharedInst; } - (id)autorelease { return sharedInst; }
其他回答
@interface MySingleton : NSObject
{
}
+ (MySingleton *)sharedSingleton;
@end
@implementation MySingleton
+ (MySingleton *)sharedSingleton
{
static MySingleton *sharedSingleton;
@synchronized(self)
{
if (!sharedSingleton)
sharedSingleton = [[MySingleton alloc] init];
return sharedSingleton;
}
}
@end
(来源)
KLSingleton是: 可子类化(到n阶) 弧相容 使用alloc和init是安全的 装载 线程安全的 无锁(使用+initialize,而不是@synchronize) Macro-free Swizzle-free 简单的
克林格尔顿
扩展一下@robbie-hanson的例子…
static MySingleton* sharedSingleton = nil;
+ (void)initialize {
static BOOL initialized = NO;
if (!initialized) {
initialized = YES;
sharedSingleton = [[self alloc] init];
}
}
- (id)init {
self = [super init];
if (self) {
// Member initialization here.
}
return self;
}
简单回答:太棒了。
长话短说:类似....
static SomeSingleton *instance = NULL;
@implementation SomeSingleton
+ (id) instance {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
if (instance == NULL){
instance = [[super allocWithZone:NULL] init];
}
});
return instance;
}
+ (id) allocWithZone:(NSZone *)paramZone {
return [[self instance] retain];
}
- (id) copyWithZone:(NSZone *)paramZone {
return self;
}
- (id) autorelease {
return self;
}
- (NSUInteger) retainCount {
return NSUIntegerMax;
}
- (id) retain {
return self;
}
@end
一定要阅读dispatch/once.h头文件以了解发生了什么。在这种情况下,标题注释比文档或手册页更适用。
对于Objective C类方法,我们可以避免以通常的方式使用单例模式,如下所示:
[[Librarian sharedInstance] openLibrary]
to:
[Librarian openLibrary]
通过将类包装到另一个只有类方法的类中,这样就不会意外地创建重复的实例,因为我们没有创建任何实例!
我在这里写了一篇更详细的博客:)