如果你的目标系统是iOS 4.0或以上
使用GCD，它是在Objective-C(线程安全)中创建单例的最好方法吗?
+ (instancetype)sharedInstance
{
static dispatch_once_t once;
static id sharedInstance;
dispatch_once(&once, ^{
sharedInstance = [[self alloc] init];
});
return sharedInstance;
}
当前回答
@interface className : NSObject{
+(className*)SingleTonShare;
}
@implementation className
+(className*)SingleTonShare{
static className* sharedObj = nil;
static dispatch_once_t once = 0;
dispatch_once(&once, ^{
if (sharedObj == nil){
sharedObj = [[className alloc] init];
}
});
return sharedObj;
}
其他回答
要创建线程安全单例，你可以这样做:
@interface SomeManager : NSObject
+ (id)sharedManager;
@end
/* thread safe */
@implementation SomeManager
static id sharedManager = nil;
+ (void)initialize {
if (self == [SomeManager class]) {
sharedManager = [[self alloc] init];
}
}
+ (id)sharedManager {
return sharedManager;
}
@end
这个博客很好地解释了objc/cocoa中的单例
@interface className : NSObject{
+(className*)SingleTonShare;
}
@implementation className
+(className*)SingleTonShare{
static className* sharedObj = nil;
static dispatch_once_t once = 0;
dispatch_once(&once, ^{
if (sharedObj == nil){
sharedObj = [[className alloc] init];
}
});
return sharedObj;
}
戴夫说得对，这完全没问题。如果类选择不使用sharedFoo方法，你可能想要查看Apple文档中关于创建单例的提示，以确保只有一个方法可以被创建。
如果你想确保[[MyClass alloc] init]返回与sharedInstance相同的对象(在我看来没有必要，但有些人想要它)，可以使用第二个dispatch_once非常容易和安全地完成:
- (instancetype)init
{
static dispatch_once_t once;
static Class *sharedInstance;
dispatch_once(&once, ^
{
// Your normal init code goes here.
sharedInstance = self;
});
return sharedInstance;
}
This allows any combination of [[MyClass alloc] init] and [MyClass sharedInstance] to return the same object; [MyClass sharedInstance] would just be a bit more efficient. How it works: [MyClass sharedInstance] will call [[MyClass alloc] init] once. Other code could call it as well, any number of times. The first caller to init will do the "normal" initialisation and store the singleton object away in the init method. Any later calls to init will completely ignore what alloc returned and return the same sharedInstance; the result of alloc will be deallocated.
+sharedInstance方法将像往常一样工作。如果它不是第一个调用[[MyClass alloc] init]的调用者，那么init的结果不是alloc调用的结果，但这是可以的。
可以避免通过覆盖alloc方法来分配类。
@implementation MyClass
static BOOL useinside = NO;
static id _sharedObject = nil;
+(id) alloc {
if (!useinside) {
@throw [NSException exceptionWithName:@"Singleton Vialotaion" reason:@"You are violating the singleton class usage. Please call +sharedInstance method" userInfo:nil];
}
else {
return [super alloc];
}
}
+(id)sharedInstance
{
static dispatch_once_t p = 0;
dispatch_once(&p, ^{
useinside = YES;
_sharedObject = [[MyClass alloc] init];
useinside = NO;
});
// returns the same object each time
return _sharedObject;
}
