单例类是一种特殊的类，在操作系统中的一个进程内只会存在一个该类的对象，在iOS应用中同样也只会出现一个对象。这种设计模式在Apple系统框架中许多地方也都使用了，如NSFileManager、UIApplication
- 在ARC的环境下，接口文件为：
//
// DVISingleton.h
//
// Copyright (c) 2014 长沙戴维营教育. All rights reserved.
//
#import <Foundation/Foundation.h>
@interface DVISingleton : NSObject
+ (instancetype)sharedSingleton;
@end
- 在ARC的环境下，实现文件:
//
// DVISingleton.h
//
// Copyright (c) 2014 长沙戴维营教育. All rights reserved.
//
#import <Foundation/Foundation.h>
@interface DVISingleton : NSObject
+ (instancetype)sharedSingleton;
@end
实现文件：
//
// DVISingleton.m
//
// Copyright (c) 2014 长沙戴维营教育. All rights reserved.
//
#import "DVISingleton.h"
@implementation DVISingleton
+ (instancetype)sharedSingleton
{
static DVISingleton *sharedObject = nil;
//线程安全，只允许执行依次
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
//使用父类的allocWithZone:方法创建对象
sharedObject = [[super allocWithZone:NULL] init];
});
return sharedObject;
}
- (id)init
{
if (self = [super init]) {
}
return self;
}
+ (id)allocWithZone:(struct _NSZone *)zone
{
return [self sharedSingleton];
}
- (id)copy
{
return self;
}
- (void)dealloc
{
}
@end
- 在非ARC环境下的实现文件：
#import "DVISingleton.h"
@implementation DVISingleton
+ (instancetype)sharedSingleton
{
static DVISingleton *sharedObject = nil;
//线程安全，只允许执行依次
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
//使用父类的allocWithZone:方法创建对象
sharedObject = [[super allocWithZone:NULL] init];
});
return sharedObject;
}
+ (id)allocWithZone:(NSZone *)zone {
return [[self sharedSingleton] retain];
}
- (id)copyWithZone:(NSZone *)zone {
return self;
}
- (id)retain {
return self;
}
- (unsigned)retainCount {
return UINT_MAX; //denotes an object that cannot be released
}
- (oneway void)release {
// never release
}
- (id)autorelease {
return self;
}
- (id)init {
if (self = [super init]) {
}
return self;
}
- (void)dealloc {
[super dealloc];
}
@end
