Cocoa設計模式之單例

在objective-c中要實現一個單例類，至少需要做以下四個步驟：
1、為單例對象實現一個靜態執行個體，並初始化，然後設定成nil，

static MTNetworkEnvironment *g_instance = nil;

2、實現一個執行個體構造方法檢查上面聲明的靜態執行個體是否為nil，如果是則建立並返回一個本類的執行個體，

+ (MTNetworkEnvironment *)sharedInstance{    @synchronized(self) {        if ( g_instance == nil ) {            g_instance = [[self alloc] init];        }    }    return g_instance;}

3、重寫allocWithZone方法，用來保證其他人直接使用alloc和init試圖獲得一個新實力的時候不產生一個新執行個體，

+ (id)allocWithZone:(NSZone *)zone{    @synchronized(self){        if (g_instance == nil) {            g_instance = [super allocWithZone:zone];            return  g_instance;        }    }    return nil;}

這裡的鎖適用於防止多線程，保證該操作只在一個線程中執行。

4、適當實現allocWitheZone，copyWithZone，release和autorelease。

標頭檔：

/** * @file         MTNetworkEnvironment.h  * @brief       MoneyTree network environment * @author      philiphu@mintcode.com * @date          2012-07-14 * @version      1.0.0  */#import <Foundation/Foundation.h>#import "Reachability.h"/** * @brief           create and manage network enviroment */@interface MTNetworkEnvironment : NSObject/** * @brief           get the signalton engine object * @return          the engine object  */+ (MTNetworkEnvironment *)sharedInstance;/** * @brief           get the network statue  */- (BOOL)isNetworkReachable;/** * @brief           Judgment wifi is connected */- (BOOL)isEnableWIFI;/** * @brief           To judge whether the 3G connection */- (BOOL)isEnable3G;@end

實現檔案：

/** * @file         MTNetworkEnvironment.m  * @brief       MoneyTree network environment * @author      philiphu@mintcode.com * @date          2012-07-14 * @version      1.0.0  */#import "MTNetworkEnvironment.h"@interface MTNetworkEnvironment(Private)@end@implementation MTNetworkEnvironmentstatic MTNetworkEnvironment *g_instance = nil;+ (id)allocWithZone:(NSZone *)zone{    @synchronized(self){        if (g_instance == nil) {            g_instance = [super allocWithZone:zone];            return  g_instance;        }    }    return nil;}- (id)init{    self = [super init];    if (self) {            }    return self;}/** * @brief           get the signalton engine object * @return          the engine object  */+ (MTNetworkEnvironment *)sharedInstance{    @synchronized(self) {        if ( g_instance == nil ) {            g_instance = [[self alloc] init];        }    }    return g_instance;}/** * @brief           get the network statue  */- (BOOL)isNetworkReachable{    BOOL isReachable = NO;    Reachability *reachability = [Reachability reachabilityWithHostName:@"www.baidu.com"];    switch ([reachability currentReachabilityStatus]) {        case NotReachable:{            isReachable = NO;        }            break;        case ReachableViaWWAN:{            isReachable = YES;        }            break;        case ReachableViaWiFi:{            isReachable = YES;           }            break;        default:            isReachable = NO;            break;    }    return isReachable;}/** * @brief           Judgment wifi is connected */- (BOOL)isEnableWIFI{     return ([[Reachability reachabilityForLocalWiFi] currentReachabilityStatus] != NotReachable);}/** * @brief           To judge whether the 3G connection */- (BOOL)isEnable3G{    return ([[Reachability reachabilityForInternetConnection] currentReachabilityStatus] != NotReachable);}@end