IOS單例模式

1,定義一個靜態全域的變數static Settings *sharedSettings = nil;2，建立一個類方法，用來返回該類執行個體+ (Settings *)sharedInstance {    @synchronized(self){if(sharedSettings  ==  nil){            sharedSettings = [[self alloc] init];                      //做一些初始化操作        }    }return  sharedSettings;}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 (sharedSettings == nil) {            sharedSettings = [super allocWithZone:zone];        }    }    return sharedSettings;}4、適當實現allocWitheZone，copyWithZone，release和autorelease+ (id)copyWithZone:(NSZone *)zone {    return self;}- (id)retain {    return self;}- (unsigned)retainCount {    return UINT_MAX;}- (oneway void)release {}- (id)autorelease {    return self;}