在之前的版本是可以使用UDID獲取iOS設備唯一標識，

NSString *udidString = [[UIDevice currentDevice] uniqueIdentifier];

但是iOS5及以后，被蘋果禁止使用了（棄用了）

而直接獲取的UUID系統不會存儲，每次調用的時候都會獲得一個新的UUID標示符
一般獲取UUID的方法如下

// 普通的獲取UUID的方法
+ (NSString *)getUUID {
    CFUUIDRef puuid = CFUUIDCreate( nil );
    CFStringRef uuidString = CFUUIDCreateString(nil, puuid);
    NSString *result = (NSString *)CFBridgingRelease(CFStringCreateCopy( NULL, uuidString));
    return result;
}

我們可以通過持久存儲這個標識符，來保證即使重新加載，刪除后重裝應用都能夠唯一標識，以下的方式通過獲取到UUID后存入系統中的keychain中，來保證以后每次可以得到相同的唯一標志。
代碼如下：

#import "UUIDTool.h"

// kUUIDKey盡量改成便于區分App的（有簡友說如果一個手機上兩個App都這樣用相同的kUUIDKey可能會存在bug，我測試幾次沒發現，但是還是建議盡量要區分開）
NSString * const kUUIDKey = @"com.myApp.uuid";

@implementation UUIDTool

#pragma mark - 獲取到UUID后存入系統中的keychain中

+ (NSString *)getUUIDInKeychain {
    // 1.直接從keychain中獲取UUID
    NSString *getUDIDInKeychain = (NSString *)[UUIDTool load:kUUIDKey];
    NSLog(@"從keychain中獲取UUID%@", getUDIDInKeychain);
    
    // 2.如果獲取不到，需要生成UUID并存入系統中的keychain
    if (!getUDIDInKeychain || [getUDIDInKeychain isEqualToString:@""] || [getUDIDInKeychain isKindOfClass:[NSNull class]]) {
        // 2.1 生成UUID
        CFUUIDRef puuid = CFUUIDCreate(nil);
        CFStringRef uuidString = CFUUIDCreateString(nil, puuid);
        NSString *result = (NSString *)CFBridgingRelease(CFStringCreateCopy(NULL, uuidString));
        CFRelease(puuid);
        CFRelease(uuidString);
        NSLog(@"生成UUID：%@",result);
        // 2.2 將生成的UUID保存到keychain中
        [UUIDTool save:kUUIDKey data:result];
        // 2.3 從keychain中獲取UUID
        getUDIDInKeychain = (NSString *)[UUIDTool load:kUUIDKey];
    }
    
    return getUDIDInKeychain;
}


#pragma mark - 刪除存儲在keychain中的UUID

+ (void)deleteKeyChain {
    [self delete:kUUIDKey];
}


#pragma mark - 私有方法

+ (NSMutableDictionary *)getKeyChainQuery:(NSString *)service {
    return [NSMutableDictionary dictionaryWithObjectsAndKeys:(id)kSecClassGenericPassword,(id)kSecClass,service,(id)kSecAttrService,service,(id)kSecAttrAccount,(id)kSecAttrAccessibleAfterFirstUnlock,(id)kSecAttrAccessible, nil];
}

// 從keychain中獲取UUID
+ (id)load:(NSString *)service {
    id ret = nil;
    NSMutableDictionary *keychainQuery = [self getKeyChainQuery:service];
    [keychainQuery setObject:(id)kCFBooleanTrue forKey:(id)kSecReturnData];
    [keychainQuery setObject:(id)kSecMatchLimitOne forKey:(id)kSecMatchLimit];
    CFDataRef keyData = NULL;
    if (SecItemCopyMatching((CFDictionaryRef)keychainQuery, (CFTypeRef *)&keyData) == noErr) {
        @try {
            ret = [NSKeyedUnarchiver unarchiveObjectWithData:(__bridge NSData *)keyData];
        }
        @catch (NSException *exception) {
            NSLog(@"Unarchive of %@ failed: %@", service, exception);
        }
        @finally {
            NSLog(@"finally");
        }
    }
    
    if (keyData) {
        CFRelease(keyData);
    }
    NSLog(@"ret = %@", ret);
    return ret;
}

+ (void)delete:(NSString *)service {
    NSMutableDictionary *keychainQuery = [self getKeyChainQuery:service];
    SecItemDelete((CFDictionaryRef)keychainQuery);
}

// 將生成的UUID保存到keychain中
+ (void)save:(NSString *)service data:(id)data {
    // Get search dictionary
    NSMutableDictionary *keychainQuery = [self getKeyChainQuery:service];
    // Delete old item before add new item
    SecItemDelete((CFDictionaryRef)keychainQuery);
    // Add new object to search dictionary(Attention:the data format)
    [keychainQuery setObject:[NSKeyedArchiver archivedDataWithRootObject:data] forKey:(id)kSecValueData];
    // Add item to keychain with the search dictionary
    SecItemAdd((CFDictionaryRef)keychainQuery, NULL);
}

另外附上GitHub上的Demo鏈接：https://github.com/rambocai/ZYUUID-Emon.git