谈一谈iOS单例模式

//非线程安全写法static UserHelper * helper = nil;+ (UserHelper *)sharedUserHelper {if (helper == nil) {helper = [[UserHelper alloc] init];} return helper;}

//线程安全写法1static UserHelper * helper = nil;+ (UserHelper *)sharedUserHelper { @synchronized(self) {    if (helper == nil) {   helper = [[UserHelper alloc] init];  } } return helper;}

+ (void)initialize {  if ([self class] == [UserHelper class]) {  helper = [[UserHelper alloc] init]; }}

//线程安全写法3（苹果推荐，主要用这个）static UserHelper * helper = nil;+ (UserHelper *)sharedUserHelper { static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{  helper = [[UserHelper alloc] init]; });  return helper;}

#import <Foundation/Foundation.h>#import "UserHelper.h" void func() {  static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ NSLog(@"haha"); });} int main(int argc, const char * argv[]) { @autoreleasepool { // [UserHelper logout];  if ([UserHelper isLogin]) {    UserHelper * helper = [UserHelper sharedUserHelper];  NSLog(@"username = %@ password = %@",helper.userName,helper.password);   } else {    char name[20];  char pwd[20];  NSLog(@"请输入用户名");  scanf("%s",name);  NSLog(@"请输入密码");  scanf("%s",pwd);    NSString * userName = [[NSString alloc] initWithUTF8String:name];  NSString * password = [[NSString alloc] initWithUTF8String:pwd];    if (userName && password) {    [UserHelper loginWithUserName:userName password:password];    UserHelper * helper = [UserHelper sharedUserHelper];  NSLog(@"username = %@ password = %@",helper.userName,helper.password);    } } // UserHelper * help1 = [UserHelper sharedUserHelper];// help1.userName = @"dahuan";// help1.password = @"123456";// NSLog(@"%p",help1);// NSLog(@"%@",help1.userName);// NSLog(@"%@",help1.password);//// // UserHelper * help2 = [UserHelper sharedUserHelper];// help2.password = @"zxc";// NSLog(@"%p",help2);// NSLog(@"%@",help1.userName);// NSLog(@"%@",help1.password);  } return 0;} //class.h#import <Foundation/Foundation.h> @interface UserHelper : NSObject //1、创建类方法，返回对象实例 shared default current + (UserHelper *)sharedUserHelper; @property (nonatomic, copy) NSString * userName; @property (nonatomic, copy) NSString * password; + (BOOL)isLogin; + (void)loginWithUserName:(NSString *)userName password:(NSString *)password; + (void)logout; @end // class.m#import "UserHelper.h" //2、创建一个全局变量 #define Path @"/Users/dahuan/Desktop/data" static UserHelper * helper = nil; @implementation UserHelper //+ (void)initialize {// // if ([self class] == [UserHelper class]) {// helper = [[UserHelper alloc] init];// }//} + (UserHelper *)sharedUserHelper {  //3、判断对象是否存在，若不存在，创建对象 //线程安全// @synchronized(self) {// // if (helper == nil) {//  helper = [[UserHelper alloc] init];// }// }  //gcd 线程安全 static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ helper = [[UserHelper alloc] init]; });  return helper;} - (instancetype)init {  if (self = [super init]) {  NSString * data = [NSString stringWithContentsOfFile:Path encoding:NSUTF8StringEncoding error:nil]; if (data) {  NSArray * array = [data componentsSeparatedByString:@"-"];  _userName = array[0];  _password = array[1]; } } return self;} + (BOOL)isLogin {  UserHelper * helper = [UserHelper sharedUserHelper]; if (helper.userName && helper.password) { return YES; } return NO;} + (void)loginWithUserName:(NSString *)userName password:(NSString *)password {  UserHelper * helper = [UserHelper sharedUserHelper]; helper.userName = userName; helper.password = password;  NSArray * array = @[userName,password]; NSString * data = [array componentsJoinedByString:@"-"]; [data writeToFile:Path atomically:YES encoding:NSUTF8StringEncoding error:nil];} + (void)logout {  NSFileManager * fm = [NSFileManager defaultManager];  if ([fm fileExistsAtPath:Path]) { [fm removeItemAtPath:Path error:nil]; }} @end