單例模式是一種常用的軟件設計模式。在它的核心結構中只包含一個被稱為單例的特殊類。通過單例模式可以保證系統中一個類只有一個實例而且該實例易于外界訪問，從而方便對實例個數的控制并節約系統資源。如果希望在系統中某個類的對象只能存在一個，單例模式是最好的解決方案。
1、書寫步驟
1）、創建類方法，返回對象實例.以shared default current開頭。
2）、創建一個全局變量用來保存對象的引用
3）、判斷對象是否存在，若不存在，創建對象
2、具體單例模式的幾種模式
第一種單例模式
//非線程安全寫法static UserHelper * helper = nil;+ (UserHelper *)sharedUserHelper {if (helper == nil) {helper = [[UserHelper alloc] init];} return helper;}第二種單例模式
//線程安全寫法1static UserHelper * helper = nil;+ (UserHelper *)sharedUserHelper { @synchronized(self) { if (helper == nil) { helper = [[UserHelper alloc] init]; } } return helper;}第三種單例模式
+ (void)initialize { if ([self class] == [UserHelper class]) { helper = [[UserHelper alloc] init]; }}第四種單例模式
//線程安全寫法3（蘋果推薦，主要用這個）static UserHelper * helper = nil;+ (UserHelper *)sharedUserHelper { static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ helper = [[UserHelper alloc] init]; }); return helper;}MRC全面實現單例寫法（了解）
#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(@"請輸入用戶名"); scanf("%s",name); NSLog(@"請輸入密碼"); 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、創建類方法，返回對象實例 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、創建一個全局變量 #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、判斷對象是否存在，若不存在，創建對象 //線程安全// @synchronized(self) {// // if (helper == nil) {// helper = [[UserHelper alloc] init];// }// } //gcd 線程安全 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以上就是關于iOS單例模式的全部內容，希望對大家的學習有所幫助。
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