單例模式是一種常用的軟件設(shè)計模式。在它的核心結(jié)構(gòu)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(tǒng)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(shù)的控制并節(jié)約系統(tǒng)資源。如果希望在系統(tǒng)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(chuàng)建類方法，返回對象實例.以shared default current開頭。
2）、創(chuàng)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(chuàng)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(xiàn)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(chuàng)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(chuàng)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(chuàng)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(guān)于iOS單例模式的全部內(nèi)容，希望對大家的學(xué)習(xí)有所幫助。
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