ios开发多线程篇——线程安全

一.多线程的安全隐患
资源共享
1块资源可能会被多个线程共享，也就是多个线程可能会访问同一块资源。
比如多个线程访问同一个对象，同一个变量，同一个文件。
当多个线程访问同一块资源时，很容易引发数据错乱和数据安全问题。
示例一:

示例二:

问题代码:

#import "ViewController.h"@interface ViewController ()//剩余票数@property(nonatomic,assign) int leftTicketsCount;@property(nonatomic,strong) NSThread *thread1;@property(nonatomic,strong) NSThread *thread2;@property(nonatomic,strong) NSThread *thread3;@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    //默认有10张票    self.leftTicketsCount = 10;    self.thread1 = [[NSThread alloc] initWithTarget:self                                           selector:@selector(sellTickets)                                             object:@"售票员A"];    self.thread1.name = @"售票员A";    self.thread2 = [[NSThread alloc] initWithTarget:self                                           selector:@selector(sellTickets)                                             object:@"售票员B"];    self.thread2.name = @"售票员B";    self.thread3 = [[NSThread alloc] initWithTarget:self                                           selector:@selector(sellTickets)                                             object:@"售票员C"];    self.thread3.name = @"售票员C";}- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}- (void)sellTickets{    while (1) {        //先检查票数        int count = self.leftTicketsCount;        if (count > 0) {            //暂停一段时间            [NSThread sleepForTimeInterval:0.02];            //票数－1            self.leftTicketsCount = count - 1;            //获取当前线程            NSThread *current = [NSThread currentThread];            NSLog(@"%@--卖了一张票，还剩余%d张票",current,self.leftTicketsCount);        }        else{            //退出线程            [NSThread exit];        }    }}-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    //开启线程    [self.thread1 start];    [self.thread2 start];    [self.thread3 start];}@end

二.如何解决
互斥锁使用格式
@synchroized(锁对象){//需要锁定的代码}
注意:锁定1份代码只用1把锁，用多把锁是无效的
代码示例:

#import "ViewController.h"@interface ViewController ()//剩余票数@property(nonatomic,assign) int leftTicketsCount;@property(nonatomic,strong) NSThread *thread1;@property(nonatomic,strong) NSThread *thread2;@property(nonatomic,strong) NSThread *thread3;@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    //默认有10张票    self.leftTicketsCount = 10;    self.thread1 = [[NSThread alloc] initWithTarget:self                                           selector:@selector(sellTickets)                                             object:@"售票员A"];    self.thread1.name = @"售票员A";    self.thread2 = [[NSThread alloc] initWithTarget:self                                           selector:@selector(sellTickets)                                             object:@"售票员B"];    self.thread2.name = @"售票员B";    self.thread3 = [[NSThread alloc] initWithTarget:self                                           selector:@selector(sellTickets)                                             object:@"售票员C"];    self.thread3.name = @"售票员C";}- (void)didReceiveMemoryWarning {    [super didReceiveMemoryWarning];    // Dispose of any resources that can be recreated.}- (void)sellTickets{    while (1) {        @synchronized (self) {            //先检查票数            int count = self.leftTicketsCount;            if (count > 0) {                //暂停一段时间                [NSThread sleepForTimeInterval:0.02];                //票数－1                self.leftTicketsCount = count - 1;                //获取当前线程                NSThread *current = [NSThread currentThread];                NSLog(@"%@--卖了一张票，还剩余%d张票",current,self.leftTicketsCount);            }            else{                //退出线程                [NSThread exit];            }        }    }}-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    //开启线程    [self.thread1 start];    [self.thread2 start];    [self.thread3 start];}@end

互斥锁的优缺点
优点：能有效防止因多线程抢夺资源造成的数据安全问题
缺点：需要消耗大量的CPU资源

互斥锁的使用前提：多条线程抢夺同一块资源
相关专业术语：线程同步，多条线程按顺序地执行
互斥锁，就是使用了线程同步技术

四.原子和非原子属性
OC在定义属性时有nonatomic和atomic两种选择
atomic：原子属性，为setter方法加锁(默认就是atomic)
nonatomic：非原子属性，不会为setter方法加锁

atomic加锁原理

@property(assign,atomic) int age;- (void)setAge:(int)age{    @synchroinzed(self){        _age = age;    }}

原子和非原子属性的选择
nonatomic和atomic对比
atomic：线程安全，需要消耗大量的资源。
nonatomic：非线程安全，适合内存小的移动设备

ios开发的建议
所有的属性都声明为nonatomic
尽量避免多线程抢夺同一块资源
尽量将枷锁，资源抢夺的业务逻辑交给服务器端处理