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