IOS development - multithreading - thread safety (3)

IOS Development of —— Multithreading - Thread safety (3)

One Multithreading resource sharing

• 1 Block resources may be shared by multiple threads , That is to say Multiple threads may access the same resource
• For example, multiple threads access the same object 、 The same variable 、 The same file
• When multiple threads access the same block of resources , It's easy to trigger Data disorder and data security problem

Two Examples of safety hazards

2.1 Example 1 saving and withdrawing money

2.2 Selling tickets

3、 ... and Multi thread ticket selling

3.1 explain

• After the program starts , Turn on 1,2,3 Three threads sell tickets
• Three threads are sold together 100 Tickets

3.2 Code

#import "ViewController.h"

@interface ViewController ()
@property(nonatomic,strong) NSThread *thread1;
@property(nonatomic,strong) NSThread *thread2;
@property(nonatomic,strong) NSThread *thread3;
@property(nonatomic,assign) int leftTicketCount;// The number of votes left 

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.leftTicketCount=100;
    
    self.thread1=[[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil];
    [email protected]"1 Window No ";
    
    self.thread2=[[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil];
    [email protected]"2 Window No ";
    
    self.thread3=[[NSThread alloc]initWithTarget:self selector:@selector(saleTicket) object:nil];
    [email protected]"3 Window No ";
    
    self.locker=[[NSObject alloc]init];
    
}
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    [self.thread1 start];
    [self.thread2 start];
    [self.thread3 start];
    
}
// Selling tickets 
-(void)saleTicket
{
    while (1) {
            int count=self.leftTicketCount;
            if (count>0) {
                [NSThread sleepForTimeInterval:0.1];
                self.leftTicketCount=count-1;
                NSLog(@"%@ Sold a ticket , The remaining %d Tickets ",[NSThread currentThread].name,count);
            }else{
                return;// Get out of the loop 
            }
    }
}
@end

3.3 The phenomenon

2022-02-20 23:10:18.593201+0800 05- Thread safety problem [45790:535673] 3 Window number sold a ticket , The remaining 2 Tickets 
2022-02-20 23:10:18.593201+0800 05- Thread safety problem [45790:535672] 2 Window number sold a ticket , The remaining 2 Tickets 
2022-02-20 23:10:18.593201+0800 05- Thread safety problem [45790:535671] 1 Window number sold a ticket , The remaining 2 Tickets 
2022-02-20 23:10:18.696806+0800 05- Thread safety problem [45790:535671] 1 Window number sold a ticket , The remaining 1 Tickets 
2022-02-20 23:10:18.696809+0800 05- Thread safety problem [45790:535672] 2 Window number sold a ticket , The remaining 1 Tickets 
2022-02-20 23:10:18.696806+0800 05- Thread safety problem [45790:535673] 3 Window number sold a ticket , The remaining 1 Tickets 

3.4 Safety hazard analysis

3.5 Security risks are solved ( The mutex )

Four The mutex

4.1 Mutex usage format

@synchronized( Lock object ){// Code that needs to be locked 
 Be careful ： lock 1 Code only 1 The lock , It is invalid to use more than one lock 
}

4.2 Advantages and disadvantages of mutex

• advantage ： It can effectively prevent data security problems caused by multithreading
• shortcoming ： It takes a lot of CPU resources

4.3 The premise of using mutex

Multiple threads scramble for the same resource

4.4 Related technical terms ： Thread synchronization

• Thread synchronization means ： Multiple threads execute on the same line ( Perform tasks in sequence )
• The mutex ： Is the use of thread synchronization technology

4.5 Atomic and non atomic properties

• OC When defining properties, there are nonatomic and atomic Two options
• Atomic： Thread safety , It takes a lot of resources
• Nonatomic： Non-thread safety , Suitable for small memory mobile devices

3.5 iOS Suggestions for development

• All properties are declared as nonatomic
• Try to avoid multiple threads seizing the same resource
• Try to lock 、 The business logic of resource grabbing is handed over to the server for processing , Reduce the pressure on mobile clients

5、 ... and Code after locking

-(void)saleTicket
{
    while (1) {
        @synchronized(self) {
            int count=self.leftTicketCount;
            if (count>0) {
                [NSThread sleepForTimeInterval:0.1];
                self.leftTicketCount=count-1;
                NSLog(@"%@ Sold a ticket , The remaining %d Tickets ",[NSThread currentThread].name,count);
            }else{
                return;// Get out of the loop 
            }
        }
    }
}