线程同步

我们演示一个经典的卖票的例子来讲NSThread的线程同步：

#import <UIKit/UIKit.h>

@class ViewController;

@interface AppDelegate : UIResponder <UIApplicationDelegate>
{
    int tickets;
    int count;
    NSThread* ticketsThreadone;
    NSThread* ticketsThreadtwo;
    NSCondition* ticketsCondition;
    NSLock *theLock;
}
@property (strong, nonatomic) UIWindow *window;

@property (strong, nonatomic) ViewController *viewController;

@end

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    tickets = 100;
    count = 0;
    theLock = [[NSLock alloc] init];
    // 锁对象
    ticketsCondition = [[NSCondition alloc] init];
    ticketsThreadone = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
    [ticketsThreadone setName:@"Thread-1"];
    [ticketsThreadone start];


    ticketsThreadtwo = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
    [ticketsThreadtwo setName:@"Thread-2"];
    [ticketsThreadtwo start];

    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];
    return YES;
}

- (void)run{
    while (TRUE) {
        // 上锁
//        [ticketsCondition lock];
        [theLock lock];
        if(tickets >= 0){
            [NSThread sleepForTimeInterval:0.09];
            count = 100 - tickets;
            NSLog(@"当前票数是:%d,售出:%d,线程名:%@",tickets,count,[[NSThread currentThread] name]);
            tickets--;
        }else{
            break;
        }
        [theLock unlock];
//        [ticketsCondition unlock];
    }
}

如果没有线程同步的lock，卖票数可能是-1.加上lock之后线程同步保证了数据的正确性。
上面例子我使用了两种锁，一种NSCondition ，一种是：NSLock。 NSCondition我已经注释了。

线程的顺序执行

他们都可以通过

        [ticketsCondition signal]; 发送信号的方式，在一个线程唤醒另外一个线程的等待。

比如：

 

#import "AppDelegate.h"

#import "ViewController.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    tickets = 100;
    count = 0;
    theLock = [[NSLock alloc] init];
    // 锁对象
    ticketsCondition = [[NSCondition alloc] init];
    ticketsThreadone = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
    [ticketsThreadone setName:@"Thread-1"];
    [ticketsThreadone start];

    ticketsThreadtwo = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
    [ticketsThreadtwo setName:@"Thread-2"];
    [ticketsThreadtwo start];

    NSThread *ticketsThreadthree = [[NSThread alloc] initWithTarget:self selector:@selector(run3) object:nil];
    [ticketsThreadthree setName:@"Thread-3"];
    [ticketsThreadthree start];
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.
    self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    self.window.rootViewController = self.viewController;
    [self.window makeKeyAndVisible];
    return YES;
}

-(void)run3{
    while (YES) {
        [ticketsCondition lock];
        [NSThread sleepForTimeInterval:3];
        [ticketsCondition signal];
        [ticketsCondition unlock];
    }
}

- (void)run{
    while (TRUE) {
        // 上锁
        [ticketsCondition lock];
        [ticketsCondition wait];
        [theLock lock];
        if(tickets >= 0){
            [NSThread sleepForTimeInterval:0.09];
            count = 100 - tickets;
            NSLog(@"当前票数是:%d,售出:%d,线程名:%@",tickets,count,[[NSThread currentThread] name]);
            tickets--;
        }else{
            break;
        }
        [theLock unlock];
        [ticketsCondition unlock];
    }
}

wait是等待，我加了一个 线程3 去唤醒其他两个线程锁中的wait

其他同步

我们可以使用指令 @synchronized 来简化 NSLock的使用，这样我们就不必显示编写创建NSLock,加锁并解锁相关代码。
- (void)doSomeThing:(id)anObj
{
    @synchronized(anObj)
    {
        // Everything between the braces is protected by the @synchronized directive.
    }
}