NSCondition实现生产者消费者模式

使用NSCondition对象来控制进程的同步，通过NSCondition对象的操作实现进程间的通信。NSCondition也实现了NSLocking协议，因此也可以调用lock、 unlock来实现线程的同步。NSCondition类提供以下3个方法：

wait——该方法让线程一直等待；signal——唤醒在此NSCondition对象上等待的单个线程；broadcast——唤醒在此NSCondition对象上等待的所有线程

以银行存取款为例：设置一个BOOL类型的flag来标识庄户中是否已有存款。NO表示没有存款，存款线程可以向下执行，当存款者把钱存入账户后，设置flag为YES，并调用sinal或broadcast方法唤醒其他线程；当存款者线程进入线程体后，若flag=YES,调用wait方法让改线程等待。当flag为YES时，表明账户中已有存款，取钱线程可以向下执行，当取出钱后，将flag设置为NO,并调用signal或broadcast方法唤醒其他线程；当取钱者进入线程后，如果标识为NO，就调用wait方法进行等待。

只有当取钱线程结束后，存款线程才可以存款；同样只有存款线程存款后取钱线程才可以取钱。

定义一个Acount类，里面定义draw和deposit方法，分别实现存取款操作；

#import "Acount.h"

@implementation Acount
NSCondition *cond;
bool flag;
-(id)initWithAccount:(NSString *)acount balanca:(double)balance
{
    if(self = [super init])
    {
        cond = [[NSCondition alloc] init];
        _acount = acount;
        _balance = balance;
    }
    return  self;
}

-(void)draw:(CGFloat)drawAmount
{
    [cond lock];//add lock
    if(!flag)
        [cond wait];
    else{
        NSLog(@"%@ 取了%f", [NSThread currentThread].name, drawAmount);
        _balance -= drawAmount;
        NSLog(@"账户余额为%f", self.balance);

        flag =NO;
        [cond broadcast];//wake up other threads
    }
    [cond unlock];
}
-(void)deposit:(CGFloat) depositAmount
{
    [cond lock];
    if(flag)
        [cond wait];
    else
    {
        NSLog(@"%@ 存了 %f", [NSThread currentThread].name, depositAmount);
        _balance += depositAmount;
        NSLog(@"账户余额为%f", _balance);

        flag = YES;
        [cond broadcast];//wake up other threads
    }
    [cond unlock];//open lock
}
@end

在ViewController.h中，生成一个上类的对象，在button的响应方法里面分别创建取钱线程、存款线程，分别调用Acount的存款方法，Acount的取款方法。

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController
FKAcount *acount;
- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
    acount = [[FKAcount alloc] initWithAccount:@"小明" balanca:0.0];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (IBAction)drawDeposit:(id)sender {
    NSThread *thread1 = [[NSThread alloc] initWithTarget:self selector:@selector(draw:) object:[NSNumber numberWithFloat:800.0]];

    NSThread *thread2 = [[NSThread alloc] initWithTarget:self selector:@selector(draw:) object:[NSNumber numberWithFloat:800.0]];

    NSThread *thread3 = [[NSThread alloc] initWithTarget:self selector:@selector(deposit:) object:[NSNumber numberWithFloat:800.0]];

    [thread1 start];
    [thread2 start];
    [thread3 start];
}
-(void)draw:(NSNumber*) drawAmount
{
    [NSThread currentThread].name = @"取";
    for(int i=0 ;i < 50; i++)
        [acount draw:drawAmount.doubleValue];
}
-(void)deposit:(NSNumber *) depositAmount
{
    [NSThread currentThread].name = @"存";
    for(int i=0; i < 50; i++)
        [acount deposit:depositAmount.doubleValue];
}
@end

程序最后被阻塞无法继续向下执行，因为创建2个取款线程共100次取款操作，而1个存钱线程50次操作，程序最后被阻塞。