C5-信号量与PV操作(iOS篇-细说信号量)

一.概述

二.核心

2.1 同步机制的概念.为什么叫同步(掌握)?

2.2 信号量和P, V操作

解说(掌握):

该进程状态置为阻塞态.

将该进程插入相应的等待对列s.queue末尾.

CPU重新调度;

P操作主要做两件事(掌握):

P的总结(掌握):

唤醒相应等待队列s.queue中等待的一个进程;

改变其状态为就绪态,并将其插入就绪队列;

V(掌握):

V的总结(掌握):

一些概念(掌握)

2.3 iOS中的买卖票例子(掌握)

//
//  ViewController.m
//  02-线程安全01-购票例子
//
//  Created by sixleaves on 15/10/18.
//  Copyright © 2015年 sixleaves. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()

@property (nonatomic, strong) NSThread * thread01;
@property (nonatomic, strong) NSThread * thread02;
@property (nonatomic, strong) NSThread * thread03;

@property (nonatomic, assign) NSInteger ticketCount;

@property (nonatomic, strong) dispatch_semaphore_t sema;
@end

@implementation ViewController

- (void)viewDidLoad {
[super viewDidLoad];

self.thread01 = [[NSThread alloc] initWithTarget:self selector:@selector(buy:) object:nil];
self.thread02 = [[NSThread alloc] initWithTarget:self selector:@selector(buy:) object:nil];
self.thread03 = [[NSThread alloc] initWithTarget:self selector:@selector(buy:) object:nil];

self.ticketCount = 1000;

// 初始化
self.sema = dispatch_semaphore_create(1);

}

- (void)buy:(NSInteger)number {

// 多线程同时操作票数, 造成的数据错乱问题.
// 解决方式: 1.加锁 2.用信号量
while (1) {

// P操作
dispatch_semaphore_wait(self.sema, DISPATCH_TIME_FOREVER);

// 1.拿到票数
NSUInteger resetTicket = self.ticketCount;

// 2.判断票是否买完
if (resetTicket > 0) { // 没卖完, 则继续卖.

// 逗号表达式, 从左向右算, 取最后一个的值为表达式值
NSLog(@"还剩下%ld张 ,买出一张票, 还剩下%ld张.", resetTicket--, resetTicket);
self.ticketCount = resetTicket;

} else {

NSLog(@"票买完了");
break;
}

// V操作
dispatch_semaphore_signal(self.sema);

}
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {

if (!([self.thread01 isExecuting] || [self.thread02 isExecuting] || [self.thread03 isExecuting])) {

[self.thread01 start];
[self.thread02 start];
[self.thread03 start];
}
}

@end