iOS -- 多线程与GCD的应用

2017-01-06  本文已影响194人  有梦想的老伯伯

本篇博客共分以下几个模块来介绍GCD的相关内容:

多线程相关概念

进程与线程
线程相关
多线程中会出现的问题

多线程编程技术的优缺点比较

GCD的抽象层次最高，使用也简单，因此，苹果也推荐使用GCD

GCD中的三种队列类型

GCD编程的核心就是dispatch队列，dispatch block的执行最终会放进某个队列中去进行。

The main queue(主线程串行队列)

dispatch_sync 同步执行任务函数，不会开启新的线程，dispatch_async 异步执行任务函数，会开启新的线程

dispatch_queue_t mainQueue = dispatch_get_main_queue();

dispatch_queue_t mainQueue = dispatch_get_main_queue();
dispatch_sync(mainQueue,^{
NSLog("MainQueue"); 
});

程序正常运行，block中的代码正常运行

dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(globalQueue, ^{
    //子线程异步执行下载任务，防止主线程卡顿
    NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
    NSError *error;
    NSString *htmlData = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error];
    if (htmlData != nil) {
        dispatch_queue_t mainQueue = dispatch_get_main_queue();
        //异步返回主线程，根据获取的数据，更新UI
        dispatch_async(mainQueue, ^{
            NSLog(@"根据更新UI界面");
        });
    } else {
        NSLog(@"error when download:%@",error);
    }
});

主线程串行队列由系统默认生成的，所以无法调用dispatch_resume()和dispatch_suspend()来控制执行继续或中断。

Global queue（全局并发队列)

耗时的操作，比如读取网络数据，IO,数据库读写等，我们会在另外一个线程中处理这些操作，然后通知主线程更新界面

//程序默认的队列级别，一般不要修改,DISPATCH_QUEUE_PRIORITY_DEFAULT == 0
dispatch_queue_t globalQueue1 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//HIGH
dispatch_queue_t globalQueue2 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
//LOW
dispatch_queue_t globalQueue3 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
//BACKGROUND
dispatch_queue_t globalQueue4 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
NSLog(@"current task");
dispatch_sync(globalQueue, ^{
    sleep(2.0);
    NSLog(@"sleep 2.0s");
});
NSLog(@"next task");

控制台输出如下：

2015-11-18 15:50:14.999 Whisper[33073:343781] current task
2015-11-18 15:50:15.000 Whisper[33073:343781] next task
2015-11-18 15:50:17.004 Whisper[33073:343841] sleep 2.0s

主线程不用等待2s钟，继续执行block代码段后面的代码。

dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
NSLog(@"current task");
dispatch_async(globalQueue, ^{
    NSLog(@"最先加入全局并发队列");
});
dispatch_async(globalQueue, ^{
    NSLog(@"次加入全局并发队列");
});
NSLog(@"next task");

控制台输出如下:

2015-11-18 16:54:52.202 Whisper[39827:403208] current task
2015-11-18 16:54:52.203 Whisper[39827:403208] next task
2015-11-18 16:54:52.205 Whisper[39827:403309] 最先加入全局并发队列
2015-11-18 16:54:52.205 Whisper[39827:403291] 次加入全局并发队列

异步线程的执行顺序是不确定的。几乎同步开始执行
全局并发队列由系统默认生成的，所以无法调用dispatch_resume()和dispatch_suspend()来控制执行继续或中断。

Custom queue (自定义队列)

自定义串行队列

dispatch_queue_t serialQueue = dispatch_queue_create("com.dullgrass.serialQueue", DISPATCH_QUEUE_SERIAL);
NSLog(@"%s",dispatch_queue_get_label(conCurrentQueue)) ;

2015-11-19 11:05:34.469 Whisper[1223:42960] com.dullgrass.serialQueue

dispatch_queue_create(const char *label, dispatch_queue_attr_t attr)函数中第一个参数是给这个queue起的标识，这个在调试的可以看到是哪个队列在执行，或者在crash日志中，也能做为提示。第二个是需要创建的队列类型，是串行的还是并发的

dispatch_queue_t serialQueue = dispatch_queue_create("com.dullgrass.serialQueue", DISPATCH_QUEUE_SERIAL);
NSLog(@"current task");
dispatch_sync(serialQueue, ^{
    NSLog(@"最先加入自定义串行队列");
    sleep(2);
});
dispatch_sync(serialQueue, ^{
    NSLog(@"次加入自定义串行队列");
});
NSLog(@"next task");

2015-11-18 17:09:40.025 Whisper[40241:416296] current task
2015-11-18 17:09:40.027 Whisper[40241:416296] 最先加入自定义串行队列
2015-11-18 17:09:43.027 Whisper[40241:416296] 次加入自定义串行队列
2015-11-18 17:09:43.027 Whisper[40241:416296] next task

当前线程等待串行队列中的子线程执行完成之后再执行，串行队列中先进来的子线程先执行任务，执行完成后，再执行队列中后面的任务。

dispatch_queue_t serialQueue = dispatch_queue_create("com.dullgrass.serialQueue", DISPATCH_QUEUE_SERIAL);
dispatch_sync(serialQueue, ^{   //该代码段后面的代码都不会执行，程序被锁定在这里
       NSLog(@"会执行的代码");
       dispatch_sync(serialQueue, ^{
           NSLog(@"代码不执行");
       });
});

dispatch_queue_t serialQueue = dispatch_queue_create("com.dullgrass.serialQueue", DISPATCH_QUEUE_SERIAL);
dispatch_async(serialQueue, ^{
        NSLog(@"会执行的代码");
        dispatch_sync(serialQueue, ^{
            NSLog(@"代码不执行");
        });
});

注意不要嵌套使用同步执行的串行队列任务

自定义并发队列

dispatch_queue_t conCurrentQueue = dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT);

dispatch_queue_t conCurrentQueue = dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"current task");
dispatch_sync(conCurrentQueue, ^{
    NSLog(@"先加入队列");
});
dispatch_sync(conCurrentQueue, ^{
    NSLog(@"次加入队列");
});
NSLog(@"next task");

2015-11-19 10:36:23.259 Whisper[827:20596] current task
2015-11-19 10:36:23.261 Whisper[827:20596] 先加入队列
2015-11-19 10:36:23.261 Whisper[827:20596] 次加入队列
2015-11-19 10:36:23.261 Whisper[827:20596] next task

dispatch_queue_t conCurrentQueue = dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"current task");
dispatch_sync(conCurrentQueue, ^{
    NSLog(@"先加入队列");
    dispatch_sync(conCurrentQueue, ^{
        NSLog(@"次加入队列");
    });
});
NSLog(@"next task");

2015-11-19 10:39:21.301 Whisper[898:22273] current task
2015-11-19 10:39:21.303 Whisper[898:22273] 先加入队列
2015-11-19 10:39:21.303 Whisper[898:22273] 次加入队列
2015-11-19 10:39:21.303 Whisper[898:22273] next task

dispatch_queue_t conCurrentQueue = dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"current task");
dispatch_async(conCurrentQueue, ^{
    NSLog(@"先加入队列");
});
dispatch_async(conCurrentQueue, ^{
    NSLog(@"次加入队列");
});
NSLog(@"next task");

2015-11-19 10:45:22.290 Whisper[1050:26445] current task
2015-11-19 10:45:22.290 Whisper[1050:26445] next task
2015-11-19 10:45:22.290 Whisper[1050:26505] 次加入队列
2015-11-19 10:45:22.290 Whisper[1050:26500] 先加入队列

异步执行任务，开启新的子线程，不影响当前线程任务的执行，并发队列中的任务，几乎是同步执行的，输出顺序不确定

Group queue (队列组)

当遇到需要执行多个线程并发执行，然后等多个线程都结束之后，再汇总执行结果时可以用group queue

dispatch_queue_t conCurrentGlobalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t mainQueue = dispatch_get_main_queue();
dispatch_group_t groupQueue = dispatch_group_create();
NSLog(@"current task");
dispatch_group_async(groupQueue, conCurrentGlobalQueue, ^{
    NSLog(@"并行任务1");
});
dispatch_group_async(groupQueue, conCurrentGlobalQueue, ^{
    NSLog(@"并行任务2");
});
dispatch_group_notify(groupQueue, mainQueue, ^{
    NSLog(@"groupQueue中的任务 都执行完成,回到主线程更新UI");
});
NSLog(@"next task");

2015-11-19 13:47:55.117 Whisper[1645:97116] current task
2015-11-19 13:47:55.117 Whisper[1645:97116] next task
2015-11-19 13:47:55.119 Whisper[1645:97178] 并行任务1
2015-11-19 13:47:55.119 Whisper[1645:97227] 并行任务2
2015-11-19 13:47:55.171 Whisper[1645:97116] groupQueue中的任务 都执行完成,回到主线程更新UI

dispatch_group_t groupQueue = dispatch_group_create();
dispatch_time_t delayTime = dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC);
dispatch_queue_t conCurrentGlobalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
NSLog(@"current task");
dispatch_group_async(groupQueue, conCurrentGlobalQueue, ^{
    
    long isExecuteOver = dispatch_group_wait(groupQueue, delayTime);
    if (isExecuteOver) {
        NSLog(@"wait over");
    } else {
        NSLog(@"not over");
    }
    NSLog(@"并行任务1");
});
dispatch_group_async(groupQueue, conCurrentGlobalQueue, ^{
    NSLog(@"并行任务2");
});

2015-11-19 14:37:29.514 Whisper[2426:126683] current task
2015-11-19 14:37:29.518 Whisper[2426:126791] 并行任务2
2015-11-19 14:37:39.515 Whisper[2426:126733] wait over
2015-11-19 14:37:39.516 Whisper[2426:126733] 并行任务1

dispatch_time(dispatch_time_t when, int64_t delta);
参数注释：
第一个参数一般是DISPATCH_TIME_NOW，表示从现在开始
第二个参数是延时的具体时间
延时1秒可以写成如下几种：
NSEC_PER_SEC----每秒有多少纳秒
dispatch_time(DISPATCH_TIME_NOW, 1NSEC_PER_SEC);
USEC_PER_SEC----每秒有多少毫秒（注意是指在纳秒的基础上）
dispatch_time(DISPATCH_TIME_NOW, 1000USEC_PER_SEC); //SEC---毫秒
NSEC_PER_USEC----每毫秒有多少纳秒。
dispatch_time(DISPATCH_TIME_NOW,
USEC_PER_SEC*NSEC_PER_USEC);SEC---纳秒

GCD中一些系统提供的常用dispatch方法

dispatch_time_t delayTime3 = dispatch_time(DISPATCH_TIME_NOW, 3*NSEC_PER_SEC);
dispatch_time_t delayTime2 = dispatch_time(DISPATCH_TIME_NOW, 2*NSEC_PER_SEC);
dispatch_queue_t mainQueue = dispatch_get_main_queue();
NSLog(@"current task");
dispatch_after(delayTime3, mainQueue, ^{
    NSLog(@"3秒之后添加到队列");
});
dispatch_after(delayTime2, mainQueue, ^{
    NSLog(@"2秒之后添加到队列");
});
NSLog(@"next task");

2015-11-19 15:50:19.369 Whisper[2725:172593] current task
2015-11-19 15:50:19.370 Whisper[2725:172593] next task
2015-11-19 15:50:21.369 Whisper[2725:172593] 2秒之后添加到队列
2015-11-19 15:50:22.654 Whisper[2725:172593] 3秒之后添加到队列

dispatch_after只是延时提交block，并不是延时后立即执行，并不能做到精确控制，需要精确控制的朋友慎用哦

dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
NSLog(@"current task");
dispatch_async(globalQueue, ^{
    dispatch_queue_t applyQueue = dispatch_get_global_queue(0, 0);
    //第一个参数，3--block执行的次数
    //第二个参数，applyQueue--block任务提交到的队列
    //第三个参数，block--需要重复执行的任务
    dispatch_apply(3, applyQueue, ^(size_t index) {
        NSLog(@"current index %@",@(index));
        sleep(1);
    });
    NSLog(@"dispatch_apply 执行完成");
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    dispatch_async(mainQueue, ^{
        NSLog(@"回到主线程更新UI");
    });
});
NSLog(@"next task");

2015-11-19 16:24:45.015 Whisper[4034:202269] current task
2015-11-19 16:24:45.016 Whisper[4034:202269] next task
2015-11-19 16:24:45.016 Whisper[4034:202347] current index 0
2015-11-19 16:24:45.016 Whisper[4034:202344] current index 1
2015-11-19 16:24:45.016 Whisper[4034:202345] current index 2
2015-11-19 16:24:46.021 Whisper[4034:202347] dispatch_apply 执行完成
2015-11-19 16:24:46.021 Whisper[4034:202269] 回到主线程更新UI

ShareManager的.h文件
#import <Foundation/Foundation.h>
@interface ShareManager : NSObject
@property (nonatomic, copy) NSString *someProperty;
+ (ShareManager *)shareManager;
+ (ShareManager *)sharedManager;
@end

ShareManager的.m文件
#import "ShareManager.h"
@implementation ShareManager
static ShareManager *sharedManager = nil;
//GCD实现单例功能
+ (ShareManager *)shareManager
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedManager = [[self alloc] init];
    });
    return sharedManager;
}
//在ARC下，非GCD，实现单例功能
+ (ShareManager *)sharedManager
{
    @synchronized(self) {
        if (!sharedManager) {
            sharedManager = [[self alloc] init];
        }
    }
    return sharedManager;
}
- (instancetype)init{
    self = [super init];
    if (self) {
        _someProperty =@"Default Property Value";
    }
    return self;
}
@end

ShareManager的使用
#import "ShareManager.h"
在需要使用的函数中，直接调用下面的方法
ShareManager *share = [ShareManager sharedManager];
NSLog(@"share is %@",share.someProperty);

dispatch_queue_t conCurrentQueue = dispatch_queue_create("com.dullgrass.conCurrentQueue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(conCurrentQueue, ^{
    NSLog(@"dispatch 1");
});
dispatch_async(conCurrentQueue, ^{
    NSLog(@"dispatch 2");
});
dispatch_barrier_async(conCurrentQueue, ^{
    NSLog(@"dispatch barrier");
});
dispatch_async(conCurrentQueue, ^{
    NSLog(@"dispatch 3");
});
dispatch_async(conCurrentQueue, ^{
    NSLog(@"dispatch 4");
});

2015-11-19 18:12:34.125 Whisper[22633:297257] dispatch 1
2015-11-19 18:12:34.125 Whisper[22633:297258] dispatch 2
2015-11-19 18:12:34.126 Whisper[22633:297258] dispatch barrier
2015-11-19 18:12:34.127 Whisper[22633:297258] dispatch 3
2015-11-19 18:12:34.127 Whisper[22633:297257] dispatch 4
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