4.4 GCD->3.0 其他用法

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本文相关目录：
==================== 所属文集：4.0 多线程 ====================
4.1 多线程基础->1.0 进程 & 线程
······················ 2.0 多线程简介
4.2 pthread
4.3 NSThread->1.0 创建线程
····················· 2.0 线程属性
····················· 3.0 线程状态/线程生命周期
····················· 4.0 多线程安全隐患
····················· 5.0 线程间通讯和常用方法
4.4 GCD->1.0 GCD简介和使用
·············· 2.0 线程间的通信
·············· 3.0 其他用法
·············· 4.0 GCD 的定时器事件
4.5 NSOperation->1.0 NSOperation简介
························ 2.0 NSOperationQueue
························ 3.0 线程间通信
························ 4.0 自定义NSOperation
4.6 RunLoop - 运行循环
===================== 所属文集：4.0 多线程 =====================

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3.1 barrier函数

作用：控制多线程的执行顺序

#import "ViewController.h"

@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
}

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

  // 这里使用全局并发队列的方式会导致 dispatch_barrier_async 功能失效
  dispatch_queue_t queue =
      dispatch_queue_create("TD", DISPATCH_QUEUE_CONCURRENT);

  dispatch_async(queue, ^{
    NSLog(@"----1-----%@", [NSThread currentThread]);
  });
  dispatch_async(queue, ^{
    NSLog(@"----2-----%@", [NSThread currentThread]);
  });

  dispatch_barrier_async(queue, ^{
    NSLog(@"----barrier-----%@", [NSThread currentThread]);
  });

  dispatch_async(queue, ^{
    NSLog(@"----3-----%@", [NSThread currentThread]);
  });
  dispatch_async(queue, ^{
    NSLog(@"----4-----%@", [NSThread currentThread]);
  });
}

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

@end

打印结果：

其他用法 - barrier函数[1419:94670] ----2-----{number = 3, name = (null)}
其他用法- barrier函数[1419:94732] ----1-----{number = 2, name = (null)}
其他用法- barrier函数[1419:94732] ----barrier-----{number = 2, name = (null)}
其他用法- barrier函数[1419:94732] ----3-----{number = 2, name = (null)}
其他用法 - barrier函数[1419:94670] ----4-----{number = 3, name = (null)}

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3.2 延迟执行

方法1：调用NSObject的方法

// 该方法在那个线程调用，那么run就在哪个线程执行（当前线程），通常是主线程
// 2秒后再调用self的run方法
[selfperformSelector:@selector(run) withObject:nilafterDelay:2.0];

方法2：使用GCD函数

// 这里是在主线程执行，如果想要在子线程执行，选择相应的队列
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        // 2秒后异步执行这里的代码...
    });

方法3：使用NSTimer

[NSTimer scheduledTimerWithTimeInterval:2.0
                                 target:self
                               selector:@selector(run)
                               userInfo:nil
                                repeats:NO];

代码示例：

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
}

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

  [self delay1];
}

#pragma mark - 方法1：调用NSObject的方法
- (void)delay1 {
  NSLog(@"touchesBegan-----");

  // 2秒后再调用self的run方法
  [self performSelector:@selector(run) withObject:nil afterDelay:2.0];
}

#pragma mark - 方法2：使用 GCD 函数
- (void)delay2 {
  NSLog(@"touchesBegan-----");

  // 这里是在主线程执行，如果想要在子线程执行，选择相应的队列
  dispatch_after(
      dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)),
      dispatch_get_main_queue(), ^{
        NSLog(@"run-----");
      });
}

#pragma mark - 方法3：使用NSTimer定时器
- (void)delay3 {
  NSLog(@"touchesBegan-----");

  [NSTimer scheduledTimerWithTimeInterval:2.0
                                   target:self
                                 selector:@selector(run)
                                 userInfo:nil
                                  repeats:NO];
}

- (void)run {
  NSLog(@"run-----");
}

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

@end

打印结果：

其他用法 - 延迟执行[1651:114465] touchesBegan-----
其他用法 - 延迟执行[1651:114465] run-----

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3.3 一次性代码

使用dispatch_once 函数能保证某段代码在程序运行过程中只被执行1次，适合做资源的加载

static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
     // 只执行1次的代码(这里面默认是线程安全的)
  });
}

---

3.4 快速迭代

使用dispatch_apply函数能进行快速迭代遍历：

dispatch_apply(<#size_t iterations#>, <#dispatch_queue_t queue#>, ^(size_t) {
    //  代码
});

dispatch_apply(10, dispatch_get_global_queue(0, 0), ^(size_t index){
    // 执行10次代码，index顺序不确定
});

代码示例：文件剪切

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
}
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
  [self apply];
}

#pragma mark - 文件剪切方法1：快速迭代
- (void)apply {

  dispatch_queue_t queue =
      dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

  NSString *from = @"/Users/TD/Desktop/From";
  NSString *to = @"/Users/TD/Desktop/To";

  NSFileManager *mgr = [NSFileManager defaultManager];
  NSArray *subpaths = [mgr subpathsAtPath:from];

  dispatch_apply(subpaths.count, queue, ^(size_t index) {

    NSString *subpath = subpaths[index];
    NSString *fromFullpath = [from stringByAppendingPathComponent:subpath];
    NSString *toFullpath = [to stringByAppendingPathComponent:subpath];

    // 剪切
    [mgr moveItemAtPath:fromFullpath toPath:toFullpath error:nil];

    NSLog(@"%@---%@", [NSThread currentThread], subpath);
  });
}

#pragma mark - 文件剪切方法2：传统方式
- (void)moveFile {
  NSString *from = @"/Users/TD/Desktop/From";
  NSString *to = @"/Users/TD/Desktop/To";

  NSFileManager *mgr = [NSFileManager defaultManager];

  //获取文件夹下的所有文件路径，包括子文件夹下的文件路径
  NSArray *subpaths = [mgr subpathsAtPath:from];

  for (NSString *subpath in subpaths) {

    //全路径
    NSString *fromFullpath = [from stringByAppendingPathComponent:subpath];
    NSString *toFullpath = [to stringByAppendingPathComponent:subpath];

    dispatch_async(
        dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
          // 剪切
          [mgr moveItemAtPath:fromFullpath toPath:toFullpath error:nil];
        });
  }
}

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

@end

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3.5 队列组/调度组

有这么一种需求：

首先：分别异步执行2个耗时的操作
其次：等2个异步操作都执行完毕后，再回到主线程执行操作

解决办法：用队列组，也叫做调度组

// 创建一个队列组
dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   // 执行1个耗时的异步操作
});

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   // 执行1个耗时的异步操作
});

dispatch_group_notify(group, dispatch_get_main_queue(),
^{
   // 等前面的异步操作都执行完毕后，回到主线程...
});

代码结构：

其他用法 - 队列组调度组.png

代码示例：

#import "ViewController.h"

@interface ViewController ()
@property(weak, nonatomic) IBOutlet UIImageView *imageView;
@property(nonatomic, strong) UIImage *image1; //图片1
@property(nonatomic, strong) UIImage *image2; //图片2
@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
}

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

  dispatch_queue_t queue =
      dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

  // 创建一个队列组
  dispatch_group_t group = dispatch_group_create();

  // 1.下载图片1
  dispatch_group_async(group, queue, ^{
    // 图片的网络路径
    NSURL *url = [NSURL URLWithString:@"http://img.pconline.com.cn/images/photoblog/9/9/8/1/9981681/200910/11/1255259355826.jpg"];
    // 加载图片
    NSData *data = [NSData dataWithContentsOfURL:url];
    // 生成图片
    self.image1 = [UIImage imageWithData:data];
  });

  // 2.下载图片2
  dispatch_group_async(group, queue, ^{
    // 图片的网络路径
    NSURL *url = [NSURL URLWithString: @"http://pic38.nipic.com/20140228/5571398_215900721128_2.jpg"];
    // 加载图片
    NSData *data = [NSData dataWithContentsOfURL:url];
    // 生成图片
    self.image2 = [UIImage imageWithData:data];
  });

  // 3.将图片1、图片2合成一张新的图片（也可以直接在此处回到主线程，只不过是因为绘制图片比较耗时，没有放在主线程而已）
  dispatch_group_notify(group, queue, ^{

    // 开启新的图形上下文
    UIGraphicsBeginImageContext(CGSizeMake(100, 100));

    // 绘制图片
    [self.image1 drawInRect:CGRectMake(0, 0, 50, 100)];
    [self.image2 drawInRect:CGRectMake(50, 0, 50, 100)];

    // 取得上下文中的图片
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

    // 结束上下文
    UIGraphicsEndImageContext();

    // 回到主线程显示图片
    dispatch_async(dispatch_get_main_queue(), ^{
      // 4.将新图片显示出来
      self.imageView.image = image;
    });
  });
}

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

@end

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本文源码 Demo 详见 Github
https://github.com/shorfng/iOS_4.0_multithreading.git

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作者：蓝田（Loto）

出处：

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