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GCD介绍
Grand Central Dispatch (GCD)是Apple开发的一个多核编程的解决方法
基于C语言,提供了非常多强大的函数
术语
同步 (Synchronous)
在当前线程中执行任务,不具备开启新线程的能力
提交的任务在执行完成后才会返回
同步函数: dispatch_sync()
异步 (Asynchronous)
在新线程中执行任务,具备开启新线程的能力
提交的任务立刻返回,在后台队列中执行
异步函数: dispatch_async()
串行 (Serial)
一个任务执行完毕后,再执行下一个任务
并发 (Concurrent)
多个任务同时执行(自动开启多个线程),
只有在异步函数下才有效
描述 | 说明 |
---|---|
queue | 队列 |
main | 主队列 |
global | 全局队列 |
dispatch_queue_t | 描述队列 |
dispatch_block_t | 描述任务 |
dispatch_once_t | 描述一次性 |
dispatch_time_t | 描述时间 |
dispatch_group_t | 描述队列组 |
dispatch_semaphore_t | 描述信号量 |
函数 | 说明 |
---|---|
dispatch_sync() | 同步执行 |
dispatch_async() | 异步执行 |
dispatch_after() | 延时执行 |
dispatch_once() | 一次性执行 |
dispatch_apply() | 提交队列 |
dispatch_queue_create() | 创建队列 |
dispatch_group_create() | 创建队列组 |
dispatch_group_async() | 提交任务到队列组 |
dispatch_group_enter() / dispatch_group_leave() | 将队列组中的任务未执行完毕的任务数目加减1(两个函数要配合使用) |
dispatch_group_notify() | 监听队列组执行完毕 |
dispatch_group_wait() | 设置等待时间(返回 0成功,1失败) |
注意:
1.所有的执行都放到队列中(queue),队列的特点是FIFO(先提交的先执行)
2.必须在主线程访问 UIKit 的类
3.并发队列只在异步函数下才有效
基本使用
NSLog(@"当前线程: %@", [NSThread currentThread]);
//获取主队列
dispatch_queue_t mainQueue = dispatch_get_main_queue();
//获取全局并发队列
dispatch_queue_t otherQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//同步函数(在当前线程中执行,不具备开启新线程的能力)
dispatch_sync(otherQueue, ^{
NSLog(@"同步 %@", [NSThread currentThread]);
});
//异步函数(在另一条线程中执行,具备开启新线程的能力)
dispatch_async(otherQueue, ^{
NSLog(@"异步 %@", [NSThread currentThread]);
});
//输出: 当前线程: <NSThread: 0x7f81bac06670>{number = 1, name = main}
//输出: 同步 <NSThread: 0x7f81bac06670>{number = 1, name = main}
//输出: 异步 <NSThread: 0x7f81bae35be0>{number = 3, name = (null)}
延时执行 dispatch_after()
dispatch_after()延迟一段时间把一项任务提交到队列中执行,返回之后就不能取消
常用来在在主队列上延迟执行一项任务
NSLog(@"当前线程 %@", [NSThread currentThread]);
//GCD延时调用(主线程)(主队列)
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"GCD延时(主线程) %@", [NSThread currentThread]);
});
//GCD延时调用(其他线程)(全局并发队列)
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"GCD延时(其他线程) %@", [NSThread currentThread]);
});
//输出: 当前线程 <NSThread: 0x7f8cb9701990>{number = 1, name = main}
//输出: GCD延时(主线程) <NSThread: 0x7f8cb9701990>{number = 1, name = main}
//输出: GCD延时(其他线程) <NSThread: 0x7f8cb9513ee0>{number = 3, name = (null)}
一次性执行 dispatch_once()
整个程序运行中,只会执行一次 (默认线程是安全的)
dispatch_once() 以线程安全的方式执行且仅执行其代码块一次
for (NSInteger i = 0; i < 10; i++) {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSLog(@"GCD一次性执行(默认线程是安全的)");
});
}
//输出: GCD一次性执行(默认线程是安全的)
//使用GCD初始化单例
+ (instancetype)sharedManager {
static PhotoManager *sharedPhotoManager = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedPhotoManager = [[PhotoManager alloc] init];
});
return sharedPhotoManager;
}
提交 dispatch_apply()
把一项任务提交到队列中多次执行,具体是并行执行还是串行执行由队列本身决定
dispatch_apply不会立刻返回,在执行完毕后才会返回,是同步的调用。
队列
任务1,任务2依次执行,所有任务都执行成功后回到主线程
(效率不高)
NSLog(@"当前线程 %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//全局并发队列
for (NSInteger i = 0; i < 5; i++) {
NSLog(@"任务1 %@", [NSThread currentThread]);
}
for (NSInteger i = 0; i < 5; i++) {
NSLog(@"任务2 %@", [NSThread currentThread]);
}
dispatch_async(dispatch_get_main_queue(), ^{
//主队列
NSLog(@"主线程执行(刷新UI) %@", [NSThread currentThread]);
});
});
//输出: 当前线程 <NSThread: 0x7fa78040b8b0>{number = 1, name = main}
//输出: 任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务1 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7f9da1416a00>{number = 3, name = (null)}
//输出: 主线程(刷新UI) <NSThread: 0x7f9da1705940>{number = 1, name = main}
队列组
任务1,任务2同时执行,所有任务都执行成功后回到主线程
(效率高)
NSLog(@"当前线程 %@", [NSThread currentThread]);
//(1)创建一个队列组
dispatch_group_t group= dispatch_group_create();
//(2)开启任务1
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for (NSInteger i = 0; i < 5; i++) {
NSLog(@"任务1 %@", [NSThread currentThread]);
}
});
//(3)开启任务2
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for (NSInteger i = 0; i < 5; i++) {
NSLog(@"任务2 %@", [NSThread currentThread]);
}
});
//(4)所有任务执行完毕,回到主线程
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"主线程(刷新UI) %@", [NSThread currentThread]);
});
//输出: 当前线程 <NSThread: 0x7fd951704e70>{number = 1, name = main}
//输出: 任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
//输出: 任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
//输出: 任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
//输出: 任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
//输出: 任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
//输出: 任务1 <NSThread: 0x7ff65a50b4d0>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
//输出: 任务2 <NSThread: 0x7ff65a697770>{number = 4, name = (null)}
//输出: 主线程(刷新UI) <NSThread: 0x7ff65a406fb0>{number = 1, name = main}
串行与并发
串行队列
一个任务执行完毕后,再执行下一个任务
主队列是GCD自带的一种特殊的串行队列,放在主队列中的任务,都会放到主线程中执行
//(1)使用dispatch_queue_create函数创建串行队列
//参数1: 队列名称
//参数2: 队列属性 (一般用NULL)
dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", NULL);
//(2)使用主队列(跟主线程相关联的队列)
dispatch_queue_t serialMainQueue = dispatch_get_main_queue();
并发队列
多个任务并发执行(自动开启多个线程同时执行任务)
<u>并发功能只有在异步(dispatch_async)函数下才有效!!!</u>
GCD默认已经提供了全局的并发队列,供整个应用使用,不需要手动创建
并发队列优先级 | 快捷值 | 优先级 |
---|---|---|
DISPATCH_QUEUE_PRIORITY_HIGH | 2 | 高 |
DISPATCH_QUEUE_PRIORITY_DEFAULT | 0 | 中(默认) |
DISPATCH_QUEUE_PRIORITY_LOW | (-2) | 低 |
DISPATCH_QUEUE_PRIORITY_BACKGROUND | INT16_MIN | 后台 |
//(1)使用dispatch_get_global_queue函数获得全局的并发队列
//参数1: 优先级
//参数2: 暂时无用参数 (传0)
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
异步函数_并发队列
(开启新线程,并发执行任务)
NSLog(@"当前线程 %@", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"任务1 %@", [NSThread currentThread]);
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"任务2 %@", [NSThread currentThread]);
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"任务3 %@", [NSThread currentThread]);
});
//输出: 当前线程 <NSThread: 0x7fbe83c067c0>{number = 1, name = main}
//输出: 任务1 <NSThread: 0x7fb84160ed90>{number = 3, name = (null)}
//输出: 任务3 <NSThread: 0x7fb841752940>{number = 4, name = (null)}
//输出: 任务2 <NSThread: 0x7fb8414167b0>{number = 5, name = (null)}
异步函数_串行队列
(开启新线程,串行执行任务)
NSLog(@"当前线程 %@", [NSThread currentThread]);
//创建串行队列
dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", NULL);
dispatch_async(serialQueue, ^{
NSLog(@"任务1 %@", [NSThread currentThread]);
});
dispatch_async(serialQueue, ^{
NSLog(@"任务2 %@", [NSThread currentThread]);
});
dispatch_async(serialQueue, ^{
NSLog(@"任务3 %@", [NSThread currentThread]);
});
//输出: 当前线程 <NSThread: 0x7fc192e05380>{number = 1, name = main}
//输出: 任务1 <NSThread: 0x7fc192c16670>{number = 3, name = (null)}
//输出: 任务2 <NSThread: 0x7fc192c16670>{number = 3, name = (null)}
//输出: 任务3 <NSThread: 0x7fc192c16670>{number = 3, name = (null)}
同步函数_并发队列
(不会开启新线程,并发执行任务失效!)
NSLog(@"当前线程 %@", [NSThread currentThread]);
dispatch_sync(dispatch_get_global_queue(0, 0), ^{
NSLog(@"任务1 %@", [NSThread currentThread]);
});
dispatch_sync(dispatch_get_global_queue(0, 0), ^{
NSLog(@"任务2 %@", [NSThread currentThread]);
});
dispatch_sync(dispatch_get_global_queue(0, 0), ^{
NSLog(@"任务3 %@", [NSThread currentThread]);
});
//输出: 当前线程 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
//输出: 任务1 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
//输出: 任务2 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
//输出: 任务3 <NSThread: 0x7fa0dbf02e20>{number = 1, name = main}
同步函数_串行队列
(不会开启新线程,串行执行任务)
NSLog(@"当前线程 %@", [NSThread currentThread]);
//创建串行队列
dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", NULL);
dispatch_sync(serialQueue, ^{
NSLog(@"任务1 %@", [NSThread currentThread]);
});
dispatch_sync(serialQueue, ^{
NSLog(@"任务2 %@", [NSThread currentThread]);
});
dispatch_sync(serialQueue, ^{
NSLog(@"任务3 %@", [NSThread currentThread]);
});
//输出: 当前线程 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
//输出: 任务1 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
//输出: 任务2 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
//输出: 任务3 <NSThread: 0x7f8c0b407f00>{number = 1, name = main}
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