转载自:http://blog.csdn.net/mad2man/article/details/21786251
1. Dispatch Queue
执行处理有两种Dispatch Queue。
一种是等待现在执行中的处理的Serial Dispatch Queue。(顺序执行)
另一种是不等待现在执行中处理的Concurrent Dispatch Queue。(并行执行)
Concurrent Dispatch Queue执行:
线程0 |
线程1 |
线程2 |
线程3 |
blk0 |
blk1 |
blk2 |
blk3 |
blk5 |
blk6 |
blk4 |
|
blk7 |
|
|
|
2. dispatch_queue_create
通过 dispatch_queue_create 函数可以生成 Dispathch Queue。
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- dispatch_queue_t myDispatchQueue = dispatch_queue_create(“com.example.queue”, NULL);
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-
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- dispatch_async(myDispatchQueue, ^{ NSLog(@“ block on queue “); });
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- dispatch_release(myDispatchQueue);
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-
-
虽然一执行完 dispatch_async 后里面释放queue,但是由于Block持有该Dispatch Queue,所以此处release后 queue 不会被废弃。
3. Main Dispatch Queue 和 Global Dispatch Queue
以上两种为系统标准提供的Dispatch Queue;
名称 |
Dispatch Queue种类 |
说明 |
Main Dispatch Queue |
Serial Dispatch Queue |
主线程执行 |
DISPATCH_QUEUE_PRIORITY_HIGH |
Concurrent Dispatch Queue |
高优先级 |
DISPATCH_QUEUE_PRIORITY_DEFAULT |
Concurrent Dispatch Queue |
默认优先级 |
DISPATCH_QUEUE_PRIORITY_LOW |
Concurrent Dispatch Queue |
低优先级 |
DISPATCH_QUEUE_PRIORITY_BACKGROUND |
Concurrent Dispatch Queue |
后台执行 |
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- dispatch_queue_t mainDispatchQueue = dispatch_get_main_queue();
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- dispatch_queue_t globalDispatchQueueHigh = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
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- dispatch_queue_t globalDispatchQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
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- dispatch_queue_t globalDispatchQueueLow = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
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- dispatch_queue_t globalDispatchQueueBackground = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
注:dispatch_retain() 和 dispatch_release() 对于这两种queue不会引起任何变化。
4. dispatch_set_target_queue
dispatch_queue_create() 函数生成的无论是Serial 还是 Concurrent ,都是默认优先级。
使用 dispatch_set_target_queue 可以变更优先级。
例如:
- dispatch_queue_t myDispatchQueue = dispatch_queue_create(“com.example.queue”, NULL);
- dispatch_queue_t globalDispatchQueueBackground = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
- dispatch_set_target_queue(myDispatchQueue, globalDispatchQueueBackground);
以上代码将 myDispatchQueue 的优先级变更为 后台优先级。
5. dispatch_after
想要在指定时间后执行处理,可使用 dispatch_after。
例如:3秒后将指定的Block 追加奥 Main Dispatch Queue 中。
- dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 33ull * NSEC_PER_SEC);
- dispatch_after(time, dispatch_get_main_queue(), ^{ NSLog(@“waited at least three seconds.”); });
注: dispatch_after 函数并不是在指定时间后执行处理,而是在指定时间后追加处理到Dispatch Queue。例如Main Dispatch Queue在主线程的RunLoop中执行。所以在比如每隔1/60秒执行的RunLoop,Block最快在3秒后执行,最慢在 3+1/60秒后执行。
6. Dispatch Group
如果想要在追加到多个Dispatch Queue中的多个处理全部结束后执行结束处理,,可使用Dispatch Group。
例如:
- dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
- dispatch_group_t group = dispatch_group_create();
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- dispatch_group_async(group, queue, ^{NSLog(@“bll0”);});
- dispatch_group_async(group, queue, ^{NSLog(@“bll1”);});
- dispatch_group_async(group, queue, ^{NSLog(@“bll2”);});
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- dispatch_group_notify(group, dispatch_get_main_queue(), ^{ NSLog(@“done”); });
- dispatch_release(group);
其中可以使用 dispatch_group_wait函数仅等待全部处理执行结束:
- dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
- dispatch_group_t group = dispatch_group_create();
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- dispatch_group_async(group, queue, ^{NSLog(@“bll0”);});
- dispatch_group_async(group, queue, ^{NSLog(@“bll1”);});
- dispatch_group_async(group, queue, ^{NSLog(@“bll2”);});
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- dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
- dispatch_release(group);
注:dispatch_group_wait 的返回值为0,标识全部处理执行结束;
返回值不为0,表示经过了指定时间,属于Dispatch Group 的某一个处理还在执行中;
由于 DISPATCH_TIME_FOREVER 标识永久等待,故而返回值恒为0;
dispatch_group_wait 可能会造成当前线程停止,直至所有处理执行结束。
7. dispatch_barrier_async
在多个并行处理之间插入指定处理后再继续多个并行处理。
例如:
- dispatch_queue_t queue = dispatch_queue_create(“com.example.queue”, DISPATCH_QUEUE_CONCURRENT);
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- dispatch_async(queue, blk0_for_reading);
- dispatch_async(queue, blk1_for_reading);
- dispatch_async(queue, blk2_for_reading);
- dispatch_async(queue, blk3_for_reading);
- dispatch_barrier_async(queue, blk_for_writing);
- dispatch_async(queue, blk4_for_reading);
- dispatch_async(queue, blk5_for_reading);
- dispatch_async(queue, blk6_for_reading);
- dispatch_async(queue, blk7_for_reading);
- dispatch_async(queue, blk8_for_reading);
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- dispatch_release(queue);
使用 Concurrent Dispatch Queue 和 dispatch_barrier_async 可实现高效率的数据库访问和文件访问。
8. dispatch_apply
该函数按指定的次数将指定的Block追加到指定的Dispatch Queue中,并等待全部处理执行结束。
例如:
- dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
- dispatch_apply(5, queue, ^(size_t index){ NSLog(@“%zu”, index); });
- NSLog(@“done”);
9. dispatch_suspend / dispatch_resume
当追加大量处理到Dispatch Queue时,在追加处理的过程中,希望不执行一追加的处理,例如验算结果被Block截获时,一些处理会对这个演算结果造成影响。
可使用以上两个函数挂起和恢复queue。
10. Dispatch Semaphore
该函数是比 dispatch_barrier_async() 更细粒度的排他控制。
先看一下例子:
- dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
- NSMutableArray *array = [[NSMutableArray alloc] init];
- for(int i = 0; i< 100000; ++i)
- {
- dispatch_async(queue, ^{ [array addObject:[NSNumber numberWithInt:i]]; });
- }
以上代码执行后由内存错误导致应用异常结束的概率很高。
Dispatch Semaphore 是持有计数的信号。当计数为0时,表示等待;计数为1或者大于1时,减去1而不等待;
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- dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
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- long result = dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
- if (result == 0)
- {
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- }else{
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- }
修改后的例子:
- dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
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- dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
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- NSMutableArray *array = [[NSMutableArray alloc] init];
- for(int i = 0; i< 100000; ++i)
- {
- dispatch_async(queue, ^{
- dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
- [array addObject:[NSNumber numberWithInt:i]];
- });
- }
- dispatch_release(semaphore);
11. dispatch_once
dispatch_once 函数保证在应用程序执行中只执行一次指定处理的API;
例如:
- static int initialized = NO;
- if (initialized == NO)
- {
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- initialized = YES;
- }
可转变成:
- static dispatch_once_t pred;
- dispatch_once(&pred, ^{
-
- });