iOS中多線程編程工具主要有:
- NSThread
- NSOperation
- GCD
這三種方法都簡單易用,各有千秋.但無疑GCD是最有誘惑力的,因爲其本身是apple爲多核的並行運算提出的解決方案.雖然當前移動平臺用雙核的不多,但不影響GCD作爲多線程編程的利器(ipad2已經是雙核了,這無疑是一個趨勢).
http://www.cnblogs.com/scorpiozj/archive/2011/07/25/2116459.html
GCD是和block緊密相連的,所以最好先了解下block(可以查看這裏).GCD是C level的函數,這意味着它也提供了C的函數指針作爲參數,方便了C程序員.
一、下面首先來看GCD的使用:
dispatch_async(dispatch_queue_t queue, dispatch_block_t block);
async表明異步運行,block代表的是你要做的事情,queue則是你把任務交給誰來處理了.(除了async,還有sync,delay,本文以async爲例).
之所以程序中會用到多線程是因爲程序往往會需要讀取數據,然後更新UI.爲了良好的用戶體驗,讀取數據的操作會傾向於在後臺運行,這樣以避免阻塞主線程.GCD裏就有三種queue來處理.
先來介紹一下 Main queue:
顧名思義,運行在主線程,由dispatch_get_main_queue獲得.和ui相關的就要使用Main Queue.
//GCD下載圖片刷新主界面的例子
/*
- (IBAction)touchUpInsideByThreadOne:(id)sender {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];
NSData * data = [[NSData alloc]initWithContentsOfURL:url];
UIImage *image = [[UIImage alloc]initWithData:data];
if (data != nil) {
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = image;
});
}
});
}*/
通過與線程池的配合,dispatch queue分爲下面兩種:而系統默認就有一個串行隊列main_queue和並行隊列global_queue:
- Serial Dispatch Queue -- 線程池只提供一個線程用來執行任務,所以後一個任務必須等到前一個任務執行結束才能開始。
- Concurrent Dispatch Queue -- 線程池提供多個線程來執行任務,所以可以按序啓動多個任務併發執行。
而系統默認就有一個串行隊列main_queue和並行隊列global_queue:
dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t mainQ = dispatch_get_main_queue();
通常,我們可以在global_queue中做一些long-running的任務,完成後在main_queue中更新UI,避免UI阻塞,無法響應用戶操作:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// long-running task
dispatch_async(dispatch_get_main_queue(), ^{
// update UI
});
});
1.Serial quque(private dispatch queue)
每次運行一個任務,可以添加多個,執行次序FIFO. 通常是指程序員生成的,比如:
NSDate *da = [NSDate date]; NSString *daStr = [da description]; const char *queueName = [daStr UTF8String]; dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_PRIORITY_DEFAULT);
下面還是下載圖片例子:
- (IBAction)touchUpInsideByThreadOne:(id)sender {
NSDate *da = [NSDate date];
NSString *daStr = [da description];
const char *queueName = [daStr UTF8String];
dispatch_queue_t myQueue = dispatch_queue_create(queueName, NULL);
dispatch_async(myQueue, ^{
NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];
NSData * data = [[NSData alloc]initWithContentsOfURL:url];
UIImage *image = [[UIImage alloc]initWithData:data];
if (data != nil) {
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = image;
});
}
});
dispatch_release(myQueue);
}
爲了驗證Serial queue的FIFO特性,寫了如下的驗證代碼:發現的確是順序執行的。
- (IBAction)touchUpInsideByThreadOne:(id)sender {
NSDate *da = [NSDate date];
NSString *daStr = [da description];
const char *queueName = [daStr UTF8String];
dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_SERIAL);
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:6];
NSLog(@"[NSThread sleepForTimeInterval:6];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"[NSThread sleepForTimeInterval:3];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"[NSThread sleepForTimeInterval:1];");
});
dispatch_release(myQueue);
}
運行結果爲:
2013-07-24 16:37:14.397 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:6];
2013-07-24 16:37:17.399 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:3];
2013-07-24 16:37:18.401 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:1];
3. Concurrent queue(global dispatch queue):
可以同時運行多個任務,每個任務的啓動時間是按照加入queue的順序,結束的順序依賴各自的任務.使用dispatch_get_global_queue獲得.
- (IBAction)touchUpInsideByThreadOne:(id)sender {
dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:6];
NSLog(@"[NSThread sleepForTimeInterval:6];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"[NSThread sleepForTimeInterval:3];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"[NSThread sleepForTimeInterval:1];");
});
dispatch_release(myQueue);
}
運行的結果爲:
2013-07-24 16:38:41.660 NSThreadAndBlockDemo[1944:12e03] [NSThread sleepForTimeInterval:1];
2013-07-24 16:38:43.660 NSThreadAndBlockDemo[1944:12b03] [NSThread sleepForTimeInterval:3];
2013-07-24 16:38:46.660 NSThreadAndBlockDemo[1944:12303] [NSThread sleepForTimeInterval:6];
二、dispatch_group_async的使用
dispatch_group_async可以實現監聽一組任務是否完成,完成後得到通知執行其他的操作。這個方法很有用,比如你執行三個下載任務,當三個任務都下載完成後你才通知界面說完成的了。下面是一段例子代碼:
- (IBAction)touchUpInsideByThreadOne:(id)sender {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:6];
NSLog(@"group1 [NSThread sleepForTimeInterval:6];");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"group2 [NSThread sleepForTimeInterval:3];");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"group3 [NSThread sleepForTimeInterval:1];");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"main thread.");
});
dispatch_release(group);
}
執行結果爲:
2013-07-24 16:48:23.063 NSThreadAndBlockDemo[2004:12e03] group3 [NSThread sleepForTimeInterval:1];
2013-07-24 16:48:25.063 NSThreadAndBlockDemo[2004:12b03] group2 [NSThread sleepForTimeInterval:3];
2013-07-24 16:48:28.063 NSThreadAndBlockDemo[2004:12303] group1 [NSThread sleepForTimeInterval:6];
2013-07-24 16:48:28.065 NSThreadAndBlockDemo[2004:11303] main thread.
果然,dispatch_group_async只會監聽最終的結果完成後,並通知main queue,那如果是我們需要順序執行的話呢?請看下面的dispatch_barrier_async。
3、dispatch_barrier_async的使用
dispatch_barrier_async是在前面的任務執行結束後它才執行,而且它後面的任務等它執行完成之後纔會執行
例子代碼如下:
- (IBAction)touchUpInsideByThreadOne:(id)sender {
dispatch_queue_t queue = dispatch_queue_create("gcdtest.rongfzh.yc", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"dispatch_async1");
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"dispatch_async2");
});
dispatch_barrier_async(queue, ^{
NSLog(@"dispatch_barrier_async");
[NSThread sleepForTimeInterval:0.5];
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"dispatch_async3");
});
}
執行結果爲:
2013-07-24 17:01:54.580 NSThreadAndBlockDemo[2153:12b03] dispatch_async2
2013-07-24 17:01:56.580 NSThreadAndBlockDemo[2153:12303] dispatch_async1
2013-07-24 17:01:56.580 NSThreadAndBlockDemo[2153:12303] dispatch_barrier_async
2013-07-24 17:01:58.083 NSThreadAndBlockDemo[2153:12303] dispatch_async3
如果使用dispatch_queue_t
queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);會發現運行結果爲:
2013-07-24 17:07:17.577 NSThreadAndBlockDemo[2247:12e03] dispatch_barrier_async
2013-07-24 17:07:18.579 NSThreadAndBlockDemo[2247:15207] dispatch_async3
2013-07-24 17:07:19.578 NSThreadAndBlockDemo[2247:12b03] dispatch_async2
2013-07-24 17:07:20.577 NSThreadAndBlockDemo[2247:12303] dispatch_async1
說明dispatch_barrier_async的順序執行還是依賴queue的類型啊,必需要queue的類型爲dispatch_queue_create創建的,而且attr參數值必需是DISPATCH_QUEUE_CONCURRENT類型,前面兩個非dispatch_barrier_async的類型的執行是依賴其本身的執行時間的,如果attr如果是DISPATCH_QUEUE_SERIAL時,那就完全是符合Serial queue的FIFO特徵了。
4、dispatch_apply
執行某個代碼片段N次。
dispatch_apply(5, globalQ, ^(size_t index) {
// 執行5次
});
5、dispatch_once
dispatch_once這個函數,它可以保證整個應用程序生命週期中某段代碼只被執行一次!
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
// code to be executed once
});
6、dispatch_after
double delayInSeconds = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
// code to be executed on the main queue after delay
});
dispatch_set_target_queue(serialQ, globalQ);