NSThread, GCD

NSThread

有三種方式可以開Thread

1. NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(test:) object:nil];

2. [NSThread detachNewThreadSelector:@selector(test:) toTarget:self withObject:@"分离子线程"];

   [self performSelectorInBackground:@selector(test:) withObject:@"后台线程"];

第一種方式可以取得該線程的instance，也可以設定Thread Priority，但要手動啟動線程

第二三種方式直接開啟線程，但無法取得instance

performSelector是比較早期的API，跟NSThreadㄧ樣呼叫一次就開一個線程 

目前Object-C大多使用GCD或是NSOperationQueue，這兩個API會自行管理線程數量

文章來源：https://www.jianshu.com/p/0d4812426f72

GCD

分作兩種模式

1. dispatch_async ：非同步
2. dispatch_sync：同步

同步模式下會先等待Block裡面的動作執行完，才繼續往下跑

個人感覺有點類似C#的async Task 搭配 await語法，會把工作丟給別的Thread去做，但會等工作做完才繼續動作

實際上使用dispatch_sync後，發現動作並沒有在其他Thread執行，而是使用main thread，爬文之後發現有人在討論這個問題

文章來源：https://stackoverflow.com/questions/26020656/dispatch-sync-always-execute-block-in-main-thread

也就是說因為main queue是serial queue，呼叫dispatch_sync必定導致線程block住，無所謂執行block內工作的線程，所以程式會直接使用main queue執行，節省開線程的資源

如果今天是在背景呼叫dispatch_sync把工作丟給main queue，就會有線程切換的動作

簡單來說 dispatch_sync 不會開新線程，但在特定情況下，會有線程切換

另外使用dispatch_async也不一定呼叫幾次就開啟多少線程，程式會自行控制

推測應該跟C#一樣使用thread pool的概念
有文章說線程上限66個，serial 與 concurrent共用這數量

使用GCD要小心Block的問題，千萬不要在同步模式（dispatch_sync) 下丟一個Block給自己的線程，自己等自己會卡住

dispatch_queue 可以分為兩種，serial queue與concurrent queue
main_queue：serial queue
global_queue：於concurrent queue
用dispatch_queue_t可以自行宣告，自行決定類型

GCD使用方法

	//sync呼叫 會在main queue執行
	dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
	for(int i=0;i<5;i++){
	NSLog(@"sync: %d %@",i, [NSThread currentThread]);
	}
	});
	//2018-04-30 16:42:57.905789+0800 NSTimer_NSRunLoop[4045:160977] sync: 0 <NSThread: 0x604000065540>{number = 1, name = main}
	//2018-04-30 16:42:57.905875+0800 NSTimer_NSRunLoop[4045:160977] sync: 1 <NSThread: 0x604000065540>{number = 1, name = main}
	//2018-04-30 16:42:57.905904+0800 NSTimer_NSRunLoop[4045:160977] sync: 2 <NSThread: 0x604000065540>{number = 1, name = main}
	//2018-04-30 16:42:57.905929+0800 NSTimer_NSRunLoop[4045:160977] sync: 3 <NSThread: 0x604000065540>{number = 1, name = main}
	//2018-04-30 16:42:57.905951+0800 NSTimer_NSRunLoop[4045:160977] sync: 4 <NSThread: 0x604000065540>{number = 1, name = main}
	sleep(1);
	//async global queue
	//global queue都是concurrent，所以印出來的結果會是亂序
	for(int i=0;i<5;i++){
	dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
	NSLog(@"global: %d %@",i, [NSThread currentThread]);
	});
	}
	//2018-04-30 16:42:58.907260+0800 NSTimer_NSRunLoop[4045:161039] global: 3 <NSThread: 0x604000460240>{number = 5, name = (null)}
	//2018-04-30 16:42:58.907258+0800 NSTimer_NSRunLoop[4045:161045] global: 1 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	//2018-04-30 16:42:58.907299+0800 NSTimer_NSRunLoop[4045:161061] global: 4 <NSThread: 0x60000007d080>{number = 6, name = (null)}
	//2018-04-30 16:42:58.907258+0800 NSTimer_NSRunLoop[4045:161040] global: 2 <NSThread: 0x60000007cf00>{number = 4, name = (null)}
	//2018-04-30 16:42:58.907258+0800 NSTimer_NSRunLoop[4045:161038] global: 0 <NSThread: 0x604000460040>{number = 2, name = (null)}
	sleep(1);
	//自行建立serial queue，會按照順序執行工作
	dispatch_queue_t serialQueue = \
	dispatch_queue_create("thread4", DISPATCH_QUEUE_SERIAL);
	for(int i=0;i<5;i++){
	dispatch_async(serialQueue, ^{
	NSLog(@"serial queue %d %@",i, [NSThread currentThread]);
	});
	}
	//2018-04-30 16:42:59.908440+0800 NSTimer_NSRunLoop[4045:161045] serial queue 0 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	//2018-04-30 16:42:59.908514+0800 NSTimer_NSRunLoop[4045:161045] serial queue 1 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	//2018-04-30 16:42:59.908541+0800 NSTimer_NSRunLoop[4045:161045] serial queue 2 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	//2018-04-30 16:42:59.908561+0800 NSTimer_NSRunLoop[4045:161045] serial queue 3 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	//2018-04-30 16:42:59.908579+0800 NSTimer_NSRunLoop[4045:161045] serial queue 4 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	sleep(1);
	//自行建立concurrent queue
	dispatch_queue_t concurrentQueue = \
	dispatch_queue_create("thread5", DISPATCH_QUEUE_CONCURRENT);
	for(int i=0;i<5;i++){
	dispatch_async(concurrentQueue, ^{
	NSLog(@"concurrent queue %d %@",i, [NSThread currentThread]);
	});
	}
	//2018-04-30 16:43:00.909678+0800 NSTimer_NSRunLoop[4045:161045] concurrent queue 0 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}
	//2018-04-30 16:43:00.909678+0800 NSTimer_NSRunLoop[4045:161040] concurrent queue 2 <NSThread: 0x60000007cf00>{number = 4, name = (null)}
	//2018-04-30 16:43:00.909746+0800 NSTimer_NSRunLoop[4045:161061] concurrent queue 3 <NSThread: 0x60000007d080>{number = 6, name = (null)}
	//2018-04-30 16:43:00.909678+0800 NSTimer_NSRunLoop[4045:161038] concurrent queue 1 <NSThread: 0x604000460040>{number = 2, name = (null)}
	//2018-04-30 16:43:00.909757+0800 NSTimer_NSRunLoop[4045:161045] concurrent queue 4 <NSThread: 0x60000007d5c0>{number = 3, name = (null)}

view raw GCD.m hosted with ❤ by GitHub

當函式使用非同步模式，又必須回傳其運作結果時，需要使用dispatch_semaphore_t
類似於async await，之後會詳細描述

http://www.iosxxx.com/blog/2016-06-02-GCD那些事.html