本文通過信號量解決了 并發線程，耗時不同，順序結束 問題。
信號量(Semaphore)，有時被稱為信號燈，是在多線程環境下使用的一種設施，是可以用來保證兩個或多個關鍵代碼段不被并發調用。在進入一個關鍵代碼段之前，線程必須獲取一個信號量；一旦該關鍵代碼段完成了，那么該線程必須釋放信號量。其它想進入該關鍵代碼段的線程必須等待直到第一個線程釋放信號量
一、API
1、創建信號量
value : 是一個不小于0的值，否則創建失敗
返回一個dispatch_semaphore_t類型的信號量。
dispatch_semaphore_t dispatch_semaphore_create(long value);
2、等待信號量
信號量-1操作
如果dsema信號量的值大于0，該函數所處線程就繼續執行下面的語句，并且將信號量的值減1；
如果desema的值為0，那么這個函數就阻塞當前線程等待timeout，如果等待期間desema的值被dispatch_semaphore_signal函數加1了，且該函數（即dispatch_semaphore_wait）所處線程獲得了信號量，那么就繼續向下執行并將信號量減1。如果等待期間沒有獲取到信號量或者信號量的值一直為0，那么等到timeout時，其所處線程自動執行其后語句。
dsema ： 信號量
timeout ：超時時間 可以使用DISPATCH_TIME_NOW or DISPATCH_TIME_FOREVER
返回值 : 成功返回0，如果timeout了返回非0。
long dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout);
3、釋放信號量
信號量+1操作
如果之前的值小于0，這個函數在返回之前喚醒了一個等待線程。
如果線程被喚醒，該函數將返回非0。否則返回0。
long dispatch_semaphore_signal(dispatch_semaphore_t dsema);
【重要】 dispatch_semaphore_signal與dispatch_semaphore_wait配對使用，dispatch_semaphore_wait先調用，等待信號量。執行完成之后釋放信號量。
二、應用
1、控制并發數，管理有限資源
線程并不是越多越好，還要考慮到線程資源競爭、內存、CPU等情況。 我們知道NSOperationQueue可以控制最大并發數，那么GCD可不可以呢？
答案是肯定的，使用信號量Dispatch Semaphore實現。
例：某個資源最多只允許6個線程同時訪問
//創建一個value為6的信號量
dispatch_semaphore_t semaphore = dispatch_semaphore_create(6);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//模擬有20個任務 等待執行
for (int i = 0; i < 20; i++)
{
//等待一個信號量
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_async(queue, ^{
NSLog(@"任務%d執行--currentThread%@",i,[NSThread currentThread]);
[NSThread sleepForTimeInterval:arc4random() % 60 / 10.0];
//執行完成之后釋放信號量
dispatch_semaphore_signal(semaphore);
});
}
【鎖】當信號量的值為1時，表示同時只有一個線程可以訪問資源，實現了一種鎖的效果
2、并發線程，耗時不同，順序結束
網上的實現方法，是每個執行完成后的任務，不斷循環判斷上一個任務的狀態。個人覺得不是很好。
現在通過信號量實現如下，
如果哪位大神發現有問題，歡迎批評指正。
【核心思想】利用 dsema 為0時dispatch_semaphore_wait會一直等待，直到dsema大于0，才會繼續執行wait以下的語句。
#import "Semaphore.h"
@interface WorkSigle: NSObject
@property (nonatomic, strong) dispatch_semaphore_t semaphore;
- (void)wait;
- (void)signal;
@end
@implementation WorkSigle
- (instancetype)init
{
self = [super init];
if (self) {
self.semaphore = dispatch_semaphore_create(0);
}
return self;
}
- (void)wait {
dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
};
- (void)signal {
dispatch_semaphore_signal(self.semaphore);
}
@end
@interface Semaphore ()
@end
@implementation Semaphore
- (void)viewDidLoad {
[super viewDidLoad];
[self worker];
return;
}
- (void)worker {
//創建一個保存信號量的數組
//假設有100個任務，那么有99個任務需要等待上一個任務結束
NSMutableArray * array = [NSMutableArray array];
for (int i = 0; i < 99; i++)
{
[array addObject:[WorkSigle new]];
}
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i = 0; i < 100; i++)
{
dispatch_async(queue, ^{
// do task
[NSThread sleepForTimeInterval:arc4random() % 60 / 10.0];
NSLog(@"do task %i",i);
//wait a before task end
if (i == 0) {
//no before task
} else {
[array[i-1] wait];
}
//tell next task i'm end
if (i == 100-1) {
//no next
//call back
NSLog(@"call back %i",i);
} else {
[array[i] signal];
//call back
NSLog(@"call back %i",i);
}
});
}
}
@end
控制臺
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2017-11-13 15:21:59.078835+0800 GCD[17635:828530] call back 90
2017-11-13 15:21:59.078846+0800 GCD[17635:828532] call back 91
2017-11-13 15:21:59.078852+0800 GCD[17635:828533] call back 92
2017-11-13 15:21:59.078862+0800 GCD[17635:828534] call back 93
2017-11-13 15:21:59.078867+0800 GCD[17635:828535] call back 94
2017-11-13 15:21:59.280196+0800 GCD[17635:828536] do task 95
2017-11-13 15:21:59.280386+0800 GCD[17635:828536] call back 95
2017-11-13 15:21:59.280394+0800 GCD[17635:828537] call back 96
2017-11-13 15:21:59.280411+0800 GCD[17635:828538] call back 97
2017-11-13 15:21:59.280416+0800 GCD[17635:828539] call back 98
2017-11-13 15:21:59.280422+0800 GCD[17635:828540] call back 99
