iOS多線程操作時一些要注意的安全問題
這次STMAssembleView https://github.com/ming1016/STMAssembleView 加入異步解析上線后發現一些線程安全方面的問題，現總結下。 先看看這段代碼
- (void)viewDidLoad {
[super viewDidLoad];
self.asStr = @"string is very first";
[self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSLog(@"before sleep %@",self.asStr);
sleep(3);
NSLog(@"after sleep %@",self.asStr);
});
}
- (void)doSomething {
self.asStr = @"string has changed";
}
執行結果
2016-08-31 20:58:43.927 HomePageTest[68018:535737] before sleep string is very first
2016-08-31 20:58:46.927 HomePageTest[68018:535737] after sleep string has changed
會發現在異步執行中如果asStr改變了，那么異步線程里的asStr也會改變這樣就沒法保證異步對資源獨占操作。
如果在異步block里創建一個str賦值如下代碼
- (void)viewDidLoad {
[super viewDidLoad];
self.asStr = @"string is very first";
[self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSString *str = self.asStr;
NSLog(@"before sleep %@",str);
sleep(3);
NSLog(@"after sleep %@",str);
});
}
- (void)doSomething {
self.asStr = @"string has changed";
}
執行結果
2016-08-31 20:59:50.094 HomePageTest[68075:537624] before sleep string is very first
2016-08-31 20:59:53.097 HomePageTest[68075:537624] after sleep string is very first
這樣新的string就不會受到外部改變的影響，但是如果在這個賦值時刻self.asStr已變成野指針那么后面的操作還是會出錯，雖然這樣情況不是那么容易出現。
如何防止這種情況呢，可以看看下面的代碼
- (void)viewDidLoad {
[super viewDidLoad];
self.asStr = @"string is very first";
[self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];
__weak __typeof(self.asStr) weakString = self.asStr;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
__strong __typeof(weakString) strongString = weakString;
if(strongString) {
NSLog(@"before sleep %@",strongString);
sleep(3);
NSLog(@"after sleep %@",strongString);
}
});
}
- (void)doSomething {
self.asStr = @"string has changed";
}
執行結果
2016-08-31 21:00:24.457 HomePageTest[68131:538976] before sleep string is very first
2016-08-31 21:00:27.461 HomePageTest[68131:538976] after sleep string is very first
weakString會在self.asStr釋放時置為nil，如果不是nil時，能夠確保對象在block調用的完整周期里面被retain，如果被搶占對strongString的執行會繼續并且會產生一樣的值，如果strongString執行到時是nil，那么block不能正確執行前已經返回，這樣就不會出現先前那樣的問題。
還可以用鎖來保證多個線程對一份資源在操作時不會被更改
@interface HomeViewController () {
pthread_mutex_t _mutex;
}
@end
@implementation HomeViewController
- (void)viewDidLoad {
[super viewDidLoad];
pthread_mutex_init(&_mutex, NULL);
self.asStr = @"string is very first";
[self performSelector:@selector(doSomething) withObject:nil afterDelay:0.5];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
pthread_mutex_lock(&_mutex);
self.asStr = @"string has changed in global queue";
NSLog(@"before sleep %@",self.asStr);
sleep(3);
NSLog(@"after sleep %@",self.asStr);
pthread_mutex_unlock(&_mutex);
});
}
- (void)doSomething {
pthread_mutex_lock(&_mutex);
self.asStr = @"string has changed";
pthread_mutex_unlock(&_mutex);
}
- (void)dealloc
{
pthread_mutex_destroy(&_mutex);
}
執行結果
2016-08-31 21:01:00.351 HomePageTest[68174:540038] before sleep string has changed in global queue
2016-08-31 21:01:03.354 HomePageTest[68174:540038] after sleep string has changed in global queue
在RAC中使用的是OSSpinLock來保證線程安全的，不過幾位蘋果工程師在swift-dev郵件列表中討論weak屬性的線程安全問題的郵件里爆出自旋鎖有bug，郵件地址： https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html 。大概就是不同優先級線程調度算法會有優先級反轉問題，比如低優先級獲鎖訪問資源，高優先級嘗試訪問時會等待，這時低優先級又沒法爭過高優先級導致任務無法完成lock釋放不了。也可以看看ReactiveCo社區的討論 https://github.com/ReactiveCocoa/ReactiveCocoa/issues/2619
本來OSSpinLock是性能最高的鎖，但是由于如果不在同一個優先級線程進行鎖操作就不能保證安全，那么dispatch_semaphore和pthread_mutex這種僅次于自旋鎖的可以作為替代方案。我注意到非死book的KVOController在2016年5月17日時的一個Commit里將所有OSSpinLock替換成了pthread_mutex，可參看這個commit https://github.com/非死book/KVOController/commit/4f5c329b26f48b151eed82da085288763e2e1761 。pthread_mutex會在新系統中性能得到很大的提升，所以可以考慮這個方案。
來自：http://www.starming.com/index.php?v=index&view=102