Thread 0 Crashed:
0 libsystem_kernel.dylib 0x00000001917718e8 __ulock_wait + 8
1 libdispatch.dylib 0x00000001916419d0 _dispatch_unfair_lock_wait + 48
2 libdispatch.dylib 0x0000000191641b6c _dispatch_gate_wait_slow + 88
3 libdispatch.dylib 0x000000019162f710 dispatch_once_f + 124
4 Demo 0x00000001004073b8 +[MyObject sharedUserInstance]
30行代码演示dispatch_once死锁
96 giveme5 关注
2017.05.31 20:25* 字数 143 阅读 45评论 0喜欢 0
如果你遇到了下面这样的崩溃,你可能也遇到了dispatch_once死锁。
Thread 0 Crashed:
0 libsystem_kernel.dylib 0x00000001917718e8 __ulock_wait + 8
1 libdispatch.dylib 0x00000001916419d0 _dispatch_unfair_lock_wait + 48
2 libdispatch.dylib 0x0000000191641b6c _dispatch_gate_wait_slow + 88
3 libdispatch.dylib 0x000000019162f710 dispatch_once_f + 124
4 Demo 0x00000001004073b8 +[MyObject sharedUserInstance]
先别着急问我怎么解决,在xcode中运行一下下面的代码,自己分析一下自然知道怎么解决了。
@interface MyObject : NSObject
+ (instancetype)sharedInstance;
@end
@implementation MyObject
+ (instancetype)sharedInstance{
static id sharedInstance = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedInstance = [[self alloc] init];
});
return sharedInstance;
}
- (instancetype)init{
self = [super init];
if (self) {
[NSThread sleepForTimeInterval:2];//等待2秒
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"永远不会打印,因为死锁了");
});
}
return self;
}
@end
@implementation ViewController
- (IBAction)clicked:(id)sender {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[MyObject sharedInstance];
});
[MyObject sharedInstance];
}
@end
0秒。点击按钮,在global_queue中执行dispatch_once 0.1秒。main_queue阻塞,等待获取once_token 2秒。global_queue结束sleep,执行dispatch_sync,阻塞,等待main_queue。至此死锁发生。