- (void) testStartingOnBackgroundThread
{
XCDYouTubeVideoOperation *operation = [[XCDYouTubeVideoOperation alloc] initWithVideoIdentifier:nil languageIdentifier:nil];
[self keyValueObservingExpectationForObject:operation keyPath:@"isFinished" handler:^BOOL(id observedObject,NSDictionary *change)
{
XCTAssertNil([observedObject video]);
XCTAssertNotNil([observedObject error]);
return YES;
}];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
XCTAssertFalse([NSThread isMainThread]);
[operation start];
});
[self waitForExpectationsWithTimeout:5 handler:nil];
}
当我在Mac上本地运行它时,此测试总是通过,但有时它会因此错误而发生在fails on Travis：
Failed: caught “NSRangeException”,“Cannot remove an observer <_XCKVOExpectation 0x1001846c0> for the key path “isFinished” from <XCDYouTubeVideoOperation 0x1001b9510> because it is not registered as an observer.”
难道我做错了什么？
解决方法
当您调用keyValueObservingExpectationForObject：keyPath：handler：时,会在引擎盖下创建_XCKVOExpectation对象.它负责观察您传递的对象/ keyPath.触发KVO通知后,将调用_safelyUnregister方法,这是删除观察者的位置.这是_safelyUnregister方法的(逆向工程)实现.
@implementation _XCKVOExpectation
- (void) _safelyUnregister
{
if (!self.hasUnregistered)
{
[self.observedObject removeObserver:self forKeyPath:self.keyPath];
self.hasUnregistered = YES;
}
}
@end
在waitForExpectationsWithTimeout：handler：的末尾再次调用此方法,并且在释放_XCKVOExpectation对象时.请注意,操作在后台线程上终止,但测试在主线程上运行.所以你有一个竞争条件：如果在后台线程上将hasUnregistered属性设置为YES之前在主线程上调用了_safelyUnregister,则会删除观察者两次,导致无法删除观察者异常.
因此,为了解决此问题,您必须使用锁保护_safelyUnregister方法.这是一个代码片段供您在测试目标中进行编译,该代码片段将负责修复此错误.
#import <objc/runtime.h>
__attribute__((constructor)) void WorkaroundXCKVOExpectationUnregistrationRaceCondition(void);
__attribute__((constructor)) void WorkaroundXCKVOExpectationUnregistrationRaceCondition(void)
{
SEL _safelyUnregisterSEL = sel_getUid("_safelyUnregister");
Method safelyUnregister = class_getInstanceMethod(objc_lookUpClass("_XCKVOExpectation"),_safelyUnregisterSEL);
void (*_safelyUnregisterIMP)(id,SEL) = (__typeof__(_safelyUnregisterIMP))method_getImplementation(safelyUnregister);
method_setImplementation(safelyUnregister,imp_implementationWithBlock(^(id self) {
@synchronized(self)
{
_safelyUnregisterIMP(self,_safelyUnregisterSEL);
}
}));
}
编辑
这个bug已经是fixed in Xcode 7 beta 4了.
