我对多线程如何与委托(delegate)一起工作感到困惑。
主线程有一个对象“A”,它创建了一个对象“B”。对象“A”是对象“B”的委托(delegate)。对象“B”使用线程来运行代码。
当对象“B”想要通知委托(delegate)时,它会:
[[self delegate] performSelectorOnMainThread:@selector(didFinish:) withObject:self waitUntilDone:[NSThread isMainThread]];
“委托(delegate)”属性是一个分配的、原子的@property。因此,根据 objective c manual,生成的 getter 将执行 [[delegate retain] autorelease]。 .
“A”的 dealloc 方法是:
- (void)dealloc
{
[b setDelegate:nil];
[b release];
[super dealloc];
}
这似乎会导致线程像这样运行的可能情况:
- 主线程:调用 [A dealloc](由于调用 [a release])
- 其他线程:b调用[A retain](由于调用[self delegate])
- 主线程:调用 [b setDelegate:nil]
- 其他线程:调用performSelectorOnMainThread
在第 2 步,似乎 retain 无法成功,因为 dealloc 已经提交 - 这是竞争条件吗?如果你在一个正在被释放的对象上调用 retain 会发生什么?它真的会发生吗?
如果是竞争条件,带委托(delegate)的多线程对象通常如何避免?
(这源于我之前提出的一个稍微相似但更简单的问题/答案,how to handle setDelegate with multiple threads。)
更新
正如已接受的答案所证明的那样,这是一个竞争条件。
我原来问题的解决方案是一起避免这种情况,我已经更新了 How to handle setDelegate: when using multipe threads来展示这一点。
最佳答案
我认为 dealloc
与 retain
/release
没有锁。下面的例子有一个 dealloc
方法,其中有一个 sleep()
(有人知道 sleep()
是否会破坏锁吗?我不知道认为它确实如此,但你永远不会知道)。一个更好的例子可能是重复实例化/销毁 A 和 B 的实例,直到出现此处提到的情况,没有 sleep()
。
View Controller ,在我的例子中,但可以是任何东西:
-(void)septhreadRetainDel
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSLog(@"[thread#2] sleep(1.f);");
sleep(1.f);
NSLog(@"[thread#2] [b retainDelegate];");
[b retainDelegate];
NSLog(@"[thread#2] sleep(2.f);");
sleep(2.f);
NSLog(@"[thread#2] [b release];");
[b release];
[pool release];
}
- (void)viewDidLoad {
NSLog(@"-(void)viewDidLoad:");
[super viewDidLoad];
NSLog(@"a = [[A alloc] init];");
a = [[A alloc] init];
NSLog(@"[a autorelease];");
[a autorelease];
NSLog(@"b = [[B alloc] init];");
b = [[B alloc] init];
NSLog(@"b.delegate = a;");
b.delegate = a;
NSLog(@"[NSThread detachNewThreadSelector:@selector(septhreadRetainDel) toTarget:self withObject:nil];");
[NSThread detachNewThreadSelector:@selector(septhreadRetainDel) toTarget:self withObject:nil];
}
答:
#import "A.h"
@implementation A
-(void)dealloc
{
NSLog(@"A: dealloc; zzz for 2s");
sleep(2.f);
NSLog(@"A: dealloc; waking up in time for my demise!");
[super dealloc];
}
-(id)retain
{
NSLog(@"A retain (%d++>%d)", self.retainCount, self.retainCount+1);
return [super retain];
}
-(void)release
{
NSLog(@"A release (%d-->%d)", self.retainCount, self.retainCount-1);
[super release];
}
@end
乙(.h):
#import "A.h"
@interface B : NSObject {
A *delegate;
}
-(void) retainDelegate;
@property (nonatomic, assign) A *delegate;
@end
B (.m):
#import "B.h"
@implementation B
@synthesize delegate;
-(void)retainDelegate
{
NSLog(@"B:: -(void)retainDelegate (delegate currently has %d retain count):", delegate.retainCount);
NSLog(@"B:: [delegate retain];");
[delegate retain];
}
-(void)releaseDelegate
{
NSLog(@"B releases delegate");
[delegate release];
delegate = nil;
}
-(void)dealloc
{
NSLog(@"B dealloc; closing shop");
[self releaseDelegate];
[super dealloc];
}
-(id)retain
{
NSLog(@"B retain (%d++>%d)", self.retainCount, self.retainCount+1);
return [super retain];
}
-(void)release
{
NSLog(@"B release (%d-->%d)", self.retainCount, self.retainCount-1);
[super release];
}
@end
程序最终在 B 的 releaseDelegate 方法处崩溃并返回 EXC_BAD_ACCESS
。以下是 NSLogs 的输出:
2010-07-10 11:49:27.044 race[832:207] -(void)viewDidLoad:
2010-07-10 11:49:27.050 race[832:207] a = [[A alloc] init];
2010-07-10 11:49:27.053 race[832:207] [a autorelease];
2010-07-10 11:49:27.056 race[832:207] b = [[B alloc] init];
2010-07-10 11:49:27.058 race[832:207] b.delegate = a;
2010-07-10 11:49:27.061 race[832:207] [NSThread detachNewThreadSelector:@selector(septhreadRetainDel) toTarget:self withObject:nil];
2010-07-10 11:49:27.064 race[832:4703] [thread#2] sleep(1.f);
2010-07-10 11:49:27.082 race[832:207] A release (1-->0)
2010-07-10 11:49:27.089 race[832:207] A: dealloc; zzz for 2s
2010-07-10 11:49:28.066 race[832:4703] [thread#2] [b retainDelegate];
2010-07-10 11:49:28.072 race[832:4703] B:: -(void)retainDelegate (delegate currently has 1 retain count):
2010-07-10 11:49:28.076 race[832:4703] B:: [delegate retain];
2010-07-10 11:49:28.079 race[832:4703] A retain (1++>2)
2010-07-10 11:49:28.081 race[832:4703] [thread#2] sleep(2.f);
2010-07-10 11:49:29.092 race[832:207] A: dealloc; waking up in time for my demise!
2010-07-10 11:49:30.084 race[832:4703] [thread#2] [b release];
2010-07-10 11:49:30.089 race[832:4703] B release (1-->0)
2010-07-10 11:49:30.094 race[832:4703] B dealloc; closing shop
2010-07-10 11:49:30.097 race[832:4703] B releases delegate
Program received signal: “EXC_BAD_ACCESS”.
一旦 -dealloc
被调用,保留计数就不再重要。该对象将被销毁(这可能是显而易见的,但我想知道如果您检查了自己的 retainCount 并且如果对象保留了不调用 [super dealloc] 会发生什么……疯狂的想法)。现在,如果我们修改 A 的 -dealloc
以首先将 B 的委托(delegate)设置为 nil
,则程序可以运行,但这只是因为我们没有设置 delegate
在 releaseDelegate
中的 B 中。
我不知道这是否真的回答了你的问题,但假设 sleep() 不会以某种方式破坏线程锁,当 dealloc
在保留
。
关于iphone - 竞争条件与否?委托(delegate)和多线程,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/3216888/