xcode 使用 performSelector 执行的方法中的 objc_retain 崩溃
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Crash in objc_retain in method performed with performSelector
提问by orj
I have this strange crash relating to ARC auto-inserting objc_retains in my code.
我在我的代码中遇到了与 ARC 自动插入 objc_retains 相关的奇怪崩溃。
I have the following two classes:
我有以下两个类：
@interface MenuItem : NSObject
@property (weak, nonatomic) id target;
@property (unsafe_unretained, nonatomic) SEL action;
@property (strong, nonatomic) id object;
- (instancetype)initWIthTarget:(id)target action:(SEL)action withObject:(id)object;
- (void)performAction;
@end
@implementation MenuItem
- (void)performAction
{
if (self.target && self.action)
{
if (self.object)
{
[self.target performSelector:self.action withObject:self.object];
}
else
{
[self.target performSelector:self.action];
}
}
}
@end
@interface Widget : NSObject
- (void)someMethod:(id)sender;
@end
At some point I instantiate a MenuItem as such:
在某些时候，我将 MenuItem 实例化为：
MenuItem *item = [MenuItem alloc] initWithTarget:widget action:@selector(someMethod:) object:nil];
Then elsewhere I invoke performActionon the menu item:
然后在其他地方我performAction在菜单项上调用：
[item performAction];
In the implementation of someMethodI get a crash:
在执行someMethod我得到一个崩溃：
@implementation Widget
- (void)someMethod:(id)sender
{
// EXEC_BAD_ACCESS crash in objc_retain
}
@end
Why is this happening?
为什么会这样？
回答by orj
The reason for the crash was because I was using the wrong performSelector.
崩溃的原因是因为我使用了错误的performSelector.
NSObjectdefines multiple versions of performSelector. The one I was invoking was:
NSObject定义了多个版本的performSelector. 我调用的是：
- (id)performSelector:(SEL)aSelector;
However the method I was invoking took an idparameter. Eg:
但是，我调用的方法采用了一个id参数。例如：
- (void)someMethod:(id)sender;
Now ARC being the nice safe memory management system that it is tries to ensure that parameters are properly retained during the execution of a method. So even though my someMethod:was empty ARC was producing code that looked like this:
现在 ARC 是一个很好的安全内存管理系统，它试图确保在方法执行期间正确保留参数。因此，即使我someMethod:是空的，ARC也会生成如下所示的代码：
- (void)someMethod:(id)sender
{
objc_retain(sender);
objc_release(sender);
}
The problem with this however was that I was invoking performSelector:and not supplying a value for the senderparameter. So senderwas pointing at random junk on the stack. Therefore when objc_retain()was invoked the app crashed.
然而，问题在于我正在调用performSelector:而不是为sender参数提供值。于是sender指着堆在随机的垃圾。因此，当objc_retain()被调用时，应用程序崩溃了。
If I change:
如果我改变：
MenuItem *item = [[MenuItem alloc] initWithTarget:widget
action:@selector(someMethod:)
object:nil];
to
到
MenuItem *item = [[MenuItem alloc] initWithTarget:widget
action:@selector(someMethod)
object:nil];
and
和
- (void)someMethod:(id)sender;
to
到
- (void)someMethod;
Then the crash goes away.
然后崩溃消失了。
Similarly I can also change
同样我也可以改变
[self.target performSelector:self.action];
to
到
[self.target performSelector:self.action withObject:nil];
if I want to follow the 'standard' form of target-action methods that take a single parameter. The benefit of the second form of performSelectoris that if I'm invoking a method that doesn't take a parameter it will still work fine.
如果我想遵循采用单个参数的目标操作方法的“标准”形式。第二种形式的好处performSelector是，如果我调用一个不带参数的方法，它仍然可以正常工作。
