我得到ARC编译器的以下警告:
"performSelector may cause a leak because its selector is unknown".
这是我正在做的:
[_controller performSelector:NSSelectorFromString(@"someMethod")];
为什么我得到这个警告?我理解编译器不能检查选择器是否存在,但为什么会导致泄漏?我怎样才能改变我的代码,让我不再得到这个警告呢?
我得到ARC编译器的以下警告:
"performSelector may cause a leak because its selector is unknown".
这是我正在做的:
[_controller performSelector:NSSelectorFromString(@"someMethod")];
为什么我得到这个警告?我理解编译器不能检查选择器是否存在,但为什么会导致泄漏?我怎样才能改变我的代码,让我不再得到这个警告呢?
当前回答
下面是基于上面给出的答案更新的宏。这应该允许您使用return语句来包装代码。
#define SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING(code) \
_Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \
code; \
_Pragma("clang diagnostic pop") \
SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING(
return [_target performSelector:_action withObject:self]
);
其他回答
下面是基于上面给出的答案更新的宏。这应该允许您使用return语句来包装代码。
#define SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING(code) \
_Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \
code; \
_Pragma("clang diagnostic pop") \
SUPPRESS_PERFORM_SELECTOR_LEAK_WARNING(
return [_target performSelector:_action withObject:self]
);
这里有很多答案,但由于这个有点不同,我把几个答案结合起来,我想我应该把它放进去。我使用一个NSObject类别检查,以确保选择器返回void,也抑制编译器警告。
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import "Debug.h" // not given; just an assert
@interface NSObject (Extras)
// Enforce the rule that the selector used must return void.
- (void) performVoidReturnSelector:(SEL)aSelector withObject:(id)object;
- (void) performVoidReturnSelector:(SEL)aSelector;
@end
@implementation NSObject (Extras)
// Apparently the reason the regular performSelect gives a compile time warning is that the system doesn't know the return type. I'm going to (a) make sure that the return type is void, and (b) disable this warning
// See http://stackoverflow.com/questions/7017281/performselector-may-cause-a-leak-because-its-selector-is-unknown
- (void) checkSelector:(SEL)aSelector {
// See http://stackoverflow.com/questions/14602854/objective-c-is-there-a-way-to-check-a-selector-return-value
Method m = class_getInstanceMethod([self class], aSelector);
char type[128];
method_getReturnType(m, type, sizeof(type));
NSString *message = [[NSString alloc] initWithFormat:@"NSObject+Extras.performVoidReturnSelector: %@.%@ selector (type: %s)", [self class], NSStringFromSelector(aSelector), type];
NSLog(@"%@", message);
if (type[0] != 'v') {
message = [[NSString alloc] initWithFormat:@"%@ was not void", message];
[Debug assertTrue:FALSE withMessage:message];
}
}
- (void) performVoidReturnSelector:(SEL)aSelector withObject:(id)object {
[self checkSelector:aSelector];
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
// Since the selector (aSelector) is returning void, it doesn't make sense to try to obtain the return result of performSelector. In fact, if we do, it crashes the app.
[self performSelector: aSelector withObject: object];
#pragma clang diagnostic pop
}
- (void) performVoidReturnSelector:(SEL)aSelector {
[self checkSelector:aSelector];
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
[self performSelector: aSelector];
#pragma clang diagnostic pop
}
@end
为了让Scott Thompson的宏更通用:
// String expander
#define MY_STRX(X) #X
#define MY_STR(X) MY_STRX(X)
#define MYSilenceWarning(FLAG, MACRO) \
_Pragma("clang diagnostic push") \
_Pragma(MY_STR(clang diagnostic ignored MY_STR(FLAG))) \
MACRO \
_Pragma("clang diagnostic pop")
然后这样使用它:
MYSilenceWarning(-Warc-performSelector-leaks,
[_target performSelector:_action withObject:self];
)
我的猜测是这样的:因为选择器对编译器是未知的,ARC不能强制执行适当的内存管理。
事实上,有时内存管理通过特定的约定与方法的名称绑定在一起。具体来说,我在考虑方便构造函数和make方法;前者按照约定返回一个自动释放的对象;后者是一个保留对象。该约定基于选择器的名称,因此如果编译器不知道选择器,那么它就不能强制执行正确的内存管理规则。
如果这是正确的,我认为你可以安全地使用你的代码,只要你确保内存管理一切正常(例如,你的方法不返回它们分配的对象)。
为了子孙后代着想,我决定参加竞选:)
最近,我看到越来越多的重构偏离了目标/选择器范式,而倾向于协议、块等。然而,我已经使用过几次的performSelector有一个替代物:
[NSApp sendAction: NSSelectorFromString(@"someMethod") to: _controller from: nil];
这些似乎是一个干净的,弧安全的,和几乎相同的替代performSelector没有太多的objc_msgSend()。
不过,我不知道iOS上是否有类似的功能。