项目中使用到了从字符串创建选择器,编译时发现警告:”performSelector may cause a leak because its selector is unknown”(因为performSelector的选择器未知可能会引起泄漏),为什么在ARC模式下会出现这个警告?
经过搜索后,在Stackoverflow上发现了一个令人满意的答案。见http://stackoverflow.com/questions/7017281/performselector-may-cause-a-leak-because-its-selector-is-unknown。
在ARC模式下,运行时需要知道如何处理你正在调用的方法的返回值。这个返回值可以是任意值,如void,int,char,NSString,id等等。ARC通过头文件的函数定义来得到这些信息。所以平时我们用到的静态选择器就不会出现这个警告。因为在编译期间,这些信息都已经确定。
如:
...
[someController performSelector:@selector(someMethod)];
...
- (void)someMethod
{
//bla bla...
}
而使用
[someController performSelector: NSSelectorFromString(@"someMethod")];
时ARC并不知道该方法的返回值是什么,以及该如何处理?该忽略?还是标记为ns_returns_retained
还是ns_returns_autoreleased
?
SEL selector = NSSelectorFromString(@"someMethod");
IMP imp = [_controller methodForSelector:selector];
void (*func)(id, SEL) = (void *)imp;
func(_controller, selector);
当有额外参数时,如
SEL selector = NSSelectorFromString(@"processRegion:ofView:");
IMP imp = [_controller methodForSelector:selector];
CGRect (*func)(id, SEL, CGRect, UIView *) = (void *)imp;
CGRect result = func(_controller, selector, someRect, someView);
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
[someController performSelector: NSSelectorFromString(@"someMethod")]
#pragma clang diagnostic pop
通过使用#pragma clang diagnostic push/pop
,你可以告诉Clang编译器仅仅为某一特定部分的代码来忽视特定警告。
如果需要忽视的警告有多处,可以定义一个宏
#define SuppressPerformSelectorLeakWarning(Stuff) \ do { \ _Pragma("clang diagnostic push") \ _Pragma("clang diagnostic ignored \"-Warc-performSelector-leaks\"") \ Stuff; \ _Pragma("clang diagnostic pop") \ } while (0)
在产生警告也就是performSelector的地方用使用该宏,如
SuppressPerformSelectorLeakWarning(
[_target performSelector:_action withObject:self]
);
如果需要performSelector返回值的话,
id result;
SuppressPerformSelectorLeakWarning(
result = [_target performSelector:_action withObject:self]
);
[self performSelector:aSelector withObject:nil afterDelay:0.0];
如果在接受范围内,允许在下一个runloop执行,可以这么做。xCode5没问题,但据反映,xCode6的话这个不能消除警告。
文章转载自:原文链接