如何強制一個未使用的變量成爲塊的一部分？

Question

考慮下面的代碼：

- (void)downloadObjectUsingURL: (NSURL *)url; 
{ 
    id file = [self createFileForURL: url]; 
    Finalization objectFinalization = ^() { 
     // we don't access url here, but it would be handy to see it in the debugger 
     [file close]; 
    } 
    id request = [self buildRequestFromURL: url]; 
    [object download: request 
       toFile: file 
     finalization: objectFinalization]; 
} 

什麼是強制url被複製爲objectFinalization的上下文的一部分最簡單的方法，複製並作爲objectFinalization更容易調試的目的，一部分通過呢？

我可以想出幾個可能的答案，但他們沒有一個讓我感覺好。我可以做NSLog(@"%@",url)，但是會導致噴出。我可以做一個NSAssert(url,...);或if (url) {}，但這兩個都沒有真正表達我想要做的事情。

Answer 1

年後，我發現了最簡單的答案。裏面的塊，使用：

(void)variable; 

例子：

- (void)downloadObjectUsingURL: (NSURL *)url; 
{ 
    id file = [self createFileForURL: url]; 
    Finalization objectFinalization = ^() { 
     (void)url; 
     [file close]; 
    } 
    id request = [self buildRequestFromURL: url]; 
    [object download: request 
       toFile: file 
     finalization: objectFinalization]; 
} 

如果只想在調試：

- (void)downloadObjectUsingURL: (NSURL *)url; 
{ 
    id file = [self createFileForURL: url]; 
    Finalization objectFinalization = ^() { 
#if DEBUG 
     (void)url; 
#endif 
     [file close]; 
    } 
    id request = [self buildRequestFromURL: url]; 
    [object download: request 
       toFile: file 
     finalization: objectFinalization]; 
} 

Answer 2

裹在#ifdef debug.. #endif

Answer 3

調用NSLog()我想到了一個更好的答案，但由於@NSResponder讓我覺得是這樣的：

#if DEBUG 
#define FORCE(a) NSParameterAssert(YES || a) 
#else 
#define FORCE(a) 
#endif 

這使得代碼如下所示：

- (void)downloadObjectUsingURL: (NSURL *)url; 
{ 
    id file = [self createFileForURL: url]; 
    Finalization objectFinalization = ^() { 
     FORCE(urL); 
     [file close]; 
    } 
    id request = [self buildRequestFromURL: url]; 
    [object download: request 
       toFile: file 
     finalization: objectFinalization]; 
}