Objective-C中的全局常量 [英] Global constants in Objective-C

问题描述

我有一个名为 Constants.h 的文件：

  extern NSString * const BASE_URL; 
  

和 Constants.m ：

  #ifdef DEBUG 
 NSString * const BASE_URL = @http://www.example.org; 
 #else 
 NSString * const BASE_URL = @http：// localhost; 
 #endif 
  

第一个问题：如何将DEBUG切换为 True 和 False ？

---

我有一个视图控制器文件 MyViewController.m ：

 ＃导入MyViewController.h
 #importConstants.h
 
 //这不起作用。请参阅上面的错误。 
 static NSString * const ANOTHER_URL = [NSString stringWithFormat：@％@％@，BASE_URL，@/ path /]; 
 
 @implementation HomeViewController 
 
 [...] 
  

代码不起作用，并返回此错误：

 错误：语义问题：初始化元素不是编译时间常数
  

我该如何解决这个问题？

$ b

---

UPDATE 1

我需要将几个全局字符串变量与其他字符串结合起来创建各种url。

现在 Constants.m 是：

  #importConstants.h
 
 #ifdef DEBUG 
 #define DEF_BASE_URLhttp://www.example.org/
 #else 
 #define DEF_BASE_URLhttp：// localhost /
 #endif 
 
 NSString * const BASE_URL =（NSString *）CFSTR（DEF_BASE_URL）; 
 NSString * const API_URL =（NSString *）CFSTR（DEF_BASE_URLapi /）; 
 NSString * const API_SETTINGS_URL =（NSString *）CFSTR（API_URLsettings /）; 
  

但最后一行有错误解析错误：预期'）' STRONG>。
也许我只能用宏使用CFSTR。我需要找到一种方法来获得我所有的全局变量。解决方案A：

p>

就个人而言，我只会使用 ANOTHER_URL 的函数。

<如果你真的想要一个常量：你应该能够通过来使用cstring连接规则， #define ，然后通过 CFSTR（）管道：

  // defs.h 
 extern NSString * const BASE_URL; 
 extern NSString * const ANOTHER_URL; 
 
 // defs.m 
 
 #ifdef DEBUG 
 #define DEF_BASE_URLhttp://www.example.org
 #else 
 #define DEF_BASE_URLhttp：// localhost
 #endif 
 
 NSString * const BASE_URL =（NSString *）CFSTR（DEF_BASE_URL）; 
 NSString * const ANOTHER_URL =（NSString *）CFSTR（DEF_BASE_URL/ path /）; 
  

解决方案C：

如果您想通过初始化创建一个，您还可以在C ++ / ObjC ++翻译中完成一个函数/方法local static（然后在需要时使用C或ObjC可见性）：

  NSString * URL（）{
 static NSString * const ANOTHER_URL = [NSString stringWithFormat：@％@％@，BASE_URL，@/ path / ]; 
返回ANOTHER_URL; 
} 
  

I've a file called Constants.h:

extern NSString * const BASE_URL;

and Constants.m:

#ifdef DEBUG
    NSString * const BASE_URL = @"http://www.example.org ";
#else
    NSString * const BASE_URL = @"http://localhost";
#endif

First question: How can I switch DEBUG to be True and False?

---

I've a view controller file MyViewController.m:

#import "MyViewController.h"
#import "Constants.h"

// this doesn't works. see above for the error.
static NSString * const ANOTHER_URL = [NSString stringWithFormat:@"%@%@", BASE_URL, @"/path/"];

@implementation HomeViewController

[...]

The code doesn't work and returns me this error:

error: Semantic Issue: Initializer element is not a compile-time constant

How can I fix this?

I need to combine several global string variabile with other strings for create various urls.

---

UPDATE 1

Now Constants.m is:

#import "Constants.h"

#ifdef DEBUG
    #define DEF_BASE_URL "http://www.example.org/"
#else
    #define DEF_BASE_URL "http://localhost/"
#endif

NSString * const BASE_URL = (NSString*)CFSTR(DEF_BASE_URL);
NSString * const API_URL = (NSString*)CFSTR(DEF_BASE_URL "api/");
NSString * const API_SETTINGS_URL = (NSString*)CFSTR(API_URL "settings/");

But there is an error on the last line Parse error: expected ')'. Probably I can use CFSTR only with macros. I need to find a way for have all my global variables.

解决方案

Solution A:

Personally, I would just use a function for ANOTHER_URL.

Solution B:

If you really want a constant: You should be able to use cstring concatenation rules via #define, then pipe that through CFSTR():

// defs.h
extern NSString * const BASE_URL;
extern NSString * const ANOTHER_URL;

// defs.m

#ifdef DEBUG
#define DEF_BASE_URL "http://www.example.org"
#else
#define DEF_BASE_URL "http://localhost"
#endif

NSString * const BASE_URL = (NSString*)CFSTR(DEF_BASE_URL);
NSString * const ANOTHER_URL = (NSString*)CFSTR(DEF_BASE_URL "/path/");

Solution C:

If you want to create just one via initialization, you can also accomplish a function/method local static in C++/ObjC++ translations (then use C or ObjC visibility, where needed):

NSString * URL() {
  static NSString * const ANOTHER_URL = [NSString stringWithFormat:@"%@%@", BASE_URL, @"/path/"];
  return ANOTHER_URL;
}

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