从NSString中删除最后一个字符

Question

我有一个NSString和我的生活我无法弄清楚如何remove spaces，然后采取last 30 characters它。

这样的错误：

3/5/13 3:38:30 PM *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFType UTF8String]: unrecognized selector sent to instance 0x160bf20'

或

3/5/13 3:30:51 PM *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFType length]: unrecognized selector sent to instance 0x160bf20'

我不明白这些错误。

代码：

NSString* m = (NSString*)CFDictionaryGetValue(diskDesc, kDADiskDescriptionVolumeUUIDKey); 
[m stringByReplacingOccurrencesOfString:@" " withString:@""]  
NSString *newString = [m substringToIndex:[m length]-30]; 

另一个更新：

NSString * m = (NSString*)CFDictionaryGetValue(diskDesc, kDADiskDescriptionDeviceModelKey); 
NSString * v = (NSString*)CFDictionaryGetValue(diskDesc, kDADiskDescriptionVolumeUUIDKey); 

if(m) 
{ 
    m = [m stringByReplacingOccurrencesOfString:@" " withString:@""]; 
} 

if(v) 
{ 
    if([v length] > 30) 
    { 
     NSString *newString = [v substringToIndex:[v length]-30]; 
    } 
} 

Answer 1

你要注意在DADisk.h头文件中的文件，指定导致字典的类型将保持一个特定的键：

extern const CFStringRef kDADiskDescriptionVolumeUUIDKey;  /* (CFUUID  ) */ 

extern const CFStringRef kDADiskDescriptionDeviceModelKey;  /* (CFString ) */ 

所有的密钥本身是NSString/CFString s，但是当您向字典索取该密钥的值时，没有任何说明它必须是字符串。右边括号中的值显示为该键保存的值的类型。

要获得该kDADiskDescriptionVolumeUUIDKey键保存CFUUID的字符串表示，你可以使用CFUUIDCreateString（）：

CFUUIDRef UUID = (CFUUIDRef)[(NSDictionary *)diskDesc 
       objectForKey:(id)kDADiskDescriptionVolumeUUIDKey]; 

NSString *UUIDString = [(id)CFUUIDCreateString(NULL, UUID) autorelease]; 

另外，为了除去在kDADiskDescriptionDeviceModelKey的多余的空格代码应该有可能是以下几点：

NSString *deviceModel = [(NSDictionary *)diskDesc 
       objectForKey:(id)kDADiskDescriptionDeviceModelKey]; 

NSLog(@"deviceModel == '%@'", deviceModel); 

NSCharacterSet *whitespaceCharacterSet = [NSCharacterSet 
            whitespaceAndNewlineCharacterSet]; 

deviceModel = [deviceModel stringByTrimmingCharactersInSet:whitespaceCharacterSet]; 

NSLog(@"deviceModel == '%@'", deviceModel); 

该代码记录了我下面的：

diskArbitrationFinagler[] deviceModel == 'WDC WD6400AAKS-41H2B0     ' 
diskArbitrationFinagler[] deviceModel == 'WDC WD6400AAKS-41H2B0' 

您的stringByReplacingOccurrencesOfString:将错误地删除WDC和WD64....之间的空格。

Answer 2

看起来像错误检查的大好机会。就像这样：

NSString * m = (NSString*)CFDictionaryGetValue(diskDesc, kDADiskDescriptionDeviceModelKey); 
if(m) 
{ 
    m = [m stringByReplacingOccurrencesOfString:@" " withString:@""] 
    if([m length] > 30) 
    { 
     NSString *newString = [m substringToIndex:[m length]-30]; 
    } else { 
     NSLog(@"m isn't even 30 characters"); 
    } 
} else { 
    NSLog(@"I didn't get anything useful from my call to CFDictionaryGetValue"); 
}