NSString is empty

问题:

How do you test if an NSString is empty? or all whitespace or nil? with a single method call?

回答1:

You can try something like this:

@implementation NSString (JRAdditions)

+ (BOOL)isStringEmpty:(NSString *)string {
   if([string length] == 0) { //string is empty or nil
       return YES;
   } 

   if(![[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length]) {
       //string is all whitespace
       return YES;
   }

   return NO;
}

@end

Check out the NSString reference on ADC.

回答2:

This is what I use, an Extension to NSString:

+ (BOOL)isEmptyString:(NSString *)string;
// Returns YES if the string is nil or equal to @""
{
    // Note that [string length] == 0 can be false when [string isEqualToString:@""] is true, because these are Unicode strings.

    if (((NSNull *) string == [NSNull null]) || (string == nil) ) {
        return YES;
    }
    string = [string stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];

    if ([string isEqualToString:@""]) {
        return YES;
    }

    return NO;  
}

回答3:

I use,

+ (BOOL ) stringIsEmpty:(NSString *) aString {

    if ((NSNull *) aString == [NSNull null]) {
        return YES;
    }

    if (aString == nil) {
        return YES;
    } else if ([aString length] == 0) {
        return YES;
    } else {
        aString = [aString stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
        if ([aString length] == 0) {
            return YES;
        }
    }

    return NO;  
}

+ (BOOL ) stringIsEmpty:(NSString *) aString shouldCleanWhiteSpace:(BOOL)cleanWhileSpace {

    if ((NSNull *) aString == [NSNull null]) {
        return YES;
    }

    if (aString == nil) {
        return YES;
    } else if ([aString length] == 0) {
        return YES;
    } 

    if (cleanWhileSpace) {
        aString = [aString stringByTrimmingCharactersInSet: [NSCharacterSet whitespaceAndNewlineCharacterSet]];
        if ([aString length] == 0) {
            return YES;
        }
    }

    return NO;  
}

回答4:

I hate to throw another log on this exceptionally old fire, but I'm leery about editing someone else's answer - especially when it's the selected answer.

Jacob asked a follow up question: How can I do this with a single method call?

The answer is, by creating a category - which basically extends the functionality of a base Objective-C class - and writing a "shorthand" method for all the other code.

However, technically, a string with white space characters is not empty - it just doesn't contain any visible glyphs (for the last couple of years I've been using a method called isEmptyString: and converted today after reading this question, answer, and comment set).

To create a category go to Option+Click -> New File... (or File -> New -> File... or just command+n) -> choose Objective-C Category. Pick a name for the category (this will help namespace it and reduce possible future conflicts) - choose NSString from the "Category on" drop down - save the file somewhere. (Note: The file will automatically be named NSString+YourCategoryName.h and .m.)

I personally appreciate the self-documenting nature of Objective-C; therefore, I have created the following category method on NSString modifying my original isEmptyString: method and opting for a more aptly declared method (I trust the compiler to compress the code later - maybe a little too much).

Header (.h):

#import <Foundation/Foundation.h>

@interface NSString (YourCategoryName)

/*! Strips the string of white space characters (inlcuding new line characters).
 @param string NSString object to be tested - if passed nil or @"" return will
     be negative
 @return BOOL if modified string length is greater than 0, returns YES; 
 otherwise, returns NO */
+ (BOOL)visibleGlyphsExistInString:(NSString *)string;

@end

Implementation (.m):

@implementation NSString (YourCategoryName)

+ (BOOL)visibleGlyphsExistInString:(NSString *)string
{
    // copying string should ensure retain count does not increase
    // it was a recommendation I saw somewhere (I think on stack),
    // made sense, but not sure if still necessary/recommended with ARC
    NSString *copy = [string copy];

    // assume the string has visible glyphs
    BOOL visibleGlyphsExist = YES;
    if (
        copy == nil
        || copy.length == 0
        || [[copy stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0
        ) {
        // if the string is nil, no visible characters would exist
        // if the string length is 0, no visible characters would exist
        // and, of course, if the length after stripping the white space
        // is 0, the string contains no visible glyphs
        visibleGlyphsExist = NO;

    }
    return visibleGlyphsExist;

}

@end

To call the method be sure to #import the NSString+MyCategoryName.h file into the .h or .m (I prefer the .m for categories) class where you are running this sort of validation and do the following:

NSString* myString = @""; // or nil, or tabs, or spaces, or something else
BOOL hasGlyphs = [NSString visibleGlyphsExistInString:myString];

Hopefully that covers all the bases. I remember when I first started developing for Objective-C the category thing was one of those "huh?" ordeals for me - but now I use them quite a bit to increase reusability.

Edit: And I suppose, technically, if we're stripping characters, this:

[[copy stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length] == 0

Is really all that is needed (it should do everything that category method does, including the copy), but I could be wrong on that score.

回答5:

I'm using this define as it works with nil strings as well as empty strings:

#define STR_EMPTY(str)  \
    str.length == 0

Actually now its like this:

#define STR_EMPTY(str)  \
    (![str isKindOfClass:[NSString class]] || str.length == 0)

回答6:

Based on the Jacob Relkin answer and Jonathan comment:

@implementation TextUtils

    + (BOOL)isEmpty:(NSString*) string {

        if([string length] == 0 || ![[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] length]) {
            return YES;
        }

        return NO;
    }

    @end

回答7:

Should be easier:

if (![[string stringByReplacingOccurencesOfString:@" " withString:@""] length]) { NSLog(@"This string is empty"); }

回答8:

Maybe you can try something like this:

+ (BOOL)stringIsEmpty:(NSString *)str
{
    return (str == nil) || (([str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]).length == 0);
}