objective-c - 如何检测字符串在Obj-C里是否为空？

Question

如何检测NSString在Objective-C里是否为空？

原问题：How do I test if a string is empty in Objective C?

Answer 1

Marc Charbonneau
You can check if [string length] == 0. This will check if it is a valid but empty string (@"") and if it is null. Because calling length with no value will also return 0.

---

Matt G
The answer above is correct. But I would like to take this opportunity to introduce isEmpty summarized by Wil Shipley, which he shared on his blog:

static inline BOOL IsEmpty(id thing) {
return thing == nil
|| ([thing respondsToSelector:@selector(length)]
&& [(NSData *)thing length] == 0)
|| ([thing respondsToSelector:@selector(count)]
&& [(NSArray *)thing count] == 0);
}

The first method is effective, but it will not work if the string has spaces (@" "). So the spaces must be cleared before testing.
The following code can remove all spaces on both sides of the string

[stringObject stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet] ];

A good way is to define a macro so that you don’t have to enter this long string of code:

#define allTrim( object ) [object stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet] ]

Now you can use:

NSString *emptyString = @"   ";
if ( [allTrim( emptyString ) length] == 0 ) NSLog(@"Is empty!");

---

Rob
One of the best workarounds I've seen (better than Matt G's) is this improved inline function, which I found in a Git Hub report:

// Check if the "thing" pass'd is empty
static inline BOOL isEmpty(id thing) {
    return thing == nil
    || [thing isKindOfClass:[NSNull class]]
    || ([thing respondsToSelector:@selector(length)]
        && [(NSData *)thing length] == 0)
    || ([thing respondsToSelector:@selector(count)]
        && [(NSArray *)thing count] == 0);
}

---

user238824
You'd better use this function:

@implementation NSString (Empty)

    - (BOOL) empty{
        return ([[self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]length] == 0);
    }

@end

---

chown
Another option is to use isEqualToString to check if it is equal to @"", like this:

if ([myString isEqualToString:@""]) {
    NSLog(@"myString IS empty!");
} else {
    NSLog(@"myString IS NOT empty, it is: %@", myString);
}

---

Jim Thio
This one I use:

@implementation NSObject (AdditionalMethod)
-(BOOL) isNotEmpty
{
    return !(self == nil
    || [self isKindOfClass:[NSNull class]]
    || ([self respondsToSelector:@selector(length)]
        && [(NSData *)self length] == 0)
    || ([self respondsToSelector:@selector(count)]
        && [(NSArray *)self count] == 0));

};
@end 

The problem is that if self has no value, this function will never be called. It will return false, which is required.

---

Samir Jwarchan
Just use one of the following if else conditions:
Method 1:

<strong>if([yourString isEqualToString:@""]){
        // yourString is empty.
    }
    else {
        // yourString has some text on it.
    } </strong>

Method 2:

if([yourString length] == 0){
    // Empty yourString
}
else {
    // yourString is not empty
}