字符串为空的几种判断方式

- (BOOL)isBlankString:(NSString *)string {
    if (string == nil || string == NULL) {
        return YES;
    }
    
    if ([string isKindOfClass:[NSNull class]]) {
        return YES;
    }
    
    if ([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0) {
        return YES;
    }
    
    if ([string isEqualToString:@""]) {
        return YES;
    }
    
    //(null)
    if (string == nil) {
        return YES;
    }
    
    //<null>
    if ([string isEqual:[NSNull null]]) {
        return YES;
    }
    
    return NO;
    
}