I have a JSON array that I parse. I parse the values from the key Staff, however sometimes the Staff object contain no values;
Expected return;
But sometimes it returns:
Which causes the app to crash because key[#"staff"][#"staff_id"] doesnt exist.
Ive tried:
if (key[#"staff"][#"staff_id"]){
//Parse staff
}
else{
//staff is empty
}
But this crashes as well because I think it still looks for the [#"staff_id"] which doesnt exist.
I've also tried
if (![key[#"staff"][#"staff_id"] isKindOfClass:[NSNull class]])
And
if (![key[#"staff"] isKindOfClass:[NSNull class]])
Any help is greatly appreciated. :)
That's a great example of shitty backend.
On the first example staff is a Dictionary, on the second example is an Array.
You need to ask your backend developer, to decide and always return either Array, or Dictionary.
BTW, you can workaround it
if ([key[#"staff"] isKindOfClass:[NSDictionary class]] && key[#"staff"][#"staff_id"]) {
id staffId = key[#"staff"][#"staff_id"];
} else {
// Staff is empty
}
You will only get object of class NSNull if the JSON contained a null value. For example a dictionary { "key": null } will contain a key/value pair with a key "key" and a value [NSNull null]. Instead of using "isKindOfClass" you can compare with [NSNull null], because there is only ever one NSNull object. If your JSON doesn't contain null values, that won't happen.
If your JSON sometimes contains a dictionary, and sometimes an array, well that's tough. Blame the guys creating the JSON. You can write for example:
id keyObject = ...;
NSDictionary* keyDictionary = keyObject;
NSArray* keyArray = keyArray;
if ([keyDictionary isKindOfClass:[NSDictionary class]]) {
.. you've got a dictionary
} else if ([keyArray isKindOfClass [NSArray class]]) {
.. you've got an array
} else if (keyObject == nil) {
.. there wasn't actually any key object
} else if (keyObject == [NSNull null]) {
.. your JSON contained "key": null
} else {
.. your JSON contained a string, number, or boolean value
}
Related
//result: ok fine
NSString *email = [dataDic objectForKey:#"email"];
if([email isEqualToString:#"null"])
email = nil;
if((![email length]) == 0)
self.emailLbl.text = email;
// result: not fine
NSString *email = [dataDic objectForKey:#"email"];
if((![email length]) == 0 || (email != nil) )
self.emailLbl.text = email;
In dictionary, email property value contained null value. My question is what is the type of nil? -> if comparison email == nil || email == [NSNull null] ?
There are several values that are different from Objective-C perspective here:
nil (aka null in other languages)
[NSNull null] (a special marker value object)
#"" (empty string)
#"null" (just a string with 4 characters)
If you write your dataDic from your app, and you know that your app handles it well, you don't have to check all the cases. Check only the ones you expect. For example, if your app only writes non-empty strings to the dictionary, but sometimes "email" is not there, you only have to check nil, because objectForKey returns nil if the value is not inside the dictionary.
On the other hand if you have obtained dataDic from a 3rd party API, decoded from JSON for example, then you should do the full checking:
[NSNull null] is placed inside the dictionary if JSON has null originally like {"email":null}
If the server API changed you might get some other structure than NSString there (although quite unlikely here).
nil is returned if you don't have the key/value at all.
You can rule out all the 3 checks at once by doing:
NSString *emailStr = nil;
id emailObj = [dataDic objectForKey:#"email"];
if ([emailObj isKindOfClass:[NSString class]]) {
emailStr = emailObj;
}
Note that you might not have to check for an empty string or nil before assigning to UILabel text, because those work fine and just erase the label text:
self.emailLbl.text = #"";
self.emailLbl.text = nil; // another way to erase
You should check whether the key #"email" contain any value or not like (if its a String)
NSString *email=dic[#"email"];
if (email) {
//do anything with email
}
if you want to check your object is Nil Or Null then do
if (!email || email == (id)[NSNull null]) {
//email unavailable
}
Alternately you can call [Obj isKindOfClass:[NSNull class]] on any object.
I get an array of dictionaries back from reading json off a web server and use the following to make sure I got a particular key in the first dictionary in the array before getting its int value:
if([jsonObject[0] objectForKey:#"votes"]!= nil)
{
int votes = [[jsonObject[0] objectForKey:#"votes"] intValue];
[[UserObject userUnique] updateVotes:votes];
}
However, my app still occasionally crashes saying I have called intValue on Null. I have also tried structuring the control statement as
if([jsonObject[0] objectForKey:#"votes"])
but this also leads to the same error/app crashing. My syntax seems in line with accepted answers on SO (Check if key exists in NSDictionary is null or not). Any suggestions for what else/how else I should check the existence of key-value pair for applying intvalue?
Thank you for any advice.
There is a difference between nil and null. nil is not an object: it's a special pointer value. null (as retuned by [NSNull null]) is an object: it's needed because it can be stored in containers like NSDictionary.
NSString *votesString = [jsonObject[0] objectForKey:#"votes"];
if (votesString != nil && votesString != [NSNull null])
{
int votes = [votesString intValue];
[[UserObject userUnique] updateVotes:votes];
}
EDIT: An answer to #SunnysideProductions question
The post you mentioned recommends a way of turning null values into nil values by creating a -safeObjectForKey: method. You are not using -safeObjectForKey:, you are using the default -objectForKey: method.
Be consecutive in your code. Don't run with methods. It would be better add more null- and type-checks in particular in working with json. Let's do it:
if (jsonObject && [jsonObject isKindOfClass:[NSArray class]])
{
NSArray *jsonArray=(NSArray *)jsonObject;
if (jsonArray.count>0)
{
id firstObject=jsonArray[0];
if ([firstObject isKindOfClass:[NSDictionary class]])
{
NSDictionary *jsonDict=(NSDictionary *)firstObject;
id votesNumber=jsonDict[#"votes"];
if (votesNumber && [votesNumber isKindOfClass:[NSNumber class]])
{
int votes=[votesNumber intValue];
[[UserObject userUnique] updateVotes:votes];
}
}
}
}
Now the code is more safe. Does it still crash?
When you call objectForKeyin nullable dictionary, app gets crashed so I fixed this from following way.
- (instancetype)initWithDictionary:(NSDictionary*)dictionary {
id object = dictionary;
if (dictionary && (object != [NSNull null])) {
self.name = [dictionary objectForKey:#"name"];
self.age = [dictionary objectForKey:#"age"];
}
return self;
}
I've got an API returning a JSON encoded string of data that returns a real number or "null" as a value. As long as the JSON contains a numeric or string value, everything works as expected. If the key:value pair value is null, the code below crashes.
How do I properly test NSDictionary objectForKey when it's getting a NULL from SBJSON?
When the API returns a null for filetype, the code below crashes at the if() line.
My Objective-C code attempts to test for expected values:
if (1 == [[task valueForKey:#"filetype"] integerValue]) {
// do something
} else {
// do something else
}
The API JSON output:
{"tclid":"3","filename":null,"filetype":null}
The NSLog() output of the NSDictionary is:
task {
filename = "<null>";
filetype = "<null>";
tclid = 3;
}
When transferring data from JSON to a Cocoa collection, the NSNull class is used to represent "no value", since Cocoa collections can't have empty slots. <null> is how NSNull prints itself.
To test for this, you can use someObject == [NSNull null]. It's a singleton -- there's only one instance of NSNull per process -- so pointer comparison works, although you may prefer to follow the usual Cocoa comparison convention and use [someObject isKindOfClass:[NSNull class]].
You're getting the crash because you're sending integerValue to that NSNull object. NSNull doesn't respond to integerValue and raises an exception.
You should first test if there is a value is null, if it is null performing the intValue method may crash your application.
Doing this should do.
if ([[task valueForKey:#"filetype"] isKindOfClass:[NSNumber Class]] && 1 == [[task valueForKey:#"filetype"] integerValue]) {
// do something
} else {
// do something else
}
I hope it helps.
I have set up my simple Xcode project with a table that is binded to an array controller. It works fine if the array controller is full of entities with a string attribute. However I want to change the attribute to a BOOL and have the table show the string "true" or "false" based on the BOOL.
I have overrided the following two methods from NSFormatter:
-(NSString*) stringForObjectValue:(id)object {
//what is the object?
NSLog(#"object is: %#", object);
if(![object isKindOfClass: [ NSString class ] ] ) {
return nil;
}
//i'm tired....just output hello in the table!!
NSString *returnStr = [[NSString alloc] initWithFormat:#"hello"];
return returnStr;
}
-(BOOL)getObjectValue: (id*)object forString:string errorDescription:(NSString**)error {
if( object ) {
return YES;
}
return NO;
}
So the table gets populated with "hello" if the attribute is a string however if I switch it to a boolean, then the table gets populated with lots of blank spaces.
I don't know if this helps but on the line where I'm outputting the object, it outputs __NSCFString if the attribute is a string and "Text Cell" if I switch the attribute to a boolean. This is something else I don't understand.
Ok, it's not 100% clear what you're trying to do from the code, but first things first - BOOL is not an object, it's basically 0 or 1, so to place BOOL values into an array, you're probably best off using NSNumber:
NSNumber *boolValue = [NSNumber numberWithBool:YES];
and placing these into your array. Now you want to change your method:
-(NSString*) stringForObjectValue:(id)object {
NSNumber *number = (NSNumber *)object;
if ([number boolValue] == YES)
return #"true";
else
return #"false";
}
There's a few things here - for example, you want to avoid passing around id references if you can (if you know all your objects in the NSArray are NSNumber, you shouldn't need to).
I need to check if an dict has a key or not. How?
objectForKey will return nil if a key doesn't exist.
if ([[dictionary allKeys] containsObject:key]) {
// contains key
}
or
if ([dictionary objectForKey:key]) {
// contains object
}
More recent versions of Objective-C and Clang have a modern syntax for this:
if (myDictionary[myKey]) {
}
You do not have to check for equality with nil, because only non-nil Objective-C objects can be stored in dictionaries(or arrays). And all Objective-C objects are truthy values. Even #NO, #0, and [NSNull null] evaluate as true.
Edit: Swift is now a thing.
For Swift you would try something like the following
if let value = myDictionary[myKey] {
}
This syntax will only execute the if block if myKey is in the dict and if it is then the value is stored in the value variable. Note that this works for even falsey values like 0.
if ([mydict objectForKey:#"mykey"]) {
// key exists.
}
else
{
// ...
}
When using JSON dictionaries:
#define isNull(value) value == nil || [value isKindOfClass:[NSNull class]]
if( isNull( dict[#"my_key"] ) )
{
// do stuff
}
I like Fernandes' answer even though you ask for the obj twice.
This should also do (more or less the same as Martin's A).
id obj;
if ((obj=[dict objectForKey:#"blah"])) {
// use obj
} else {
// Do something else like creating the obj and add the kv pair to the dict
}
Martin's and this answer both work on iPad2 iOS 5.0.1 9A405
One very nasty gotcha which just wasted a bit of my time debugging - you may find yourself prompted by auto-complete to try using doesContain which seems to work.
Except, doesContain uses an id comparison instead of the hash comparison used by objectForKey so if you have a dictionary with string keys it will return NO to a doesContain.
NSMutableDictionary* keysByName = [[NSMutableDictionary alloc] init];
keysByName[#"fred"] = #1;
NSString* test = #"fred";
if ([keysByName objectForKey:test] != nil)
NSLog(#"\nit works for key lookups"); // OK
else
NSLog(#"\nsod it");
if (keysByName[test] != nil)
NSLog(#"\nit works for key lookups using indexed syntax"); // OK
else
NSLog(#"\nsod it");
if ([keysByName doesContain:#"fred"])
NSLog(#"\n doesContain works literally");
else
NSLog(#"\nsod it"); // this one fails because of id comparison used by doesContain
Using Swift, it would be:
if myDic[KEY] != nil {
// key exists
}
Yes. This kind of errors are very common and lead to app crash. So I use to add NSDictionary in each project as below:
//.h file code :
#interface NSDictionary (AppDictionary)
- (id)objectForKeyNotNull : (id)key;
#end
//.m file code is as below
#import "NSDictionary+WKDictionary.h"
#implementation NSDictionary (WKDictionary)
- (id)objectForKeyNotNull:(id)key {
id object = [self objectForKey:key];
if (object == [NSNull null])
return nil;
return object;
}
#end
In code you can use as below:
NSStrting *testString = [dict objectForKeyNotNull:#"blah"];
For checking existence of key in NSDictionary:
if([dictionary objectForKey:#"Replace your key here"] != nil)
NSLog(#"Key Exists");
else
NSLog(#"Key not Exists");
Because nil cannot be stored in Foundation data structures NSNull is sometimes to represent a nil. Because NSNull is a singleton object you can check to see if NSNull is the value stored in dictionary by using direct pointer comparison:
if ((NSNull *)[user objectForKey:#"myKey"] == [NSNull null]) { }
Solution for swift 4.2
So, if you just want to answer the question whether the dictionary contains the key, ask:
let keyExists = dict[key] != nil
If you want the value and you know the dictionary contains the key, say:
let val = dict[key]!
But if, as usually happens, you don't know it contains the key - you want to fetch it and use it, but only if it exists - then use something like if let:
if let val = dict[key] {
// now val is not nil and the Optional has been unwrapped, so use it
}
I'd suggest you store the result of the lookup in a temp variable, test if the temp variable is nil and then use it. That way you don't look the same object up twice:
id obj = [dict objectForKey:#"blah"];
if (obj) {
// use obj
} else {
// Do something else
}
if ([MyDictionary objectForKey:MyKey]) {
// "Key Exist"
}
As Adirael suggested objectForKey to check key existance but When you call objectForKeyin nullable dictionary, app gets crashed so I fixed this from following way.
- (instancetype)initWithDictionary:(NSDictionary*)dictionary {
id object = dictionary;
if (dictionary && (object != [NSNull null])) {
self.name = [dictionary objectForKey:#"name"];
self.age = [dictionary objectForKey:#"age"];
}
return self;
}
if ( [dictionary[#"data"][#"action"] isKindOfClass:NSNull.class ] ){
//do something if doesn't exist
}
This is for nested dictionary structure