Implementing Key Value coding in Objective c

Question

I am time implementing the key value coding in my program so i am not much comfortable with it . I am having the controller class in which the parsing methods are implemented in start document i have initialize the object and keys. All data members of class Universal are type id.

            univ=[[Universal alloc] init];
            [univ setValue:univ.datamember1 forKey:@"1"];
            [univ setValue:univ.datamember2 forKey:@"2"];
            [univ setValue:univ.datamember3 forKey:@"3"];
            [univ setValue:univ.datamember4 forKey:@"4"];
            [univ setValue:univ.datamember5 forKey:@"5"];

I am setting the value on data member in end element method :

         id temp1=[univ valueForKey:@"1"];
        temp1=strVal;
        [resultArray addObject:univ];

and i am getting the following exception for this code:

'NSUnknownKeyException', reason: '[<Universal 0x6e994d0> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key 1

what is the reason for this. Is there missing something or everything gone wrong??

Ken Thomases · Accepted Answer · 2012-05-23 10:45:40Z

2

For the most part, you don't have to implement Key-Value Coding. Instead, you create your classes to comply with various conventions and then Key-Value Coding works for it automatically.

The conventions are about how you name methods in order to make a property accessible via KVC.

Your code:

        univ=[[Universal alloc] init];
        [univ setValue:univ.datamember1 forKey:@"1"];
        [univ setValue:univ.datamember2 forKey:@"2"];
        [univ setValue:univ.datamember3 forKey:@"3"];
        [univ setValue:univ.datamember4 forKey:@"4"];
        [univ setValue:univ.datamember5 forKey:@"5"];

is strange. What are you trying to do here? This code looks like it's trying to set properties on the univ object from other properties of the univ object. That would be redundant.

I suspect you think that you're establishing synonyms or aliases here – that you're making key "1" map to the property datamember1. That's not correct. The -setValue:forKey: method is for actually setting a property's value. It's very much like calling a setter method; in fact, it usually does result in calling a setter method.

So, what you've written is very much like the following:

        univ=[[Universal alloc] init];
        [univ set1:univ.datamember1];
        [univ set2:univ.datamember2];
        [univ set3:univ.datamember3];
        [univ set4:univ.datamember4];
        [univ set5:univ.datamember5];

I doubt that's anything that you meant to do. It might help you to explain what you're trying to do in non-KVC terms. What properties are you trying change (if you are)? What values would you like to assign to them? How would you do that using just ordinary setters and getters?

Later, you posted this code:

    id temp1=[univ valueForKey:@"1"];
    temp1=strVal;
    [resultArray addObject:univ];

Again, this is strange and confusing. I suspect you're trying to change a property value on univ, but that's not what that code accomplishes, even if it could be made to work. The first line attempts to get the value of a property of the univ object, where the property's name is "1". That's not a valid name for a property. Anyway, it stores the retrieved value into a local variable temp1. The second line simply throws away the result of the first line and stores a different value into the local temp1 variable. The temp1 variable is independent of the univ object, even though a moment ago it was storing a result retrieved from the univ object. Changing temp1 can't change univ (although messaging the object pointed to by temp1 could change that object and it might also be pointed to by univ).

It seems to me that you aren't yet ready to be using Key-Value Coding. You need a better understanding of the basics. Also, it's almost never necessary to use Key-Value Coding with a static, known-at-compile-time key. Key-Value Coding is for dynamic access to properties of an object when you don't know the name of that property at compile time but you'll have the name as data at run time.

answered May 23, 2012 at 10:45

Ken Thomases

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3 Comments

V-Xtreme Over a year ago

THANK YOU very much for this nice explanation. Actually i am also don't want to use kvc. But in my application i have to use it although i am not comfortable with it. But i will go through it again as you suggested.

Ken Thomases Over a year ago

You're welcome. If you have to use KVC, perhaps you can edit the question (or ask another) to explain why you have to. What is the requirement that you're trying to satisfy? How is KVC going to be used?

Ken Thomases Over a year ago

I found your other question that probably motivated this one and posted an answer there.

NSResponder · Accepted Answer · 2012-05-23 10:30:47Z

0

You haven't implemented setters and getters for the keys you're trying to access, and you haven't implemented -setValue:forUndefinedKey:.

At runtime, NSObject's implementation of -setValue:forKey: looks for a method named "set1:", and if it can't find that, it looks for an ivar named "1" (which isn't a legal name for an ivar, but still), and failing that, it attempts to send -setValue:forUndefinedKey:, which you haven't implemented.

Read the documentation on key-value coding, and find out how to make your class KVC compliant.

answered May 23, 2012 at 10:30

NSResponder

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Ken Thomases · Accepted Answer · 2012-05-23 10:47:56Z

0

Please read the section in Apple's documentation on ensuring KVC compliance.

It says this:

For properties that are an attribute or a to-one relationship, this requires that your class:

Implement a method named -<key>, -is<Key>, or have an instance variable <key> or _<key>.

If the property is mutable, then it should also implement -set<Key>:.

Synthesizing a property also qualifies as implementing a method, so your Universal class needs a method called 1 (and 2 etc) and another method called set1: which it certainly doesn't have since it is illegal to have method names starting with a number.

edited May 23, 2012 at 10:47

Ken Thomases

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answered May 23, 2012 at 10:28

JeremyP

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