Re: Custom Attribute Questions.....
Re: Custom Attribute Questions.....
- Subject: Re: Custom Attribute Questions.....
- From: Hugi Thordarson <email@hidden>
- Date: Wed, 15 Jun 2005 15:59:29 +0000
The short answer:
Make your attribute type immutable. It is the right thing to do for
such a simple class and will save you a lot of work in the long run.
Plus, you get to keep all the hair you would otherwise lose while
tracking down hard-to-find bugs :-).
The long answer:
I have a framework that uses a mutable attribute type, basically, my
own subclass of NSMutableDictionary. I tried really hard to make the
attribute's mutability work out without having to make any special
acommodations outside of the value class itself, but it really just
doesn't. EOF wants it's attributes nice, clean and immutable.
What I did to get around it was to make my dictionary attribute keep a
reference to it's EO and attribute keypath. So every time the
dictionary is changed, it replaces the EO's instance with a clone of
itself. It works, but it's dirty as hell, slow, and is really only an
option if you have complete control over your framework, i.e. you're
using it in your own application. So, caveats aplenty.
In your class (I dropped accessors/mutators for brevity):
public class EncryptedEOAttribute {
public EOEnterprise Object owningEO;
public String keyPath;
public void setValue( Object newValue ) {
// first do the stuff you need to do with (encrypt) your new value
here, and then...:
if( owningEO != null ) {
owningEO.takeValueForKeyPath( this.clone(), keyPath );
}
}
}
In the EO class:
public class MyEO extends EOGenericRecord {
public void awakeFromInsertion( EOEditingContext ec ) {
super.awakeFromInsertion( ec );
setMyAttribute( new EncryptedEOAttribute() );
myAttribute.owningEO = this;
myAttribute.keyPath = "myAttribute";
}
public void awakeFromFetch( EOEditingContext ec ) {
super.awakeFromFetch( ec );
myAttribute.owningEO = this;
myAttribute.keyPath = "myAttribute";
}
}
Sorry for inflicting this abomination upon you - but you asked :-).
Cheers,
Hugi
// Hugi Thordarson
// Landeyða
// http://hugi.karlmenn.is/
On 15.6.2005, at 15:24, Kieran Kelleher wrote:
I need a little Customer EO Attribute advice so I don't end up missing
the beach this weekend like Chuck Hill did on page 43 of PWO :-)
I have a custom class, EncryptedEOAttribute, that wraps a String ivar
named _decodedValue
The class has value() and setValue() that return and set the raw clear
string.
It has a conversion method public String toEncodedString() that
encodes the string into the database.
It has a factory method that decodes an encoded string from the
database to create an instance (wrapping the clear string):
public static EncryptedEOAttribute createInstanceFromEncodedString(
String encodedString )
It has a display method toString() that masks the clear string by
replacing most of the characters with an asterisk.
In my app, I plan to simply use setValue to update the instance's
_decodedValue ivar.
For a class like this, do I have to worry about the object mutability
snapshot saving problem since changes only occur when setValue is
called and the String ivar _decodedValue is changed?
If so, how would I approach it ..... do I need to design my
EncryptedEOAttribute class to be immutable (basically remove setValue
method) and create a new instance every time a new _decodedValue is to
be stored? Does EOF just compare the object reference to the snapshot
reference or does it use the conversion method return value for
snapshot comparison before deciding to save.
Regards,
-Kieran
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