On 23/10/2009, at 12:27 PM, Michael de Haan wrote:
Let me start by saying I **think** I understand why my first
approach did not work, but am interested if my working approach is
correct in concept.
Background.
As a **very long** detour from Hillegass's chapter 18 challenge, I
am following along with mmalc's excellent series of Binding
examples. Thus, now creating a table of 4 columns without bindings.
(He uses 1 column, but I added a few).
In order to create the NSMutableDictionary object, which will be
added to an NSDoc Array ivar, (which will be used by the datasource
methods of a table) I have created a
Record Object with one ivar ( _record ( An NSMutableDictionary).)
which returns:
First approach: ( did **not** work)
-(id) init
{
self = ....snip...usual code
NSString * _name = @"John Doe";
NSString * _city
....snip...other string properties
OK, these are NOT "properties". They are simply local variables.
(Also, don't snip code, it makes it really hard to tell whether what
you've done is right, as you have to mentally put back what should be
there - who knows if it matches what you actually have?)
_record = [ NSMutableDictionary dictionaryWithObjectsAndKeys:
_name, MDHNameKey,
....snip...
nil;
}
return self;
}
As I learnt, self returns an empty object. ( If I understand it
correctly, the local variables _name etc are no longer in scope once
self is returned.)
This should not return an empty object, but it should return one
having the ivar _record. One question is whether you're using garbage
collection or not, because if not, then _record is shortly going to
get autoreleased, leading to a stale reference and almost certain
disaster (unless you've also snipped out vital memory management calls
as well as the closing square bracket).
If you are truly getting an empty object, then you'll have to show all
the code you've snipped to be able to say why.
Second approach.
Record Object with 4 ivars viz
_name
_city etc etc
Self now returns an object whose attributes are now, in the calling
method ( the "AddRecord") used to create the NSMutableDictionary
object and added "model" array, and the table populates as expected.
OK, read this about twenty times now and still have no clear idea what
you mean. What mutable dictionary? Having done it this way, you no
longer need one as far as I can see.
May I ask if this is the correct approach? ...or should one still
try and aim to **return** a dictionary object to the calling method?
Personally I'd say your second approach is more usual. You *could*
store your properties in a dictionary - this storage is an
implementation detail of your record class. However it's usually
easier just to have a separate ivar for each one, which is how the KVC
system is typically implemented. If you exposed your 'record' property
as a property in itself, you could access individual properties using
a keypath such as yourObject.record.someKey, but typically you'd just
want yourObject.someKey so that the implementation of the property
storage is not exposed.
--Graham
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