[fpc-devel] Reading empty collection from stream

Marc Weustink marc.weustink at cuperus.nl
Mon Sep 11 14:34:23 CEST 2006

Michael Van Canneyt wrote:
> On Thu, 7 Sep 2006, Marc Weustink wrote:
>> Vincent Snijders wrote:
>>> Hi,
>>> I am investigating Lazarus issue 7305. The TSynEdit class has a 
>>> KeyStrokes property of the type TSynEditKeyStrokes, which is a 
>>> TCollection descendant. The TSynEdit constructor fills the collection 
>>> with some default items. If I remove the items and stream the 
>>> TSynEdit, the following line is shown in the lfm:
>>>     Keystrokes = <>
>>> If I read this lfm, I don't get a empty KeyStrokes collection, but 
>>> one filled with the default value.
>>> I suspect the following lines in TReader.ReadCollection cause this 
>>> behaviour:
>>>     if not EndOfList then
>>>       Collection.Clear;
>>> Is this a bug or by design? How do I load an empty collection from a 
>>> stream, if the collection has a default value?
> You can't.
>> Delphi has the same construction, which is IMO a design flaw.
>> IMO when a collection is "default" is shouldn't be streamed, so there 
>> would be nothing written.
> There is no way to know whether the collection is default or not, since 
> that would imply keeping a copy of the collection as it was just after 
> creation.
>> I don't know why it is there, since when a "default" collection is 
>> written, all elements are written. So it never will be <>.
>>> Note, if I remove the 'Keystrokes = <>' line from the lfm, I will get 
>>> the default collection too, so I wonder why there are two ways to get 
>>> the default collection.
>> I propose to remove the "if not EndOfList then Collection.Clear;" line.
> You may not,

I always can propose ;-)

> because then inherited form streaming will be broken: a 
> collection of "<>" means: don't touch, keep the collection of
> the parent form.

There you have a point. Didn't think of that.

And indeed, a collection itself doesn't know if it is default or not. 
The owning class usually knows this.
To solve this problem, I think the owning class needs to know when it is 
loading or not, so that it can add the defaults when it is not loaded, 
but created.
I think the cleanest way is to introduce a Loading virtual method as 
counterpart of the Loaded method. This way the owning class can decide 
that when loading, the collection should be cleared.
AFAIK, there is no other way a component knows on creation if it gets 
loaded in the future or not.


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