[Development] Binding evaluation during ListView delegate remove animation

[Stephen Kelly]

On 19/01/16 14:17, Stephen Kelly wrote:
>
> Hi,
>
> I have uploaded a testcase to
>
>  https://github.com/ske-ableton/public_sscce/tree/master/delegate_data
>
> which demonstrates a problem that can occur with ListView delegates 
> that can be animated away on removal.
>

I got feedback that what is happening here may not be described clearly 
enough. So, I made a bug report too:

  https://bugreports.qt.io/browse/QTBUG-50542

The remove animation happens as a result of the 
QAbstractItemModel::rowsRemoved signal, so there is a 'tension' between 
the requirements:

 1. An attempt in the delegate to access the data while it fades out.
 2. The row in the model providing access to that data has just been
    removed.

The solution could be to cache data retrieved from the model.

This seems to me to be a design bug in the QML ListView. It seems to be 
a simple enough case (and easy enough to hit) that it should be easy to 
do correctly without the client needing to implement workarounds for it.

I have some ideas on how to implement a fix for this, so that the 
ListView caches data it retrieves from the model.

Before I implement a patch to implement caching, I wonder if anyone has 
any better/alternative ideas for how to fix this, or if you think this 
is 'not a bug' or any other thoughts you have. Some concerns could be 
behavior backward compatibility, API, new behavior in a new QtQuick 2.x 
version of the ListView etc.

Thanks,

Steve.


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