RFR: 8277785: ListView scrollTo jumps to wrong location when CellHeight is changed [v4]

[eduardsdv]

On Wed, 30 Mar 2022 13:27:40 GMT, Johan Vos <jvos at openjdk.org> wrote:

>> When the size of a ListCell is changed and a scrollTo method is invoked without having a layout calculation in between, the old (wrong) size is used to calculcate the total estimate. This happens e.g. when the size is changed in the `updateItem` method.
>> This PR will immediately resize the cell and sets the new value in the cache containing the cellsizes.
>
> Johan Vos has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Don't shift cells if we are already showing the lowest index cell.

I don't believe that it is possible to position the content and scrollbar **exactly**, if the VirtualFlow **estimates** the size of the cells.
For that case we need an external cell size provider, that I, as an application developer, can provide to the e.g. ListView.

I thought of something like this. The existing estimation logic can then be implemented as a default cell size provider.


interface CellSizeProvider {

 /**
  * Returns the size of all cells.
  * IMPORTANT: Be carefully by implementing this method. It can drastically degrade the performance of a component.
  */
 double getTotalSize();

 /**
  * Returns the size of the cell for an item given by its index. 
  * IMPORTANT: Be carefully by implementing this method. It can drastically degrade the performance of a component.
  */
 double getCellSize(int);

 /**
  * The callback which is set by the VirtualFlow. It allows to calculate the cell size for an item given by its index. 
  * IMPORTANT: This callback returns the exact value, but it may be slow.
  */
 void setCellSizeCalculator(Function<Integer, Double> cellSizeCalculator);

}

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PR: https://git.openjdk.java.net/jfx/pull/712