[Review Request] Support fixed cell heights in JavaFX UI controls

[Jonathan Giles]

Hi all,

https://javafx-jira.kenai.com/browse/RT-30476

In our ongoing effort to make JavaFX fast, especially on constrained 
systems such as the Raspberry Pi, I'd like to propose the addition of 
API to the ListView, TreeView, TreeTableView and TableView controls that 
allows for developers to specify a fixed row height for all rows. Should 
a developer set this, they would be explicitly telling JavaFX that their 
cells will not dynamically resize. In return for giving up this 
functionality, JavaFX can do two things that massively helps performance:

1) The virtualisation code can short-circuit all height measurements 
(and many resizing operations). This saves JavaFX from having to perform 
massive amounts of string measurement, which is a big bottleneck.

2) The TableView and TreeTableView controls can virtualise in the 
horizontal direction. In other words, for all columns that are not 
visible (as the width of all columns exceeds the width of the TableView 
itself), the cells contained within obstructed columns will be removed 
from the scenegraph (and added back in as necessary as the user scrolls 
horizontally). This saves JavaFX from having to perform CSS / layout on 
all of these cells. The reason why this is enabled only with fixed cell 
height is because we can not know whether cells outside the visible area 
have an impact on the row height.

In our tests enabling a fixed row height has massive gains in runtime 
and startup performance. It won't be useful to everyone, but I would 
imagine that it would be applicable for most people. However, because 
enabling this by default would potentially break a lot of people, this 
functionality will _not_ be enabled by default.

I'm slightly conscious of the fact that using the term 'fixed cell 
_height_' is confusing in the case of a ListView which is laid out 
horizontally. Therefore, my suggested API would be either a 
fixedCellHeight double property or a fixedCellLength double property 
(where length is the term used internally to differentiate from the cell 
breadth). From an ease-of-understanding point of view I much prefer 
fixedCellHeight however - most people won't be using horizontal 
ListViews, so it is unlikely to be confusing. My preferred approach 
would be to have the property default to Region.USE_COMPUTED_SIZE to 
indicate that fixed cell height should not be used. To enable the 
functionality, a positive value can be set. I'm not wed to using 
Region.USE_COMPUTED_SIZE, so if that feels confusing we could just state 
a negative value will disable the value.

Thoughts?

-- 
-- Jonathan