Circling back to a new layout algorithm

[Scott Palmer]

> On Nov 12, 2021, at 7:48 PM, Kevin Rushforth <kevin.rushforth at oracle.com> wrote:
> 
> I think this will be a good improvement to implement to the JavaFX layout algorithm. I see that you filed JDK-8276671 [1] to track this enhancement.
> 
> As noted, the main concern will be making sure we don't break existing applications, so testing this will be key.
> 
> One other issue that will need to be addressed is what a good the threshold would be for the maximum number of iterations, such that if it doesn't converge we will stop anyway (and log a warning). This is a trade-off between performance and accuracy, although as noted in the JBS issue, if the layout doesn't converge it's a bug. The value we ultimately choose can be discussed during the review and testing. Regardless of what the threshold is, I recommend a system property (named something like "com.sun.javafx.scene.layout.threshold"). This will be helpful during testing.
> 
> What do other developers think?

Sounds good to me. I’m all for it.

> Would this be a useful problem to solve in layout?

Absolutely.

I have dialogs that have really irritating layout issues that magically correct themselves when you interact with the controls.

Scott 

> 
> -- Kevin
> 
> [1] https://bugs.openjdk.java.net/browse/JDK-8276671
> 
> 
>> On 11/4/2021 7:31 PM, Michael Strauß wrote:
>> I previously proposed a new iterative layout algorithm [1] that
>> supports baseline alignment and introduces new APIs to give developers
>> control over the way nodes are aligned. This is a solution to the
>> long-standing problem that JavaFX cannot reliably lay out nodes that
>> are aligned on their baseline [2]. The new layout algorithm might also
>> fix some issues where the scene graph layout only settles after
>> interacting with the controls (for example, by clicking).
>> 
>> I've created a small application that shows the new APIs and a
>> correctly working baseline-aligned layout [3]. In addition to that, I
>> also built SceneBuilder with both the old and new layout system, and
>> played around with it to find out whether there were any regressions
>> or visual differences. So far, I haven't found any.
>> 
>> In order to move this forward, I think it would be a good idea to test
>> the latest version of the new layout system in more real-world JavaFX
>> applications. Any help from JavaFX application developers is greatly
>> appreciated. It's as easy as checking out the JavaFX sources from the
>> PR [1], building a local SDK and linking your application with the
>> binaries.
>> 
>> 
>> Finally, here's a high-level overview of the new algorithm:
>> 
>> When Parent::layout() is called on a layout root (i.e. a scene root or
>> an unmanaged node), it will lay out its children in a loop until the
>> scene graph under the layout root is fully laid out, which means it is
>> clean and doesn't require further layout. The totality of layout
>> activity for a single layout root is called "layout cycle". A layout
>> cycle will often take a few layout passes to finish (but not more than
>> 2 in most cases). There is no limit on how often Parent::layout() will
>> iterate to lay out its children, so in principle, this could lead to
>> an infinite layout loop.
>> 
>> One source of infinite layout loops are incorrectly implemented controls:
>> 
>>     class PathologicalControl extends Region {
>>         final Text text = new Text("foo");
>> 
>>         PathologicalControl() {
>>             getChildren().add(text);
>>         }
>> 
>>         @Override
>>         protected void layoutChildren() {
>>             text.relocate(0, text.getLayoutY() + 10);
>>         }
>>     }
>> 
>> In this example, each call to layoutChildren() moves down the text
>> node another 10 pixels from where it was, which causes the layout
>> algorithm to schedule yet another layout pass. It's an infinite loop.
>> 
>> The layout system detects this by tracking how often a node
>> invalidates the scene graph in a single layout cycle. If a node
>> exceeds a threshold of 100 invalidations, it will be suspended from
>> layout and can no longer invalidate the scene graph in the current
>> layout cycle. A warning will be logged to notify developers of that
>> fact.
>> 
>> 
>> [1] https://github.com/openjdk/jfx/pull/433
>> [2] https://bugs.openjdk.java.net/browse/JDK-8090261
>> [3] https://github.com/mstr2/jfx-layout-sample
>