Allow LazyLoad to use a custom ComponentObserver and visibility ranges

[markhammond]

Firstly, thank you for sharing LazyLoad and ComponentObserver. Working well with resize events after adapting LazyLoad to test partial visibility

Would be nice to make the default ComponentObserver policy used by LazyLoad more explicit. The default policy implies a threshold range of [0, 1] and requires observed components to become fully visible. For my case it was necessary to consider partially visible components, using a threshold range of 0.0 (consistent with the w3c default policy) and relax the test for fully visible components.

[vaadin-miki]

hello @markhammond and thank you for using my components! I am glad you found them useful :)

this sounds like a great feature to have, I will look into it - if it is not too much for my skills, I will include it in the next release (hopefully later this month) - otherwise it will require some time to be done

[vaadin-miki]

@markhammond current development branch has this feature merged, please feel free to try it out before I make a new release some time in the coming week - and of course please suggest improvements or report errors :)

basically how it works is that there is an additional parameter that can be passed to a LazyLoad constructor and that parameter is a Supplier<ComponentObserver> - by default it is simply ComponentObserver::new, but now you should be able to pass custom visibility ranges (new LazyLoad(content, () -> new ComponentObserver(start, end));)

[markhammond]

Nice @vaadin-miki . The remaining issue is that isFullyVisible() tests for a visible proportion of 1.0. In my case I wanted to allow for partial visibility, that is, not equal to zero (consistent with the w3c default threshold of 0.0).

It is difficult to customise the current behaviour as the method performs the check is private.

[vaadin-miki]

@markhammond alrighty, let me think of a nice api for that, I am thinking of allowing custom triggers for considering if a component is visible or not, based on the ComponentObservationEvent (with defaults being isFullyVisible and isNotVisible)

[vaadin-miki]

@markhammond alrighty, current development now contains LazyLoad#setContentVisibilityRanges(forHiding, forShowing), which should be what you need :) by default the ranges are 0 and 1, so hiding on fully invisible and showing on fully visible, but now it should be possible to trigger hide/show on 0 and 0.5, for example

new release coming when #502 is done