Added callbacks for backdrop events

[vlytvyne]

I find it very useful to know when the front layer expands or collapses. It might come in handy if you want to change or do smt reactively. In my case, I needed to change toolbar title when state of frontLayer changes, so I added this feature.

[daadu]

There are some changes to be done:

• rename the callbacks
• proper indentations
• callback only when the event happens (put inside the if condition)

[daadu]

add trailing comma

[daadu]

refactor it

[daadu]

@WieFel Please review this? I think this feature is useful.

[daadu]

@vlytvyne refactor properly

[vlytvyne]

I doubt if onFrontLayerVisible is an appropriate variable name. onExpanded in my opinion is way more familiar and intuitive, also it's unclear what onFrontLayerVisible means as front layer is always visible at least partly. I'll rename variables, but write if u changed ur mind.

[daadu]

@vlytvyne We already expose showFrontLayer and showBackLayer helper methods with Backdrop.of(context), then nomenclature is to keep consistency with that.

Yes it is not the most appropriate. But if we are planning to change the names then it should be everywhere - keeping backward compatibility in mind

I would say following names would be most appropriate.

• showFrontLayer -> backdropClose
• showBackLayer -> backdropOpen
• onFrontLayerVisible -> onBackdropClose
• onBackLayerVisible -> onBackdropOpen

@WieFel What do you think about it?

[vlytvyne]

Please note that "backdrop" it's a composition of both front and back layers so backdropOpen and similar variants are puzzling.

From https://material.io/components/backdrop#usage :

"A backdrop is composed of two surfaces: a back layer and a front layer. The back layer displays actions and context, and these control and inform the front layer's content."

[daadu]

@vlytvyne

MDG uses following terms for this - "Revealing the back layer" and "Concealing the back layer".

Keeping that in mind, the name should be as following:

• showFrontLayer -> concealBackLayer
• showBackLayer -> revealBackLayer
• onFrontLayerVisible -> onBackLayerConcealed
• onBackLayerVisible -> onBackLayerRevealed

[vlytvyne]

I definitely like it better this way.

[daadu]

@WieFel Let us know your views.

[daadu]

@vlytvyne @WieFel I have done the changes. Please check.

[WieFel]

I think this is a good improvement! :)

There is only one slight change I'd make:
I would change the occurrences of

  @Deprecated("..."
      "This feature was deprecated after v1.0.0.")

to "deprecated after v0.3.3", because this should indicate from which version on the feature was deprecated, or did I get that wrong?

[daadu]

@WieFel Yeah, you are right.

[daadu]

@WieFel Done requested changes.

[daadu]

This feature have landed in pub.dev with v0.3.3.

@vlytvyne Check yourself out in Contributors Section in README.md!!