How to ignore one of observables in combineLatest, but have its latest available in the combiner function?

Question

I have this real world scenario.

I have a ReaderConfig object, which is a dumb tuple of ReaderTheme, ReaderFont, ReaderFontSize. Its changes will trigger the readerConfigObservable which then redraws stuff.

So the solution sofar is

combineLatest(themeObservable, fontObservable, fontSizeObservable, 
    (theme, font, fontSize) -> new ReaderConfig(theme, font, fontSize))

However, because this is android, ReaderTheme sets some android R.theme. stuff, which need to restart Activity for them to take effect. So I need to expose the themeObservable by it self also

themeObservable.subscribe(__ -> activity.recreate())

Since it restarts the activity, theres no need for it to trigger the combineLatest, which triggers redraw and allocates resources, only to be disposed of 1 second later because of the restart.

So only fontObservable and fontSizeObservable changes should trigger the combineLatest, HOWEVER I do need the latest themeObservable value in order to construct the ReaderConfig object.

My workaround is this

public Observable<ReaderConfig> readerConfigObservable() {
        return Observable.combineLatest(
                mFontRelay, mFontSizeRelay, (__, ___) -> createReaderConfig());
    }

    public ReaderConfig createReaderConfig() {
        return new ReaderConfig(mThemeRelay.getValue(), mFontRelay.getValue(), mFontSizeRelay.getValue());
    }

So basically it pulls the themeObservable in the combiner function which is not really reactive, is there any proper solution for this exception use case?

ESala · Accepted Answer · 2018-01-07T20:09:54.137

5

I think this could be solved using withLatestFrom(...):

Merges the specified ObservableSource into this ObservableSource sequence by using the resultSelector function only when the source ObservableSource (this instance) emits an item.

See the marbles diagram: http://rxmarbles.com/#withLatestFrom

Example:

.combineLatest(fontObservable, fontSizeObservable, 
    (font, size) -> new FontConfig(font, size))
.withLatestFrom(themeObservable,
    (fontConfig, theme) -> new ReaderConfig(theme, fontConfig.font, fontConfig.size))

edited Jan 07 '18 at 20:09

answered Jan 07 '18 at 18:38

ESala

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Doesnt withLatestFrom emit when themeObservable emits? – urSus Jan 07 '18 at 20:03
@urSus no, it only emits when the source observable emits. – ESala Jan 07 '18 at 20:06
oh thats exactly what I need! I thought it was just a shorthand for combinelatest, i.e. symetry for zipWith, mergeWith, etc. – urSus Jan 07 '18 at 20:06
@urSus I added the link to the marbles diagram so its easier to see :) – ESala Jan 07 '18 at 20:10
yea, when I google "rxjava withLatestFrom" combineLatest comes up hence my assumption, thanks! – urSus Jan 07 '18 at 20:18

How to ignore one of observables in combineLatest, but have its latest available in the combiner function?

1 Answers1