I am usingobservable-fns (which can also work withthreads.js). Not sure how powerfulRxJS andRxJava can be...
Still missing one important feature though, cancellability.
I am working on SQLite-SQLite and SQLite-MongoDB syncing.
/** * DbSqlite needs three basic columns * * [uid] TEXT PRIMARY KEY, * date_created DATETIME DEFAULT CURRENT_TIMESTAMP, * date_sync DATETIME, */functionreplicate(from:DbSqlite,to:DbSqlite,uids?:string[]){returnnewObservable<{message:stringpercent?:number}>((obs)=>{(async()=>{constgetDateSync=(r:any)=>Math.max(r.date_created,r.date_sync||0)awaitto.transaction(()=>{to.sql.db.parallelize(()=>{constsyncTable=(tableName:string)=>{from.sql.each(/*sql*/` SELECT * FROM${safeColumnName(tableName)}`,(_:any,r1:any)=>{constuid=r1.uidif(uids&&!uids.includes(uid)){return}to.sql.db.get(/*sql*/` SELECT date_created, date_sync FROM${safeColumnName(tableName)} WHERE [uid] = @uid`,{uid},(_:any,r2:any)=>{constupdateSync=()=>{r1.date_sync=+newDate()to.sql.db.run(/*sql*/` REPLACE INTO${safeColumnName(tableName)} (${Object.keys(r1).map(safeColumnName)}) VALUES (${Object.keys(r1).map((c)=>`@${c}`)})`,r1)}if(r2){if(getDateSync(r1)>getDateSync(r2)){updateSync()}}else{updateSync()}})})}for(consttableNameof['user','card','quiz','lesson','deck']){obs.next({message:`Uploading table:${tableName}`})syncTable(tableName)}})})obs.complete()})().catch(obs.error)})}If you can curious on what is safe columnName / tableName in SQLite, it is
Observables can be joined.
functionimport(filename:string){returnnewObservable<{message:stringpercent?:number}>((obs)=>{(async()=>{obs.next({message:`Opening:${filename}`})constsrcDb=awaitDbSqlite.open(filename)DbSqlite.replicate(srcDb,this).subscribe(obs.next,obs.error,obs.complete)})().catch(obs.error)})}Observables can be polled via WebSockets.
conn.socket.on('message',async(msg:string)=>{const{id,filename}=JSON.parse(msg)constisNew=!socketMap.has(id)socketMap.set(id,(json:any)=>{conn.socket.send(JSON.stringify(json))})if(isNew){constobservable=db.import(filename)logger.info(`Start processing:${filename}`)observable.subscribe((status)=>{socketMap.get(id)!({id,status})logger.info(`Processing status:${filename}:${status}`)},(err)=>{socketMap.get(id)!({id,error:err.message})logger.error(`Processing error:${filename}:${err.message}`)},()=>{socketMap.get(id)!({id,status:'done'})logger.info(`Finished processing:${filename}`)})}})What is remaining is whether Observables can be cancelled via WebSockets?
Top comments(9)
- LocationBerlin, Germany
- Joined
Hi! Andy here, the author of observable-fns and threads.js.
Just stumbled upon this blog post. Nice job!
About the cancellation: Observables are kind-of cancellable – you can unsubscribe from them.
Observables are "cold" by default which means every new subscriber will get their own fresh instance of this observable, which also means that the code innew Observable(/* ... */) will be run on every.subscribe() call (!). If a subscriber unsubscribes, the upstream subscription is terminated (cancellation) – just that there may be numerous more subscriptions of the same sort.
You can make observables "hot" by using themulticast() function. Then there will only ever be one upstream subscription that all downstream subscribers share. You can unsubscribe, too, just that the single shared upstream subscription will live on until the last subscriber has unsubscribed.
What does that mean for your use case?
- Use
multicast()😉 - Then enjoy
.unsubscribe()acting as cancellation - Profit $$$
Hope that helps 🙂
- LocationThailand
- EducationYes
- Joined
How do I listen to.once('all-unsubscribed') event? Like this one package -p-cancelable.
Otherwise, it is possible to useSubject, as inthread.js? (I wouldn't want to use Worker, as I write to SQLite as well, and it is to supposed to be written from multiple threads.)
- LocationBerlin, Germany
- Joined
How do I listen to .once('all-unsubscribed') event?
functionsubscribeToThings(){returnmulticast(newObservable(observer=>{// <Subscribe to things>constterminateSubscription=()=>{// <-- THIS IS THE "all-unsubscribed" CODE// Remember? multicast() makes that all downstream subscriptions are bundled into one,// so this is the callback function that will be run when this bundled subscription is terminated}returnterminateSubscription}))}subscribeToThings().subscribe(/* ... */)Otherwise, it is possible to use Subject, as in thread.js?
Sure, who would stop you? ;)
- LocationThailand
- EducationYes
- Joined
Currently, I use the signature,
Observable<{value?:QueryItemPartiali:numbercancelFunction:CallbackType}>Will it work?
- LocationBerlin, Germany
- Joined
That doesn't make much sense. Why would you want to emit a new cancellation function for every new value?
You shouldn't have this non-standard custom cancellation approach when there is already a standardized one (observable.subscribe(),subscription.unsubscribe()). Keep in mind that emitting functions is generally considered to be an anti-pattern.
Maybe the observable-fns readme lacks a link to some documentation about observables in general… I guess this is one of your first times using observables? (No offense – I'm seriously interested :) )
- LocationThailand
- EducationYes
- Joined
Yes. Only promises and eventemitters.
Should I start with RxJS?
- LocationBerlin, Germany
- Joined
Yeah, maybe that's the best to get started. You should definitely start with RxJS's documentation – it's pretty good!
Which library you then use doesn't really matter too much as their APIs are very similar to almost identical.
- LocationThailand
- EducationYes
- Joined
const terminateSubscription = () =>
Do you actually mean
newObservable((obs)=>{returnterminateSubscription// I actually use// return () => { isUnsubscribed = true }})- LocationBerlin, Germany
- Joined
Yes, my bad! I updated my code sample accordingly.
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