Parse Local Datastore + Network Sync

I am using Parse.com with my iOS application (written in Swift) since 6 month and I would like to use Parse local Datastore for many reasons :

• Make my application usable (retrievable) offline
• Reduce data usage (many queries returning « non updated data »)
• Reduce loading time (mainly when starting application and loading all data from network)

In order to do so I would like to write a global function handeling these scenarios for all the query I do from my application.

I already have a specific idea of what the function should do but I don’t know how to technically write this function :)

Scenarios :

• Sign Up / Log In (chain multiple query) :

  1. Get data from Network
  2. Save date inside « lastUpdateLocalDatastore » variable in NSUserDefaults
  3. Pin data in Local Datastore
  4. Display data from Local Datastore —> RETURN & update TableView

• Loading App (chain multiple query) :

  1. Display data from Local Datastore —> RETURN & update TableView
  2. Get data from Network (where « lastUpdateDate » in Parse is newer than « lastUpdateLocalDatastore » from NSUserDefault)
  3. Pin data in Local Datastore
  4. Display updated data from Local Datastore —> RETURN & update TableView

• Trigger update (simple query) :

  1. Get data from Network (where « lastUpdateDate » in Parse is newer than « lastUpdateLocalDatastore » from NSUserDefault)
  2. Pin data in Local Datastore
  3. Display updated data from Local Datastore —> RETURN & update TableView

• Log Out :

  1. Unpin all data in Local Datastore
  2. Clear « lastUpdate » values in NSUserDefault

Function structure :

IF ( "First login" -> Local Datastore is empty ) {

    Get data from Network
    Pin data in Local Datastore
    Save « lastUpdateLocalDatastore » in NSUSerDefaults
    —> RETURN data in Cache

} ELSE {

    IF ( "Launching application" -> Cache is empty ) {
        Get data from Local Datastore
        —> RETURN data in Cache
    } ELSE IF ( "trigger update" ) {
       Get data from Network
       Pin new data in Local Datastore
       Save « lastUpdateLocalDatastore » in NSUSerDefaults
       —> RETURN data in Cache
    }
}

Problems :

1. How to handle multiple (asynchronous) returns
2. How to make a function capable of chaining multiple queries (for example I need to retrieve data from 6 different queries when I load my app)

Finally I found a way to do it based on this GitHub topic : https://github.com/ParsePlatform/ParseUI-iOS/issues/53

Here is the function I use :

func findObjectsLocallyThenRemotely(query: PFQuery!, block:[AnyObject]! -> Void) {

    let localQuery = (query.copy() as! PFQuery).fromLocalDatastore()
    localQuery.findObjectsInBackgroundWithBlock({ (locals, error) -> Void in
        if (error == nil) {
            println("Success : Local Query", msg: "\(query.parseClassName)")
            block(locals)
        } else {
            println("Error : Local Query", msg: "\(query.parseClassName)", error: error)
        }
        query.findObjectsInBackgroundWithBlock({ (objects, error) -> Void in
            if(error == nil) {
                println("Success : Network Query", msg: "\(query.parseClassName)")
                PFObject.unpinAllInBackground(locals, block: { (success, error) -> Void in
                    if (error == nil) {
                        println("Success : Unpin Local Query", msg: "\(query.parseClassName)")
                        block(objects)
                        PFObject.pinAllInBackground(objects, block: { (success, error) -> Void in
                            if (error == nil) {
                                println("Success : Pin Query Result", msg: "\(query.parseClassName)")
                            } else {
                                println("Error : Pin Query Result", msg: "\(query.parseClassName)", error: error)
                            }
                        })
                    } else {
                        println("Error : Unpin Local Query", msg: "\(query.parseClassName)", error: error)
                    }
                })
            } else {
                println("Error : Network Query", msg: "\(query.parseClassName)", error: error)
            }
        })
    })
}

TO DO : I need to add the "lastUpdateDate" option to fetch only modified data from network.