@rtsdk/lance-topia/src/syncStrategies/SyncStrategy.js

A Node.js based real-time multiplayer game server

export default class SyncStrategy {

    constructor(clientEngine, inputOptions) {
        this.clientEngine = clientEngine;
        this.gameEngine = clientEngine.gameEngine;
        this.needFirstSync = true;
        this.options = Object.assign({}, inputOptions);
        this.gameEngine.on('client__postStep', this.syncStep.bind(this));
        this.gameEngine.on('client__syncReceived', this.collectSync.bind(this));
        this.requiredSyncs = [];
        this.SYNC_APPLIED = 'SYNC_APPLIED';
        this.STEP_DRIFT_THRESHOLDS = {
            onServerSync: { MAX_LEAD: 1, MAX_LAG: 3 }, // max step lead/lag allowed after every server sync
            onEveryStep: { MAX_LEAD: 7, MAX_LAG: 8 }, // max step lead/lag allowed at every step
            clientReset: 20 // if we are behind this many steps, just reset the step counter
        };
    }

    // collect a sync and its events
    // maintain a "lastSync" member which describes the last sync we received from
    // the server.  the lastSync object contains:
    //  - syncObjects: all events in the sync indexed by the id of the object involved
    //  - syncSteps: all events in the sync indexed by the step on which they occurred
    //  - objCount
    //  - eventCount
    //  - stepCount
    collectSync(e) {

        // on first connect we need to wait for a full world update
        if (this.needFirstSync) {
            if (!e.fullUpdate)
                return;
        } else {

            // TODO: there is a problem below in the case where the client is 10 steps behind the server,
            // and the syncs that arrive are always in the future and never get processed.  To address this
            // we may need to store more than one sync.

            // ignore syncs which are older than the latest
            if (this.lastSync && this.lastSync.stepCount && this.lastSync.stepCount > e.stepCount)
                return;
        }

        // before we overwrite the last sync, check if it was a required sync
        // syncs that create or delete objects are saved because they must be applied.
        if (this.lastSync && this.lastSync.required) {
            this.requiredSyncs.push(this.lastSync);
        }

        // build new sync object
        let lastSync = this.lastSync = {
            stepCount: e.stepCount,
            fullUpdate: e.fullUpdate,
            syncObjects: {},
            syncSteps: {}
        };

        e.syncEvents.forEach(sEvent => {

            // keep a reference of events by object id
            if (sEvent.objectInstance) {
                let objectId = sEvent.objectInstance.id;
                if (!lastSync.syncObjects[objectId]) lastSync.syncObjects[objectId] = [];
                lastSync.syncObjects[objectId].push(sEvent);
            }

            // keep a reference of events by step
            let stepCount = sEvent.stepCount;
            let eventName = sEvent.eventName;
            if (eventName === 'objectDestroy' || eventName === 'objectCreate')
                lastSync.required = true;

            if (!lastSync.syncSteps[stepCount]) lastSync.syncSteps[stepCount] = {};
            if (!lastSync.syncSteps[stepCount][eventName]) lastSync.syncSteps[stepCount][eventName] = [];
            lastSync.syncSteps[stepCount][eventName].push(sEvent);
        });

        let eventCount = e.syncEvents.length;
        let objCount = (Object.keys(lastSync.syncObjects)).length;
        let stepCount = (Object.keys(lastSync.syncSteps)).length;
        this.gameEngine.trace.debug(() => `sync contains ${objCount} objects ${eventCount} events ${stepCount} steps`);
    }

    // add an object to our world
    addNewObject(objId, newObj, options) {

        let curObj = new newObj.constructor(this.gameEngine, {
            id: objId
        });

        // enforce object implementations of syncTo
        if (!curObj.__proto__.hasOwnProperty('syncTo')) {
            throw `GameObject of type ${curObj.class} does not implement the syncTo() method, which must copy the netscheme`;
        }

        curObj.syncTo(newObj);
        this.gameEngine.addObjectToWorld(curObj);
        if (this.clientEngine.options.verbose)
            console.log(`adding new object ${curObj}`);

        return curObj;
    }

    // sync to step, by applying bending, and applying the latest sync
    syncStep(stepDesc) {

        // apply incremental bending
        this.gameEngine.world.forEachObject((id, o) => {
            if (typeof o.applyIncrementalBending === 'function') {
                o.applyIncrementalBending(stepDesc);
                o.refreshToPhysics();
            }
        });

        // apply all pending required syncs
        while (this.requiredSyncs.length) {

            let requiredStep = this.requiredSyncs[0].stepCount;

            // if we haven't reached the corresponding step, it's too soon to apply syncs
            if (requiredStep > this.gameEngine.world.stepCount)
                return;

            this.gameEngine.trace.trace(() => `applying a required sync ${requiredStep}`);
            this.applySync(this.requiredSyncs.shift(), true);
        }

        // apply the sync and delete it on success
        if (this.lastSync) {
            let rc = this.applySync(this.lastSync, false);
            if (rc === this.SYNC_APPLIED)
                this.lastSync = null;
        }
    }
}