Add an order state cache to filter out duplicate events

packages/0x.js/src/order_watcher/order_state_watcher.ts

@@ -43,6 +43,10 @@ interface OrderByOrderHash {
     [orderHash: string]: SignedOrder;
 }
 
+interface OrderStateByOrderHash {
+    [orderHash: string]: OrderState;
+}
+
 /**
  * This class includes all the functionality related to watching a set of orders
  * for potential changes in order validity/fillability. The orderWatcher notifies
@@ -50,6 +54,7 @@ interface OrderByOrderHash {
  * the order should be deemed invalid.
  */
 export class OrderStateWatcher {
+    private _orderStateByOrderHashCache: OrderStateByOrderHash = {};
     private _orderByOrderHash: OrderByOrderHash = {};
     private _dependentOrderHashes: DependentOrderHashes = {};
     private _callbackIfExists?: OnOrderStateChangeCallback;
@@ -96,6 +101,7 @@ export class OrderStateWatcher {
             return; // noop
         }
         delete this._orderByOrderHash[orderHash];
+        delete this._orderStateByOrderHashCache[orderHash];
         const exchange = (this._orderFilledCancelledLazyStore as any).exchange as ExchangeWrapper;
         const zrxTokenAddress = await exchange.getZRXTokenAddressAsync();
         this.removeFromDependentOrderHashes(signedOrder.maker, zrxTokenAddress, orderHash);
@@ -210,6 +216,12 @@ export class OrderStateWatcher {
             if (_.isUndefined(this._callbackIfExists)) {
                 break; // Unsubscribe was called
             }
+            if (_.isEqual(orderState, this._orderStateByOrderHashCache[orderHash])) {
+                // Actual order state didn't change
+                continue;
+            } else {
+                this._orderStateByOrderHashCache[orderHash] = orderState;
+            }
             this._callbackIfExists(orderState);
         }
     }