[Events Orderbook] Refactor applyEvent Logic (#725)

The refactor that was promised! This PR refactors `applyEvents` switch statement to be more "dumb" by forwarding each event to their individual functions.

This has the added benefit of using the `Event` types so that we have type safety on the event `returnValues` property.

### Test Plan

This PR is just a refactor so unit tests should verify that nothing broke.

src/streamed/events.ts

@@ -2,12 +2,12 @@ import { Contract, EventData } from "web3-eth-contract"
 import { ContractEvent } from "../../build/types/types"
 
 /**
- * Event type specified by name.
+ * Event data type specified by name.
  */
 export type Event<
   C extends Contract,
   T extends Exclude<keyof C["events"], "allEvents">,
-> = EventMetadata & EventDiscriminant<C, T>
+> = EventValues<C["events"][T]>
 
 /**
  * Concrete event type with known properties based on the event name.

src/streamed/index.ts

@@ -14,10 +14,10 @@
  */
 
 import Web3 from "web3"
-import { TransactionReceipt } from "web3-core"
-import { EventData } from "web3-eth-contract"
+import { BlockNumber, TransactionReceipt } from "web3-core"
 import { AbiItem } from "web3-utils"
 import { BatchExchange, BatchExchangeArtifact } from "../.."
+import { AnyEvent } from "./events"
 import { AuctionState } from "./state"
 
 /**
@@ -88,7 +88,7 @@ export const BATCH_DURATION = 300
  */
 export class StreamedOrderbook {
   private readonly state: AuctionState;
-  private pendingEvents: EventData[] = [];
+  private pendingEvents: AnyEvent<BatchExchange>[] = [];
 
   private invalidState?: InvalidAuctionStateError;
 
@@ -150,11 +150,8 @@ export class StreamedOrderbook {
         endBlock,
       )
 
-      this.options.logger?.debug(`fetching page ${fromBlock}-${toBlock}`)
-      const events = await this.contract.getPastEvents(
-        "allEvents",
-        { fromBlock, toBlock },
-      )
+      this.options.logger?.debug(`fetching past events from ${fromBlock}-${toBlock}`)
+      const events = await this.getPastEvents({ fromBlock, toBlock })
 
       this.options.logger?.debug(`applying ${events.length} past events`)
       this.state.applyEvents(events)
@@ -180,7 +177,7 @@ export class StreamedOrderbook {
 
     const fromBlock = this.state.nextBlock
     this.options.logger?.debug(`fetching new events from ${fromBlock}-latest`)
-    const events = await this.contract.getPastEvents("allEvents", { fromBlock, toBlock: "latest" })
+    const events = await this.getPastEvents({ fromBlock })
 
     const latestBlock = await this.web3.eth.getBlockNumber()
     const confirmedBlock = latestBlock - this.options.blockConfirmations
@@ -201,6 +198,16 @@ export class StreamedOrderbook {
     }
     this.pendingEvents = pendingEvents
   }
+
+  private async getPastEvents(
+    options: { fromBlock: BlockNumber; toBlock?: BlockNumber },
+  ): Promise<AnyEvent<BatchExchange>[]> {
+    const events = await this.contract.getPastEvents("allEvents", {
+      toBlock: "latest",
+      ...options,
+    })
+    return events as AnyEvent<BatchExchange>[]
+  }
 }
 
 /**