Expose close event

Add the close event to the DbEvents list, so close event can be subscribed by doing this,
db.on('close'). This close event only fires if the database is unexpectedly closed.

src/classes/dexie/dexie-open.ts

@@ -100,6 +100,10 @@ export function dexieOpen (db: Dexie) {
               db.on("versionchange").fire(ev);
           });
 
+          idbdb.onclose = wrap(ev => {
+              db.on("close").fire(ev);
+          });
+
           _onDatabaseCreated(db._deps, dbName);
 
           resolve();

src/classes/dexie/dexie.ts

@@ -126,7 +126,7 @@ export class Dexie implements IDexie {
     });
     this._state = state;
     this.name = name;
-    this.on = Events(this, "populate", "blocked", "versionchange", { ready: [promisableChain, nop] }) as DbEvents;
+    this.on = Events(this, "populate", "blocked", "versionchange", "close", { ready: [promisableChain, nop] }) as DbEvents;
     this.on.ready.subscribe = override(this.on.ready.subscribe, subscribe => {
       return (subscriber, bSticky) => {
         (Dexie as any as DexieConstructor).vip(() => {

src/public/types/db-events.d.ts

@@ -21,15 +21,23 @@ export interface DexiePopulateEvent {
   fire(trans: Transaction): any;
 }
 
+export interface DexieCloseEvent {
+  subscribe(fn: (event: Event) => any): void;
+  unsubscribe(fn: (event: Event) => any): void;
+  fire(event: Event): any;
+}
+
 export interface DbEvents extends DexieEventSet {
   (eventName: 'ready', subscriber: () => any, bSticky?: boolean): void;
   (eventName: 'populate', subscriber: (trans: Transaction) => any): void;
   (eventName: 'blocked', subscriber: (event: IDBVersionChangeEvent) => any): void;
   (eventName: 'versionchange', subscriber: (event: IDBVersionChangeEvent) => any): void;
+  (eventName: 'close', subscriber: (event: Event) => any): void;
   ready: DexieOnReadyEvent;
   populate: DexiePopulateEvent;
   blocked: DexieEvent;
   versionchange: DexieVersionChangeEvent;
+  close: DexieCloseEvent;
 }
 
 export type ObservabilitySet = {
@@ -45,6 +53,6 @@ export interface DexieOnTxCommittedEvent {
 }
 
 export interface GlobalDexieEvents extends DexieEventSet {
-  (eventName: 'txcommitted', subscriber: (parts: ObservabilitySet)=>any): void;
+  (eventName: 'txcommitted', subscriber: (parts: ObservabilitySet) => any): void;
   txcommitted: DexieOnTxCommittedEvent;
 }