refactor: 令 lastEmit 相关的数组类型声明为 ReadonlyArray

...并调整 QueryToken 里的 lastEmit 字段初始值，避免使用不易与实际推出
结果区分的 `[]`，使用 undefined；traces 接口在遇到 lastEmit 尚未设值的
情况时，返回意为全新结果集的 TraceResult。

Author: chuan6

src/storage/modules/QueryToken.ts

@@ -4,10 +4,18 @@ import { Selector } from './Selector'
 import { ProxySelector } from './ProxySelector'
 import { assert } from '../../utils/assert'
 import { TokenConsumed } from '../../exception/token'
-import { diff, Ops, OpsType } from '../../utils/diff'
+import { diff, Ops, OpsType, OpType } from '../../utils/diff'
 
 export type TraceResult<T> = Ops & {
-  result: T[]
+  result: ReadonlyArray<T>
+}
+
+function initialTraceResult<T>(list: ReadonlyArray<T>): TraceResult<T> {
+  return {
+    type: OpsType.Success,
+    ops: list.map((_value, index) => ({ type: OpType.New, index })),
+    result: list
+  }
 }
 
 export type SelectorMeta<T> = Selector<T> | ProxySelector<T>
@@ -20,14 +28,16 @@ export class QueryToken<T> {
   selector$: Observable<SelectorMeta<T>>
 
   private consumed = false
-  private lastEmit: T[] = []
 
-  constructor(selector$: Observable<SelectorMeta<T>>, lastEmit?: T[]) {
+  constructor(
+    selector$: Observable<SelectorMeta<T>>,
+    private lastEmit?: ReadonlyArray<T>
+  ) {
     this.selector$ = selector$.pipe(
       publishReplay(1),
       refCount(),
     )
-    this.lastEmit = lastEmit || []
+    this.lastEmit = lastEmit
   }
 
   setLastEmit(data: T[]) {
@@ -59,6 +69,9 @@ export class QueryToken<T> {
   traces(pk?: string): Observable<TraceResult<T>> {
     return this.changes().pipe(
       map((result: T[]) => {
+        if (!this.lastEmit) {
+          return initialTraceResult(result)
+        }
         const ops = diff(this.lastEmit, result, pk)
         return { result, ...ops }
       }),

src/utils/diff.ts

@@ -26,7 +26,7 @@ export type Ops = {
 }
 
 // as an example, use diff to patch data
-export const patch = <T>(ops: Op[], oldList: T[], newList: T[]) => {
+export const patch = <T>(ops: ReadonlyArray<Op>, oldList: ReadonlyArray<T>, newList: ReadonlyArray<T>) => {
   if (!oldList.length) {
     return newList
   }
@@ -42,7 +42,7 @@ export const patch = <T>(ops: Op[], oldList: T[], newList: T[]) => {
   })
 }
 
-export const getPatchResult = <T>(oldList: T[], newList: T[], ops: Ops): T[] => {
+export const getPatchResult = <T>(oldList: ReadonlyArray<T>, newList: ReadonlyArray<T>, ops: Ops): ReadonlyArray<T> => {
   switch (ops.type) {
     case OpsType.Error:
       return newList
@@ -120,7 +120,7 @@ function fastEqual(left: object, right: object) {
   return left !== left && right !== right
 }
 
-export function diff<T>(oldList: T[], newList: T[], pk = '_id'): Ops {
+export function diff<T>(oldList: ReadonlyArray<T>, newList: ReadonlyArray<T>, pk = '_id'): Ops {
   const prev = oldList
   const curr = newList