Support IComputedOptions in createTransformer

In the same vain as #215, it would be great to use a custom equals
function with createTransformer, to avoid bubbling bubbling recomputes
of transformers that return arrays.

This commit passes the options through to the internal `computed`.  The
name option is not passed through, since that can be changed with
the `debugNameGenerator` option.

src/create-transformer.ts

@@ -1,12 +1,14 @@
-import { computed, onBecomeUnobserved, IComputedValue } from "mobx"
+import { computed, onBecomeUnobserved, IComputedValue, IComputedValueOptions } from "mobx"
 import { invariant, addHiddenProp } from "./utils"
 
 export type ITransformer<A, B> = (object: A) => B
 
-export interface ITransformerParams<A, B> {
-    onCleanup?: (resultObject: B | undefined, sourceObject?: A) => void
-    debugNameGenerator?: (sourceObject?: A) => string
-}
+export type ITransformerParams<A, B> =
+    | {
+          onCleanup?: (resultObject: B | undefined, sourceObject?: A) => void
+          debugNameGenerator?: (sourceObject?: A) => string
+      }
+    | Exclude<IComputedValueOptions<B>, "name">
 
 let memoizationId = 0
 
@@ -40,9 +42,11 @@ export function createTransformer<A, B>(
 
     function createView(sourceIdentifier: number, sourceObject: A) {
         let latestValue: B
+        let computedValueOptions = {}
         if (typeof arg2 === "object") {
             onCleanup = arg2.onCleanup
             debugNameGenerator = arg2.debugNameGenerator
+            computedValueOptions = arg2
         } else if (typeof arg2 === "function") {
             onCleanup = arg2
         } else {
@@ -57,6 +61,7 @@ export function createTransformer<A, B>(
                 return (latestValue = transformer(sourceObject))
             },
             {
+                ...computedValueOptions,
                 name: prettifiedName
             }
         )

test/create-transformer.ts

@@ -1354,3 +1354,18 @@ test("should respect debugNameGenerator argument", () => {
     objectName = m.getObserverTree(state, "title").observers[0].name
     expect(objectName).toBe("COFFEE-DEBUG")
 })
+
+test("supports computed value options", () => {
+    const events: number[][] = []
+    const xs = m.observable([1, 2, 3])
+    const xsLessThan = createTransformer(n => xs.filter(x => x < n), {
+        equals: m.comparer.structural
+    })
+
+    m.autorun(() => events.push(xsLessThan(3)))
+    expect(events).toEqual([[1, 2]])
+
+    events.length = 0
+    xs.push(4)
+    expect(events).toEqual([])
+})