Validate state tree instances instead of snapshots in the SnapshotProcessor.is override

__tests__/core/snapshotProcessor.test.ts

@@ -642,6 +642,28 @@ describe("snapshotProcessor", () => {
     expect(ProcessedModel.is(Model)).toBe(false)
   })
 
+  test(".is checks instances against the underlying type", () => {
+    const ModelA = types.model({ x: types.number })
+    const ModelB = types.model({ x: types.number })
+    const modelA = ModelA.create({ x: 1 })
+    const modelB = ModelB.create({ x: 2 })
+
+    // despite having the same snapshot type, .is is false for the instance of one against the other because they are not the same type
+    expect(ModelA.is(modelA)).toBe(true)
+    expect(ModelB.is(modelA)).toBe(false)
+    expect(ModelA.is(modelB)).toBe(false)
+    expect(ModelB.is(modelB)).toBe(true)
+
+    const ProcessedModel = types.snapshotProcessor(ModelA, {})
+
+    const processedModel = ProcessedModel.create({ x: 3 })
+    expect(ProcessedModel.is(processedModel)).toBe(true)
+    expect(ModelA.is(processedModel)).toBe(true)
+    expect(ModelB.is(processedModel)).toBe(false)
+    expect(ProcessedModel.is(modelA)).toBe(true)
+    expect(ProcessedModel.is(modelB)).toBe(false)
+  })
+
   describe("1776 - reconciliation in an array", () => {
     test("model with transformed property is reconciled", () => {
       const SP = types.snapshotProcessor(

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mobx-state-tree",
-  "version": "6.0.1",
+  "version": "7.0.0-pre.1",
   "description": "Opinionated, transactional, MobX powered state container",
   "main": "dist/mobx-state-tree.js",
   "umd:main": "dist/mobx-state-tree.umd.js",

src/types/utility-types/snapshotProcessor.ts

@@ -149,11 +149,28 @@ class SnapshotProcessor<IT extends IAnyType, CustomC, CustomS> extends BaseType<
     return this._subtype
   }
 
+  /**
+   * MST considers a given value to "be" of a subtype is the value is either:
+   *
+   * 1. And instance of the subtype
+   * 2. A valid snapshot *in* of the subtype
+   *
+   * Before v7, we used to also consider processed models (as in, SnapshotOut values of this).
+   * This is no longer the case, and is more in line with our overall "is" philosophy, which you can
+   * see in `src/core/type/type.ts:104` (assuming lines don't change too much).
+   *
+   * For additonal commentary, see discussion in https://github.com/mobxjs/mobx-state-tree/pull/2182
+   *
+   * The `is` function specifically checks for `SnapshotIn` or `Instance` of a given type.
+   *
+   * @param thing
+   * @returns
+   */
   is(thing: any): thing is any {
     const value = isType(thing)
       ? this._subtype
       : isStateTreeNode(thing)
-      ? getSnapshot(thing, false)
+      ? thing
       : this.preProcessSnapshotSafe(thing)
     if (value === $preProcessorFailed) {
       return false