Add lazy type for lazy importing of models

packages/mobx-state-tree/__tests__/core/api.test.ts

@@ -116,6 +116,7 @@ const TYPES = stringToArray(`
     identifier,
     identifierNumber,
     late,
+    lazy,
     undefined,
     null,
     snapshotProcessor

packages/mobx-state-tree/__tests__/core/lazy.test.ts

@@ -0,0 +1,111 @@
+import { when } from "mobx"
+import { getRoot, types } from "../../src"
+
+interface IRootModel {
+    shouldLoad: boolean
+}
+
+test("it should load the correct type", async () => {
+    const LazyModel = types
+        .model("LazyModel", {
+            width: types.number,
+            height: types.number
+        })
+        .views((self) => ({
+            get area() {
+                return self.height * self.width
+            }
+        }))
+
+    const Root = types
+        .model("Root", {
+            shouldLoad: types.optional(types.boolean, false),
+            lazyModel: types.lazy<typeof LazyModel, IRootModel>("lazy", {
+                loadType: () => Promise.resolve(LazyModel),
+                shouldLoadPredicate: (parent) => parent.shouldLoad == true
+            })
+        })
+        .actions((self) => ({
+            load: () => {
+                self.shouldLoad = true
+            }
+        }))
+
+    const store = Root.create({
+        lazyModel: {
+            width: 3,
+            height: 2
+        }
+    })
+
+    expect(store.lazyModel.width).toBe(3)
+    expect(store.lazyModel.height).toBe(2)
+    expect(store.lazyModel.area).toBeUndefined()
+    store.load()
+    const promise = new Promise<number>((resolve, reject) => {
+        when(
+            () => store.lazyModel && store.lazyModel.area !== undefined,
+            () => resolve(store.lazyModel.area)
+        )
+
+        setTimeout(reject, 2000)
+    })
+
+    await expect(promise).resolves.toBe(6)
+})
+
+test("maintains the tree structure when loaded", async () => {
+    const LazyModel = types
+        .model("LazyModel", {
+            width: types.number,
+            height: types.number
+        })
+        .views((self) => ({
+            get area() {
+                const root = getRoot<{ rootValue: number }>(self)
+                return self.height * self.width * root.rootValue
+            }
+        }))
+
+    const Root = types
+        .model("Root", {
+            shouldLoad: types.optional(types.boolean, false),
+            lazyModel: types.lazy<typeof LazyModel, IRootModel>("lazy", {
+                loadType: () => Promise.resolve(LazyModel),
+                shouldLoadPredicate: (parent) => parent.shouldLoad == true
+            })
+        })
+        .views(() => ({
+            get rootValue() {
+                return 5
+            }
+        }))
+        .actions((self) => ({
+            load: () => {
+                self.shouldLoad = true
+            }
+        }))
+
+    const store = Root.create({
+        lazyModel: {
+            width: 3,
+            height: 2
+        }
+    })
+
+    expect(store.lazyModel.width).toBe(3)
+    expect(store.lazyModel.height).toBe(2)
+    expect(store.rootValue).toEqual(5)
+    expect(store.lazyModel.area).toBeUndefined()
+    store.load()
+    const promise = new Promise<number>((resolve, reject) => {
+        when(
+            () => store.lazyModel && store.lazyModel.area !== undefined,
+            () => resolve(store.lazyModel.area)
+        )
+
+        setTimeout(reject, 2000)
+    })
+
+    await expect(promise).resolves.toBe(30)
+})

packages/mobx-state-tree/src/core/type/type.ts

@@ -49,7 +49,8 @@ export enum TypeFlags {
     Undefined = 1 << 16,
     Integer = 1 << 17,
     Custom = 1 << 18,
-    SnapshotProcessor = 1 << 19
+    SnapshotProcessor = 1 << 19,
+    Lazy = 1 << 20
 }
 
 /**

packages/mobx-state-tree/src/internal.ts

@@ -35,6 +35,7 @@ export * from "./types/utility-types/union"
 export * from "./types/utility-types/optional"
 export * from "./types/utility-types/maybe"
 export * from "./types/utility-types/late"
+export * from "./types/utility-types/lazy"
 export * from "./types/utility-types/frozen"
 export * from "./types/utility-types/reference"
 export * from "./types/utility-types/identifier"

packages/mobx-state-tree/src/types/index.ts

@@ -23,6 +23,7 @@ import {
     identifier,
     identifierNumber,
     late,
+    lazy,
     undefinedType,
     nullType,
     snapshotProcessor
@@ -54,5 +55,6 @@ export const types = {
     late,
     undefined: undefinedType,
     null: nullType,
-    snapshotProcessor
+    snapshotProcessor,
+    lazy
 }

packages/mobx-state-tree/src/types/utility-types/lazy.ts

@@ -0,0 +1,120 @@
+import { action, IObservableArray, observable, when } from "mobx"
+import { AnyNode } from "../../core/node/BaseNode"
+import { IType } from "../../core/type/type"
+import {
+    IValidationContext,
+    IValidationResult,
+    TypeFlags,
+    typeCheckSuccess,
+    AnyObjectNode,
+    SimpleType,
+    createScalarNode,
+    deepFreeze,
+    isSerializable,
+    typeCheckFailure
+} from "../../internal"
+
+interface LazyOptions<T extends IType<any, any, any>, U> {
+    loadType: () => Promise<T>
+    shouldLoadPredicate: (parent: U) => boolean
+}
+
+export function lazy<T extends IType<any, any, any>, U>(
+    name: string,
+    options: LazyOptions<T, U>
+): T {
+    // TODO: fix this unknown casting to be stricter
+    return (new Lazy(name, options) as unknown) as T
+}
+
+/**
+ * @internal
+ * @hidden
+ */
+export class Lazy<T extends IType<any, any, any>, U> extends SimpleType<T, T, T> {
+    flags = TypeFlags.Lazy
+
+    private loadedType: T | null = null
+    private pendingNodeList: IObservableArray<AnyNode> = observable.array()
+
+    constructor(name: string, private readonly options: LazyOptions<T, U>) {
+        super(name)
+
+        when(
+            () =>
+                this.pendingNodeList.length > 0 &&
+                this.pendingNodeList.some(
+                    (node) =>
+                        node.isAlive &&
+                        this.options.shouldLoadPredicate(node.parent ? node.parent.value : null)
+                ),
+            () => {
+                this.options.loadType().then(
+                    action((type: T) => {
+                        this.loadedType = type
+                        this.pendingNodeList.forEach((node) => {
+                            if (!node.parent) return
+                            if (!this.loadedType) return
+
+                            node.parent.applyPatches([
+                                {
+                                    op: "replace",
+                                    path: `/${node.subpath}`,
+                                    value: node.snapshot
+                                }
+                            ])
+                        })
+                    })
+                )
+            }
+        )
+    }
+
+    describe() {
+        return `<lazy ${this.name}>`
+    }
+
+    instantiate(
+        parent: AnyObjectNode | null,
+        subpath: string,
+        environment: any,
+        value: this["C"]
+    ): this["N"] {
+        if (this.loadedType) {
+            return this.loadedType.instantiate(parent, subpath, environment, value) as this["N"]
+        }
+
+        const node = createScalarNode(this, parent, subpath, environment, deepFreeze(value))
+        this.pendingNodeList.push(node)
+
+        when(
+            () => !node.isAlive,
+            () => this.pendingNodeList.splice(this.pendingNodeList.indexOf(node), 1)
+        )
+
+        return node
+    }
+
+    isValidSnapshot(value: this["C"], context: IValidationContext): IValidationResult {
+        if (this.loadedType) {
+            return this.loadedType.validate(value, context)
+        }
+        if (!isSerializable(value)) {
+            return typeCheckFailure(context, value, "Value is not serializable and cannot be lazy")
+        }
+        return typeCheckSuccess()
+    }
+
+    reconcile(current: this["N"], value: T, parent: AnyObjectNode, subpath: string): this["N"] {
+        if (this.loadedType) {
+            current.die()
+            return this.loadedType.instantiate(
+                parent,
+                subpath,
+                parent.environment,
+                value
+            ) as this["N"]
+        }
+        return super.reconcile(current, value, parent, subpath)
+    }
+}