Migrated from this commit in MobX-State-Tree in the work to undo the monorepo set up for MST.

Prebuilt middlewares

The MST package ships with some prebuilt middlewares, which serves mainly as examples on how to write your own middleware. The source of each middleware can be found in this github directory, you are encouraged to read them!

The middlewares are bundled separately to keep the core package small, and can be included using:

1import { MiddlewareName } from "mst-middlewares";

The middlewares serve as example and are supported on a best effort bases. The goal of these middlewares is that if they are critical to your system, you can simply copy paste them and further tailor them towards your specific needs.

For the exact description of all middleware events offered by MST, see the api docs

Contributing

Feel free to contribute to these middlewares and improve them on your experience. The middlewares must be written in TypeScript. Any additional test for your middleware should be written inside the test folder

---

simple-action-logger

This is the most basic of middlewares: It logs all direct action invocations. Example:

1import { simpleActionLogger } from "mst-middlewares";
2
3// .. type definitions ...
4
5const store = Store.create({
6  todos: [{ title: "test " }],
7});
8
9mst.addMiddleware(store, simpleActionLogger);
10
11store.todos[0].setTitle("hello world");
12
13// Prints:
14// [MST] /todos/0/setTitle

For a more sophisticated logger, see action-logger which also logs process invocations and continuations

---

action-logger

This is a little more sophisticated middlewares: It logs all direct action invocations and also every flow that spawns, returns or throws. Example:

1import { actionLogger } from "mst-middlewares";
2
3// .. type definitions ...
4
5const store = Store.create({
6  todos: [{ title: "test " }],
7});
8
9mst.addMiddleware(store, actionLogger);
10
11store.todos[0].setTitle("hello world");

This will print something like `[MST] -

[MST] #5 action - /todos/0/setTitle
[MST] #5 flow_spawn - /todos/0/setTitle
[MST] #5 flow_spawn - /todos/0/helper2
[MST] #5 flow_return - /todos/0/helper2
[MST] #5 flow_return - /todos/0/setTitle

The number ("#5") indicates the id of the original action invocation that lead directly or indirectly to the flow being spawned. For more details on the meaning of the action types see the middleware docs.

---

atomic

This middleware rolls back if a synchronous or asynchronous action process fails.

The exception itself is not eaten, but any modifications that are made during the sync/async action will be rollback, by reverse applying any pending patches. Can be connected to a model by using either addMiddleware or decorate

Example:

1import { types, addMiddleware, flow } from "mobx-state-tree";
2import { atomic } from "mst-middlewares";
3
4const TestModel = types
5  .model({
6    z: 1,
7  })
8  // example with addMiddleware
9  .actions((self) => {
10    addMiddleware(self, atomic);
11
12    return {
13      inc: flow(function* (x) {
14        yield delay(2);
15        self.z += x;
16        yield delay(2);
17        self.z += x;
18        throw "Oops";
19      }),
20    };
21  })
22  // example with decorate
23  .actions((self) => {
24    return {
25      inc: decorate(
26        atomic,
27        flow(function* (x) {
28          yield delay(2);
29          self.z += x;
30          yield delay(2);
31          self.z += x;
32          throw "Oops";
33        })
34      ),
35    };
36  });
37
38const m = TestModel.create();
39m.inc(3).catch((error) => {
40  t.is(error, "Oops");
41  t.is(m.z, 1); // Not 7! The change was rolled back
42});

---

TimeTraveller

This built in model can be used as stand alone store or as part of your state tree and adds time travelling capabilities. It records all emitted snapshots by a tree and exposes the following methods / views:

• canUndo: boolean
• canRedo: boolean
• undo()
• redo()
• history: array with all recorded states

The state of the TimeTraveller itself is stored in a Mobx state tree, meaning that you can freely snapshot your state including its history. This means that it is possible to store your app state including the undo stack in for example local storage. (but beware that stringify-ing will not benefit from structural sharing).

Usage inside a state tree:

1import { TimeTraveller } from "mst-middlewares";
2
3export const Store = types.model({
4  todos: types.array(Todo),
5  history: types.optional(TimeTraveller, { targetPath: "../todos" }),
6});
7
8const store = Store.create();
9
10// later:
11if (store.history.canUndo) store.history.undo();
12// etc

Note that the targetPath is a path relative to the TimeTraveller instance that will indicate which part of the tree will be snapshotted. Please make sure the targetPath doesn't point to a parent of the time traveller, as that would start recording it's own history..... In other words, targetPath: "../" -> Boom💥

To instantiate the TimeTraveller as a stand-alone state tree, pass in the the store through context:

1import { TimeTraveller } from "mst-middlewares";
2
3export const Store = types.model({
4  todos: types.array(Todo),
5});
6
7const store = Store.create();
8const timeTraveller = TimeTraveller.create({}, { targetStore: store });
9
10// later:
11if (timeTraveller.canUndo) timeTraveller.undo();
12// etc

---

UndoManager

The UndoManager is the more fine grained TimeTraveller. Because it records patches instead of snapshots, it is better at dealing with concurrent and asynchronous processes. The differences to the TimeTraveller make it useful to implement end-user undo / redo.

For an in-depth explanation why undo / redo should be patch, not snapshot, based, check out the second half of the React Next talk: MobX-state-tree, React but for data

Differences to the TimeTraveller:

1. It records patches for actions / processes, not snapshots. It's therefore using less memory.
2. undo / redo applies all inverted patches / patches for a recorded action / process instead of snapshots.
3. An undo state is only comitted to the history if the action / process is finished. Ongoing processes can't be undone.
4. Failing processes do not add an undo state.
5. Multiple concurrent processes only undo their own changes and don't touch the changes caused by other actions - unlike snapshots would.
6. Can be used declaratively within the models.

API:

• history: { patches: [], inversePatches [] }[]
• canUndo: boolean true if there is at least one undo level available
• canRedo: boolean true if there is at least one redo level available
• undoLevels: number number of undo levels available
• redoLevels: number number of redo levels available
• undo() undo the last operation
• redo() redo the last operation
• withoutUndo(() => fn) patches for actions / processes within the fn are not recorded.
• withoutUndoFlow(fn*) patches the fn* are not recorded.
• startGroup(() => fn) can be used to start a group, all patches within a group are saved as one history entry.
• stopGroup() can be used to stop the recording of patches for the grouped history entry.
• clear({ undo?: true, redo: true }?) clear the history.

Setup and API usage examples:

The setup is very similar to the one of the TimeTraveller. The UndoManager automatically records all the actions within the tree it is attached to.

If you want the history to be a part of your store:

1import { UndoManager } from "mst-middlewares";
2
3export const Store = types
4  .model({
5    todos: types.array(Todo),
6    history: types.optional(UndoManager, {}),
7  })
8  .actions((self) => {
9    // you could create your undoManger anywhere but before your first needed action within the undoManager
10    setUndoManager(self);
11
12    return {
13      addTodo(todo) {
14        self.todos.push(todo);
15      },
16      // to use the undoManager to wrap the afterCreate action
17      // of the StoreModel it's necessary to set it within the store model like above
18      // afterCreate: () => undoManager.withoutUndo(() => { action() })
19    };
20  });
21
22export let undoManager = {};
23export const setUndoManager = (targetStore) => {
24  undoManager = targetStore.history;
25};
26const store = Store.create();

To record the changes into a separate tree:

1import { UndoManager } from "mst-middlewares";
2
3export const Store = types
4  .model({
5    todos: types.array(Todo),
6  })
7  .actions((self) => {
8    // you could create your undoManger anywhere but before your first needed action within the undoManager
9    setUndoManager(self);
10
11    return {
12      addTodo(todo) {
13        self.todos.push(todo);
14      },
15      // to use the undoManager to wrap the afterCreate action
16      // of the StoreModel it's necessary to set it within the store model like above
17      // afterCreate: () => undoManager.withoutUndo(() => { action() })
18    };
19  });
20
21export let undoManager = {};
22export const setUndoManager = (targetStore) => {
23  undoManager = UndoManager.create({}, { targetStore });
24};
25const store = Store.create();

If you want a limited history of actions to be recorded, for example to keep track of the last 10 actions only, you must provide a maxHistoryLength value through environment data.

1// with history in your store
2const store = Store.create({}, { maxHistoryLength: 10 });
3
4// with history in a separate store
5export const setUndoManager = (targetStore) => {
6  undoManager = UndoManager.create({}, { targetStore, maxHistoryLength: 10 });
7};

If you want to record changes in lifecycle hooks as well, you must provide an includeHooks flag:

1import { UndoManager } from "mst-middlewares";
2
3export const Store = types
4  .model({
5    todos: types.array(Todo),
6  })
7  .actions((self) => {
8    setUndoManager(self);
9
10    return {
11      afterCreate() {
12        self.todos.push({ title: "New Todo" });
13      },
14      addTodo(todo) {
15        self.todos.push(todo);
16      },
17    };
18  });
19
20export let undoManager = {};
21export const setUndoManager = (targetStore) => {
22  undoManager = UndoManager.create({}, { targetStore, includeHooks: true });
23};
24const store = Store.create();

Undo/ Redo:

1import { undoManager } from "../Store";
2
3// if the undoManger is created within another tree
4const undo = () => undoManager.canUndo && undoManager.undo();
5const redo = () => undoManager.canRedo && undoManager.redo();

WithoutUndo - within a react component:

1import {undoManger} from '../Store'
2
3...
4
5setPersonName = () => {
6    // the action setPersonName won't be saved onto the history, you could add more than one action.
7    undoManger.withoutUndo(() => store.setPersonName('firstName', 'lastName'))
8}
9
10render() {
11    return (
12        <div onClick={this.setPersonName}>
13            SetPersonName
14        </div>
15    )
16}
17
18...

WithoutUndo - declarative:

1import { types } from "mobx-state-tree";
2import { UndoManager } from "mst-middlewares";
3
4const PersonModel = types
5  .model("PersonModel", {
6    firstName: types.string,
7    lastName: types.string,
8  })
9  .actions((self) => {
10    return {
11      // setPersonName won't be recorded anymore in general
12      setPersonName: (firstName, lastName) =>
13        undoManager.withoutUndo(() => {
14          self.firstName = firstName;
15          self.lastName = lastName;
16        }),
17    };
18  });
19
20const StoreModel = types
21  .model("StoreModel", {
22    persons: types.map(PersonModel),
23  })
24  .actions((self) => {
25    setUndoManager(self);
26
27    return {
28      addPerson(firstName, lastName) {
29        persons.put({ firstName, lastName });
30      },
31    };
32  });
33
34export let undoManager = {};
35export const setUndoManager = (targetStore) => {
36  undoManager = UndoManager.create({}, { targetStore });
37};
38export const Store = StoreModel.create({});

WithoutUndoFlow - declarative:

1import {undoManager} from './Store/'
2
3...
4
5.actions(self => {
6    function updateBooks(json) {
7        self.books.values().forEach(book => (book.isAvailable = false))
8        json.forEach(bookJson => {
9            self.books.put(bookJson)
10            self.books.get(bookJson.id).isAvailable = true
11        })
12    }
13
14    function* loadBooks() {
15        try {
16          const json = yield self.shop.fetch("/books.json")
17          updateBooks(json)
18        } catch (err) {
19          console.error("Failed to load books ", err)
20        }
21    }
22
23    return {
24        loadBooks: () => undoManager.withoutUndoFlow(loadBooks)()
25        // same as: undoManager.withoutUndo(() => flow(loadBooks))()
26    }
27})

StartGroup, StopGroup - within a react component:

1import {undoManager} from '../Store'
2...
3handleStop = (mousePosition, { dx, dy }) => {
4  this.stopTrackingDrag();
5  undoManager.stopGroup();
6}
7
8handleDrag = (mousePosition, { dx, dy }) => {
9  const { view, parentNode } = this.props;
10  // only one history entry will be created for the whole dragging
11  // therefore all patches will be merged to one history entry while the group is active
12  undoManager.startGroup(() =>
13    parentNode.moveSelectedNodes({ dx: dx / view.zoom, dy: dy / view.zoom })
14  );
15}
16...

---

redux

The Redux 'middleware' is not literally middleware, but provides two useful methods for Redux interoperability:

asReduxStore

asReduxStore(mstStore, middlewares?) creates a tiny proxy around a MST tree that conforms to the redux store api. This makes it possible to use MST inside a redux application.

See the redux-todomvc example for more details.

connectReduxDevtools

connectReduxDevtools(remoteDevDependency, mstStore, options?) connects a MST tree to the Redux devtools. Pass in the remoteDev dependency to set up the connect (only one at a time). See this example for a setup example.

The options object is optional and has the following options:

• logIdempotentActionSteps: true by default due to possible performance penalty because of the internal usage of onPatch. When set to false it will skip reporting of actions and flow action "steps" that do not end up in an actual change in the model (except when an error is thrown), thus reducing the amount of noise in the logs.
• logChildActions: false by default. When set to true it will report actions that are executed inside a root actions. When set to false it will not.
• logArgsNearName: true by default. When true it will log the arguments near the action name (truncated if too long), when false it won't.