Patches · MobX-state-tree

egghead.io lesson 3: Test mobx-state-tree Models by Recording Snapshots or Patches

Modifying a model does not only result in a new snapshot, but also in a stream of JSON-patches describing which modifications were made. Patches have the following signature:

export interface IJsonPatch {
    op: "replace" | "add" | "remove"
    path: string
    value?: any
}

Patches are constructed according to JSON-Patch, RFC 6902
Patches are emitted immediately when a mutation is made and don't respect transaction boundaries (like snapshots)
Patch listeners can be used to achieve deep observing
The path attribute of a patch contains the path of the event relative to the place where the event listener is attached
A single mutation can result in multiple patches, for example when splicing an array
Patches can be reverse applied, which enables many powerful patterns like undo / redo

Useful methods:

onPatch(model, listener) attaches a patch listener to the provided model, which will be invoked whenever the model or any of its descendants is mutated
applyPatch(model, patch) applies a patch (or array of patches) to the provided model