customMiddlewares

Overview

Besides the very specific onActionMiddleware (which only tracks top-level actions and is usually used to record actions to be later replicated via applyAction), there are two additional ways to create your own custom middleware, the low-level addActionMiddleware, which should be rarely needed, and the more high-level but friendlier actionTrackingMiddleware.

actionTrackingMiddleware

Creates an action tracking middleware, which is a simplified version of the standard action middleware.

It takes two parameters, being the first one the root target model object and the second one the hooks in the form of an ActionTrackingMiddleware object.

It returns a disposer function.

The ActionTrackingMiddleware object has the following structure:

• filter?(ctx: SimpleActionContext): boolean

  Filter function called whenever each action starts, and only then. Takes as parameter a simplified action context (more on that later). Returns true to accept the action and false to skip it.

  If the action is accepted then onStart, onResume, onSuspend and onFinish for that particular action will be called.

  All actions are accepted by default if no filter function is present.

• onStart?(ctx: SimpleActionContext): void | ActionTrackingReturn

  Called when an action just started. Takes as parameter a simplified action context (more on that later). Can optionally return a result that will cancel the original action and finish it with the returned value / error to be thrown. In either case case resume / suspend / finish will still be called normally.

• onResume?(ctx: SimpleActionContext): void

  Called when an action just resumed a synchronous piece of code execution. Gets called once for sync actions and multiple times for flows. Takes as parameter a simplified action context (more on that later).

• onSuspend?(ctx: SimpleActionContext): void

  Called when an action just finished a synchronous pice of code execution. Note that this doesn't necessarily mean the action is finished. Gets called once for sync actions and multiple times for flows. Takes as parameter a simplified action context (more on that later).

• onFinish?(ctx: SimpleActionContext, ret: ActionTrackingReturn): void | ActionTrackingReturn

  Called when an action just finished, either by returning normally or by throwing an error.

  Takes as parameters:

  • ctx - Simplified action context (more on that later).
  • ret: { result: ActionTackingResult; value: any } - If the action finished normally or due to a thrown error and the returned / thrown value.

  Can optionally return a new return / error value to override the result of the action.

SimpleActionContext

Simplified version of action context, which includes the following readonly data:

• actionName: string - Action name.
• type: ActionContextActionType - Action type, sync or async.
• target: AnyModel - Action target model instance.
• args: ReadonlyArray<any> - Array of action arguments.
• parentContext?: SimpleActionContext - Parent action context, if any.
• rootContext: SimpleActionContext - Root action context, or itself if the root.
• data: any - Custom data for the action context to be set by middlewares, an object. It is advised to use symbols as keys whenever possible to avoid name clashing between middlewares.

It is simplified as in it treats all single synchronous steps of asynchronous actions as if they were in the same context, therefore eliminating the differences between the action contexts of sync actions and flows.

addActionMiddleware

addActionMiddleware adds a global action middleware to be run when an action is performed.

It takes a single parameter, an ActionMiddleware object and returns a disposer function.

The ActionMiddleware object has the following structure:

• subtreeRoot: object

  Subtree (object and child objects) this middleware will run for. This target "filter" will be run before the custom filter.

• filter?(ctx: ActionContext): boolean

  A filter function to decide if an action middleware function should be run or not.

• middleware(ctx: ActionContext, next: () => any): any

  An action middleware function. Remember to return next() if you want to continue the action or throw if you want to cancel it.

ActionContext

Low-level action context, which includes the following readonly data:

• actionName: string - Action name.
• type: ActionContextActionType - Action type, sync or async.
• target: AnyModel - Action target model instance.
• args: ReadonlyArray<any> - Array of action arguments.
• parentContext?: SimpleActionContext - Parent action context, if any.
• rootContext: SimpleActionContext - Root action context, or itself if the root.
• previousAsyncStepContext?: ActionContext - Previous async step context, undefined for sync actions or the first action of a flow.
• spawnAsyncStepContext?: ActionContext - Spawn async step context, or undefined for sync actions.
• asyncStepType?: ActionContextAsyncStepType - Async step type, or undefined for sync actions.
• data: any - Custom data for the action context to be set by middlewares, an object. It is advised to use symbols as keys whenever possible to avoid name clashing between middlewares.