Disclaimer: These are planning documents. The functionalities described here may be unimplemented, partially implemented, or implemented differently than the original design.

Canopy 0.2


The goal of Canopy is to provide a container for saplings - custom client and server applications for Splinter application - and system-like calls for things like users and private keys.

The current architecture of Canopy allows for only a single sapling to reside in the container at any point in time which requires a refresh of the page to switch to alternate saplings. Additionally, it leaves it to the sapling to install itself into the container (described as “mounting”). This creates a set of issues around installing new saplings at run-time, as well as introduces a number of issues with state management for the existing saplings.

This design introduces a new system where Canopy controls the life-cycle of saplings and can install or remove saplings at run-time.

Guide-level explanation

This new design for Canopy introduces new concepts and configurations: a new model for describing saplings, a new way to install/launch saplings, and an intent system for communicating between saplings. It also removes prior distinction between user saplings and configuration saplings.

Intent system

Much of what will power canopy is the intent system. This system is a specialized event/messaging system. The events consist of the event name and an object that contains arbitrary data relative to that event. The arbitrary intent data may be merged with canopy data, depending on the type of intent.

Sapling descriptors

Saplings will be described by a manifest that contains details about an installer script and a set of core scripts. These manifests may be given to canopy in a list of currently enabled saplings during initialization, or one at a time, as new saplings are installed at run-time.

Installing saplings

When a sapling is installed using its manifest, canopy will call the function declared within the manifest. It will provide the installer function a subset of the canopy system API to perform actions, such as adding navigation items, configuring drop-down entries or scheduling background tasks. They may also perform standard JS tasks, such as starting web workers or service workers to manage their own background tasks (though, these workers will not have access to canopy system API functions).

The installer function is expected to be a quickly-executing function, and should make no async calls that will block its completion.

Launching saplings

Saplings are launched when an intent they have registered for has been sent via the canopy system API. It is up to the sapling’s core code to decide what is done with the given intent. It may render code in the main canopy view area or it may trigger a background operation.

Canopy will intercept all intents for a given sapling and ensure that the core files have been loaded and installed, before the intent handler is run.

Reference-level explanation

Intent system

The intent system is modeled after loosely coupled application interactions where applications can register to handle specific “intents”. An intent can be sent by one component, either Canopy itself or another sapling, and will be handled by the registered sapling. Intents can be general requests that may be handled by multiple saplings, where the actual executing sapling is chosen by the user.

For example, a sapling may register for the intent canopy::login to provide user authentication client behaviour. This sapling would provide a set of UIs and behaviours appropriate for its back-end service provider, be it Splinter Biome, or OAuth, etc.

Canopy will have a set of common intents that saplings may register for. The current list is

  • canopy::login - this intent is fired if a request to authenticate is made.

A intent handler is called with the following signature:

function (intent, intentData) {
  // ...

Canopy will wrap these handlers and potentially inject values into the intentData object provided above.

Sapling descriptors

The manifest for saplings changes from a declaration of what canopy can display (icons, etc) to a more general declaration of details about a given sapling. Beyond providing a name, version, and intent namespace (more on that below), the files are divided into to main parts: the installer and the core files.

The installer is declared with a single source file. This source file should contain a light-weight javascript file that will install what it needs to configure canopy to use the sapling. This file will be loaded after all manifests have been read.

The core files include one or more sources, which defines and handles any intents that the system has registered to use. These files will not be loaded immediately.

The following is an example of a manifest:

  "manifestVersion": 1,
  "name": "Sapling 1",
  "version": 1,
  "intentNS": "sapling1"
  "installer": {
    "source": {
      "src": "https://example.com/sapling1-install.js",
      "hash": "<sha512 of src file>"
    "installFn": "installSapling",
  "core": {
    "sources": [
        "src": "https://example.com/sapling1-core1.js",
        "hash": "<sha512 of src file>"

Multiple sapling manifests can be specified in an JSON array.

Installing saplings

Once a manifest is loaded by canopy, the system will load the installer src file. Canopy will call the installFn value on the window. This install function will take a subset of the canopy system API that is relavent at install time.

The API will include the following (all optional fields or arguments will be prefixed with ?):

  • registerIntentHandler(intent, handler)

    This function registers an intent handler for the sapling. The handler handler must take into account that the actual logic may not yet be loaded at the time of this call.

    More on the intent system below.

  • addNavIcon({iconImg, shortName, longName, intent, ?intentData})

    This function adds an item to the navigation pane provided by canopy. The provided intent and its optional data will be emitted when the icon is clicked.

    All navigation items will have the following merged into the optionally provided intent data:

        // The root DOM node where content may be mounted.
        rootDomNode: <HTMLDomElement>,
        // The canopy system API.
        canopySystem: <CanopySystemAPI>,
  • addMenuItem(menuName, {itemName, intent, ?intentdata, ?updatesUI})

    This function adds an item to the drop-down menu provided by canopy. The provided intent and its optional data will be emitted when the icon is clicked.

    All menu items will have the following merged into the optionally provided intent data (with optionally available items prefixed with a ?:

        // The canopy system API.
        canopySystem: <CanopySystemAPI>,
        // The root DOM node where content may be mounted, if the item is marked
        // as `updatesUI = true`
        ?rootDomNode: <HTMLDomElement>,
  • scheduleBackgroundEffect({intent, ?intentData, interval, ?delay})

    This function schedules an intent to be emitted periodically. If a delay is specified, the intent will not be fired until the delay has passed. If no delay is fired, it will begin in the next tick after all manifests have been installed.

While the install function for a sapling may make async calls, canopy will not wait for the calls to complete before it moves on to the next sapling manifest.

Launching saplings

Saplings are launched on the first invocation of an intent handler. At launch time, the files specified in the core section of the manifest will be loaded.

This does mean that on first execution of an intent handler may experience some loading delay. All subsequent loads should be O(1).

The registered handler for the sapling will receive an object that has the canopy system API. In addition to the methods provided on installation, the canopy system API has the following functions:

  • sendIntent(intent, ?intentData)

    This function will send an intent that may be handled by any other sapling or core canopy handlers in the system.

  • getUser()

    This function will return the current user details, if a user has been authenticated, undefined otherwise.

    The details of the user object returned is beyond the scope of this document.

  • setUser(user)

    This function will set the current user details. This is useful for saplings that provide authentication services. A null or undefined user will unset the current user.

    The details of the user object expected is beyond the scope of this document.

  • getKeys()

    This function will return the current array of signing keys for the authenticated user, undefined otherwise.

    The details of the keys value is beyond the scope of this document.

  • setKeys(keys)

    This function will set the current array of signing keys for the authenticated user. A value of null or undefined for keys will unset the current keys.

    The details of the keys value is beyond the scope of this document.


This design removes the sandboxed nature of the current canopy implementation. In that iteration, when a sapling is loaded, the whole page is reloaded, in order to effectively give it process isolation of its data. The new system does not make any serious effort to sandbox each saplings data.

This sandboxed model does have a number of limitations on how saplings may interact with each other, as they have no ability to send data to other saplings, nor does it allow for pluggable features like authentication alternatives.

Prior art

The sapling concept is a continuation and improvement over what currently is implemented in splinter-canopyjs and splinter-saplingjs.

In this model, saplings take on more of the behaviour of plugins in many common systems. They don’t drive canopy but are driven by it.

The intent system is inspired by Android.

Unresolved questions

This document does not address the following:

  • Unloading saplings

    Saplings could be removed dynamically, and so this leaves open the question of how much needs to be unloaded from the running canopy state.