Data in Foundry progresses through many stages to be rendered in a document sheet. This guide is intended to be a reference for developers trying to understand where they should define data the order in which it is processed.
This guide will reference file locations rather than the official API docs because this is focusing on the internal workings of methods that are not well-commented. All file paths are all relative to foundryInstallPath/resources/app. Technically your installation just loads the general public/scripts/foundry.mjs file, but the files in client and common are smaller and easier to follow.
Legend
Class.method is a static method or property on the given class
Class#method is an instance method or property
¶ Loading from the Database
The server loads up data from the various databases as part of opening a world from the setup screen, transmitting it to users who login to or refresh the /game page.
¶ Game.getData
Foundry begins its path with client/client.mjs, which adds an event listener, DOMContentLoaded, that fires after all of the files Foundry is pulling in have loaded (e.g. all the esmodules). Game.create is called and the returned reference is stashed in globalThis.game. This method establishes a socket connection, uses it to fetch the raw data from the world databases with Game.getData(), and constructs the Game instance, putting all the raw database info in game.data - the first step of our Document journey.
¶ Calling the Document Constructor
The next step is Game#initialize, where we get that init Hook call and "Foundry VTT | Initializing Foundry Virtual Tabletop Game" printed to console. This method eventually calls Game##initializeGameView, which is really just a license-check wrapper for Game#setupGame. Inside that method we get Game#initializePacks and Game#initializeDocuments, the second step of our Document journey (after which the setup hook is called).
The first of these two methods, initializePacks, only instantiates the indices of the packs. The second, initializeDocuments, loops through all of the document data stashed in game.data, calls the appropriate constructor, and then puts it in the appropriate world collection. For example, game.data.actors gets fed into new CONFIG.Actor.documentClass and the result is poured inside game.actors. When all of these documents are loaded "Foundry VTT | Prepared World Documents in ${Math.round(dt)}ms" is printed to console.debug.
The following sections describe what happens inside these constructor calls as well as what happens after a database update.
¶ DataModel
API Reference: https://foundryvtt.com/api/classes/foundry.abstract.DataModel.html
The top level inheritance for documents traces back to the DataModel class, found in common/abstract/data.mjs, which controls the earliest steps of processing. These are run
¶ DataModel#_initializeSource
In the DataModel constructor, the data is first run through DataModel#_initializeSource and then stored in _source. This method applies DataModel#migrateDataSafe, DataModel#cleanData, and DataModel#shimData, ensuring the constructed document matches the specifications from its schema property before saving it in _source.
DataModel then calls _configure, which does nothing in the base class but in Document is responsible for assigning the parentCollection and pack properties. After these adjustments, DataModel#validate runs for one last check.
Validation: Most validation occurs on the DataField level. If a validator returns a boolean, no further checks are needed - e.g. _validateSpecial will return either true or false if the value is null or undefined, but if it's neither then it will return void to pass the value on to be checked by _validateType. Unlike the other steps in _initializeSource, validate is also called every update.
¶ DataModel#_initialize
This method copies data from _source to the top level, using the schema field as a guide. Each property of the schema gets run through the appropriate DataField#initialize method, which performs operations like transforming a stored ID string in ForeignDocumentField#initialize to a pointer to the document instance. Unlike _initializeSource, this runs after every document update.
TypeDataField: This field stands out because it grabs a reference to CONFIG?.[this.documentName]?.dataModels?.[type] to check if there's a class to instantiate for the system property, otherwise it's created as a plain object. This means that system is baseline an open-ended ObjectField; it will save any data so long as you don't directly try to save 'doc.system': false or some kind of non-object value.
If you're only using template.json to define properties, any missing properties will get filled in from the template but otherwise it provides no other validation or handling; if a user updates a property to be a boolean but your template.json value is a string, Foundry won't reject the update, but if a user deletes a property provided by template.json then it will replace that property with the default.
¶ ClientDocument
API Reference: https://foundryvtt.com/api/classes/foundry.ClientDocument.html
The next step is specific to client documents, as these processes are not run in the server. The ClientDocumentMixin is found in client/documents/abstract/client-document.mjs and takes a Document as its argument, for example BaseActor. This document provides the necessary schema information for all of the other processes to work.
Our main function of concern here is ClientDocument#prepareData, which is wrapped by ClientDocument#_safePrepareData with error-catching protections. The actual function is quite straightforward:
prepareData() {
const isTypeData = this.system instanceof foundry.abstract.TypeDataModel;
if ( isTypeData ) this.system.prepareBaseData();
this.prepareBaseData();
this.prepareEmbeddedDocuments();
if ( isTypeData ) this.system.prepareDerivedData();
this.prepareDerivedData();
}
On initial world load, this call is deferred until after ALL documents have been created. Afterwards, it's called for a document after it's created or updated. It can be manually called again as a side effect of other operations that wouldn't otherwise trigger it.
Everything here should be performed using standard javascript assignment (=). Calling update within prepareData or its subsidiary methods can easily trigger an infinite loop, as that update call then creates another prepareData call.
¶ prepareBaseData
This is a good place to initialize any values that aren't stored in the database but are targets for active effects, such as creating a blank array or setting a value to 0. More complicated calculations should usually wait until prepareDerivedData after active effects have been applied, but your specific use case may disagree.
¶ TypeDataModel#prepareBaseData
If you have a system data model, you can run type-specific logic here. Keep in mind that you're operating within the system object, so you'll need to call this.parent to access the actual document properties, e.g. this.parent.items to access the items collection.
¶ ClientDocument#prepareBaseData
Most documents do not do anything here natively; only ActiveEffect, Scene, and Token natively do things here, and ActiveEffect's handling covers a deprecated field.
¶ ClientDocument#prepareEmbeddedDocuments
Baseline, this loops through each of the Document's collections and calls prepareData on each document. Each document has a defined hierarchy to handle possible multiple collections; in the case of actors, Items are prepared before ActiveEffects. The Actor class follows this up with a call to applyActiveEffects, which provides native handling for data modifications.
Developers usually don't need to make direct edits here; they're better off making changes in the appropriate prepareData methods of the embedded documents or in applyActiveEffects.
¶ prepareDerivedData
This is the final place to manipulate a document's data in a way that is generally accessible within the foundry API. Derived data is the place to calculate modifiers, encumbrance, and all of the other pieces of information you want available. What actually needs to be derived depends enormously on what you're building; trad games like D&D, PF, or SWADE have lots of derived data, while more rules light games like various PbtAs have very little derived data.
¶ TypeDataModel#prepareDerivedData
If you have a system data model, you can run type-specific logic here. Keep in mind that you're operating within the system object, so you'll need to call this.parent to access the actual document properties, e.g. this.parent.items to access the items collection.
¶ ClientDocument#prepareDerivedData
Many document classes have their own native handling here that you should keep in mind to call super.prepareDerivedData() for:
- ActiveEffect updates its duration
- Cards ensure all of their faces have images to display
- Chat messages construct rolls from their roll data
- Combats sorts turns
- Combatants pull their display image and update their displayed tracked resource
- Playlists update their
playingproperty - Tiles derive their dimensions
- Users ensure they have images available.
¶ Setup
After all documents have prepared their data for the first time the setup hook fires.
¶ DocumentSheet
The final step in the journey to displaying a document involves the application class stack. Many classes extend DocumentSheet, which can be found in client\apps\form.js. This extends FormApplication which extends Application. There's a lot of logic here that isn't relevant to our goal, which is "How does the document data we've defined in the previous sections get displayed to an end user"; other features are covered in Application.
¶ Application#render
The journey starts with a call to Application#render(true), which "forces" the application to render; by default, render will only refresh an already displayed application. This encapsulates _render with some error handling.
Inside Application#_render, there are several stages of processing possible application render states, including printing "Foundry VTT | Rendering ${this.constructor.name}" and merging in the options until finally a call is made to Application#getData. The return of this is used by Application#_renderInner to call renderTemplate(this.template, data), constructing the html that is eventually displayed.
¶ DocumentSheet#getData
For a DocumentSheet, there are many layers of overrides to that getData call. Between it and FormApplication, you end up with a javascript object that looks like this:
const isEditable = this.isEditable;
{
object: this.object,
options: this.options,
title: this.title,
cssClass: isEditable ? "editable" : "locked",
editable: isEditable,
document: this.document,
data: data,
limited: this.document.limited,
options: this.options,
owner: this.document.isOwner,
title: this.title
}
(note: object is the target of the FormApplication and is the same as document for a DocumentSheet)
Classes like ActorSheet or ItemSheet often add pieces to this, e.g. adding another pointer named after the document class. What happens here though is ultimately only for the purposes of being fed into a renderTemplate call, and so everything that happens here is for display logic.
Things that are good to do in getData
- Associate labels
- Enrich editor data
- Split embedded documents up into categories (e.g. items by type and active effects by enabled)
- Sort embedded documents (ActorSheet already does this for items)
- Construct
selectOptionsfor dropdown fields
After the template is rendered, the context is handed off to the renderDocumentSheet hook and then evaporates, which is why any non-display logic should be processed in the previous steps.
¶ DocumentSheetV2
Alternatively, for HandlebarsApplicationMixin(DocumentSheetV2), the relevant rendering processes involve three classes; ApplicationV2, DocumentSheetV2, and HandlebarsApplicationMixin, all of which can be found in the resources/app/client-esm/applications/api directory.
¶ ApplicationV2#render
The journey starts with ApplicationV2#render({ force: true }); this options object is of type ApplicationRenderOptions. Like AppV1, force controls whether to freshly-render an application not displayed on the screen. Unlike AppV1, render is asynchronous and returns a promise that resolves when the application is fully rendered.
The render method then calls its private partner, ApplicationV2##render. This calls a series of protected functions, only some of which are used by the three classes used above. You can always extend or override these; just keep in mind that the choice of if and where to call super can impact expected functionality.
_canRender, which in DocumentSheetV2 checks against the user'sviewPermission._configureRenderOptions, which is used by ApplicationV2 to update the window and position properties and by HandlebarsApplicationMixin to decide what parts to show (by default all)._prepareContext, which will prepare tabs for applications with a single tab group, and in DocumentSheetV2 will assign commonly referenced properties likedocumentandeditable. (details below)._preFirstRender, which is blank in ApplicationV2 & DocumentSheetV2 but implemented by some core applications._preRender, which is used by HandlebarsApplicationMixin to load the additional templates necessary for rendering._renderFrame, which is used by both ApplicationV2 and DocumentSheetV2 to initialize key application properties such as the close and UUID buttons._attachFrameListeners, which is used by ApplicationV2 to attach listeners to the whole frame such as Form Submission handling._renderHTML, which is implemented by HandlebarsApplicationMixin to generate the HTML of all the parts. This function calls_preparePartContextfor additional support._replaceHTML, which is implemented by HandlebarsApplicationMixin and replaces any previously rendered HTML for a part with the freshly generated HTML._insertElement, which is used by ApplicationV2 to add the Application to the window._updateFrame, which is used by ApplicationV2 to update the header bar properties._onFirstRender, which is used by DocumentSheetV2 to add the application to the document'sappsrecord._onRender, which is used by DocumentSheetV2 to disable inputs if the document is not editable.- The
renderApplicationV2hook is called but not awaited, so any asynchronous callbacks are not guaranteed to finish before the render proceeds. _postRender, which is blank in ApplicationV2 & DocumentSheetV2 but implemented by some core applications.
Broadly speaking, ApplicationV2 handles the window frame + header, while HandlebarsApplicationMixin is responsible for the internal HTML. Everything except _replaceHTML, _insertElement, and _updateFrame is asynchronous and awaited.
¶ _prepareContext and _preparePartContext
The most important step for the typical developer and one that almost certainly needs overrides are the asynchronous functions _prepareContext (added by ApplicationV2) and _preparePartContext (added by HandlebarsApplicationV2).
The reason this step is important is that the context object returned by these functions is fed into a renderTemplate call by HandlebarsApplicationMixin#_renderHTML. This is where all display logic should be handled, such as:
- Associate labels
- Enrich editor data
- Split embedded documents up into categories (e.g. items by type and active effects by enabled)
- Sort embedded documents
- Construct
selectOptionsfor dropdown fields
A very basic _prepareContext call should look something like the following. this.document and this.isEditable are two getters defined by DocumentSheetV2.
async _prepareContext(options) {
const context = await super._prepareContext(options)
return Object.assign(context, {
system: this.document.system,
systemFields: this.document.system.schema.fields,
})
}
Then, in _preparePartContext, you can wrap your logic in a switch(partId) {} statement to prepare elements as needed. This can both reduce the amount of work if you're only selectively rendering parts (such as if a user only has limited permissions) as well as let you cleanly redefine variables across different parts.
Don't be afraid to delegate complex preparation logic to protected or private functions;
context.items = await this._prepareItems(context, options)can help keep your_prepareContextfunction readable.
After the context from _preparePartContext is used in the renderTemplate call it's discarded, so non-display logic should always happen in the earlier document preparation steps. Similarly, while the context object from _prepareContext is passed into many of the functions outlined earlier (e.g. _onFirstRender), it is not "saved" anywhere.
¶ Conclusion
This hopefully was a helpful walkthrough of how information flows from the database to actually being rendered on a user's screen.