@storybook/preview-web

Preview (Web)

This is the main API for the (web) version of the Storybook Preview.

The preview's job is:

1. Read and update the URL (via the URL Store)

2. Listen to instructions on the channel and emit events as things occur.

3. Render the current selection to the web view in either story or docs mode.

V7 Store vs Legacy (V6)

The story store is designed to load stories 'on demand', and will operate in this fashion if the storyStoreV7 feature is enabled.

However, for back-compat reasons, in v6 mode, we need to load all stories, synchronously on bootup, emitting the SET_STORIES event.

In V7 mode we do not emit that event, instead preferring the STORY_PREPARED event, with the data for the single story being rendered.

Initialization

The preview is initialized in two ways.

V7 Mode:

• importFn - is an async import() function

• getProjectAnnotations - is a simple function that evaluations preview.js and addon config files and combines them. If it errors, the Preview will show the error.

• No getStoryIndex function is passed, instead the preview creates a StoryIndexClient that pulls stories.json from node and watches the event stream for invalidation events.

V6 Mode

• importFn - is a simulated import() function, that is synchronous, see client-api for details.
• getProjectAnnotations - also evaluates preview.js et al, but watches for calls to setStories, and passes them to the ClientApi
• getStoryIndex is a local function (that must be called after getProjectAnnotations) that gets the list of stories added.

See client-api for more details on this process.

Story Rendering and interruptions

The Preview is split into three parts responsible for state management:

• PreviewWeb - which story is rendered, receives events and (maybe) changes/re-renders stories
• StoryRender - (imports +) prepares the story, renders it through the various phases
• DocsRender - if a story renders in docs mode, it is "transformed" into a DocsRender once we know.

A rendering story goes through these phases:

• preparing - (maybe async) import the story file and prepare the story function.
• loading - async loaders are running
• rendering - the renderToDOM function for the framework is running
• playing - the play function is running
• completed - the story is done.

It also has two error states:

• aborted - the story was stopped midway (see below)
• errored - there was an error thrown somewhere along the way.

Re-rendering and aborting

A story may re-render due to various events, which can have implications if the story is not in the completed phase:

• UPDATE_STORY_ARGS / UPDATE_GLOBALS -- change of inputs
• FORCE_RE_RENDER - re-render unchanged

If these events happen during a render:

• if the story is preparing or loading, leave thing unchanged and let the new args/globals be picked up by the render phase

• otherwise, use the result of the previous loaders run, and simply re-render over the top

• FORCE_REMOUNT - remount (or equivalent) the component and re-render.

If this happens during a render, treat loading similarly, but:

• if the story is rendering, start a new render and abort the previous render immediately afterwards
• if the story is playing, attempt to abort the previous play function, and start a new render.

Changing story

Also the SET_CURRENT_STORY event may change the current story. We need to check:

• If the storyId changed
• If the viewMode changed
• If the story implementation changed (i.e if HMR occurred).

If the previous story is still preparing, we cannot know if the implementation changed, so we abort the preparing immediately, and let the new story take over.

Otherwise, if all of the above are the same, we do nothing.

If they are different, and the old story is not completed, we try to abort it immediately. If that fails (e.g. the play function doesn't respond to the abort event), then we reload the window.