feat(AssetsController): Basic functionality

[Kriys94]

Explanation

What is the current state of things and why does it need to change?

The AssetsController always uses all balance data sources (BackendWebsocket, AccountsApi, Snap, RPC) for balance fetch and subscribe, and the API for metadata and price. Extension and mobile need a way to run in a "basic functionality" mode where only RPC is used for balance and token/price APIs are not used (e.g. when the user opts in or in constrained environments).

What is the solution your changes offer and how does it work?

• isBasicFunctionality — Optional constructor option: a getter () => boolean. No value is stored in the controller; the getter is invoked when deciding which data sources to use.
• When the getter returns true: only balance fetch and balance subscribe use RPC (RpcDataSource only). Token and price APIs are not used: the force-update fetch path omits token and price middlewares (no metadata/price from API), and subscribeAssetsPrice returns early (no price subscription). So in basic mode, getAssets / getAssetsBalance / getAssetsPrice and price subscription do not call the token or price API.
• handleBasicFunctionalityChange(isBasic: boolean) — New public method. The consumer (extension/mobile) calls it when the user toggles the setting; it stops current subscriptions and re-subscribes so the next run uses the current getter value (RPC-only vs full balance sources).
• The logic that lived in a #subscriptionBalanceDataSources getter is inlined into #subscribeAssetsBalance (basic: [this.#rpcDataSource], otherwise this.#allBalanceDataSources).

Are there any changes whose purpose might not be obvious?

• The isBasic parameter to handleBasicFunctionalityChange is for call-site clarity only; the getter remains the source of truth when the controller runs.
• #allBalanceDataSources is still used in #stop() so we can unsubscribe from every balance source when tearing down (e.g. when switching from full to basic).

If your primary goal was to update one package but you found you had to update another one along the way, why did you do so?

N/A — only @metamask/assets-controller was modified.

If you had to upgrade a dependency, why did you do so?

No dependency upgrades in this PR.

References

• Changelog updated in this repo: packages/assets-controller/CHANGELOG.md.
• No related issues or client/consumer PRs linked; additive API for extension/mobile to adopt when they add the basic functionality toggle.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Medium Risk
Touches core asset fetch/subscription routing and selectively disables price/metadata enrichment, which could change observed balances/prices and subscription lifecycles if the toggle is miswired or races with initialization.

Overview
Adds an opt-in basic functionality mode to AssetsController via new constructor options isBasicFunctionality() and subscribeToBasicFunctionalityChange(onChange), plus a public handleBasicFunctionalityChange to tear down and re-subscribe when the setting flips.

When basic functionality is off, the controller switches to RPC-only balance subscriptions, skips token/price middlewares during forceUpdate fetches, and disables subscribeAssetsPrice entirely; cleanup logic is updated to unsubscribe from all balance sources regardless of mode. Tests and docs are updated, and @metamask/preferences-controller is added as a dependency/type reference.

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[Prithpal-Sooriya]

nit - Hmmm unsure if types are lost, can we add strict types?

Suggested change

	const [{ state = {}, isBasicFunctionality }, fn] =
	args.length === 2 ? args : [{}, args[0]];
	const [{ state = {}, isBasicFunctionality }, fn]: [
	WithControllerOptions,
	WithControllerCallback<ReturnValue>,
	] = args.length === 2 ? args : [{}, args[0]];

[Prithpal-Sooriya]

nit - can this be simplified?

Suggested change

	...(isBasicFunctionality !== undefined && { isBasicFunctionality }),
	isBasicFunctionality,

[Prithpal-Sooriya]

Same double check for me, I think this is inverted lol

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.