A few days ago, we [wrote a post about upcoming changes to our legacy lifecycle methods](/blog/2018/03/27/update-on-async-rendering.html), including gradual migration strategies. In React 16.3.0, we are adding a few new lifecycle methods to assist with that migration. We are also introducing new APIs for long requested features: an official context API, a ref forwarding API, and an ergonomic ref API.
For many years, React has offered an experimental API for context. Although it was a powerful tool, its use was discouraged because of inherent problems in the API, and because we always intended to replace the experimental API with a better one.
Previously, React provided two ways of managing refs: the legacy string ref API and the callback API. Although the string ref API was the more convenient of the two, it had [several downsides](https://github.com/facebook/react/issues/1373) and so our official recommendation was to use the callback form instead.
Generally, React components are declarative, but sometimes imperative access to the component instances and the underlying DOM nodes is necessary. This is common for use cases like managing focus, selection, or animations. React provides [refs](/docs/refs-and-the-dom.html) as a way to solve this problem. However, component encapsulation poses some challenges with refs.
For example, if you replace a `<button>` with a custom `<FancyButton>` component, the `ref` attribute on it will start pointing at the wrapper component instead of the DOM node (and would be `null` for function components). While this is desirable for "application-level" components like `FeedStory` or `Comment` that need to be encapsulated, it can be annoying for "leaf" components such as `FancyButton` or `MyTextInput` that are typically used like their DOM counterparts, and might need to expose their DOM nodes.
Ref forwarding is a new opt-in feature that lets some components take a `ref` they receive, and pass it further down (in other words, "forward" it) to a child. In the example below, `FancyButton` forwards its ref to a DOM `button` that it renders:
This way, components using `FancyButton` can get a ref to the underlying `button` DOM node and access it if necessary—just like if they used a DOM `button` directly.
Ref forwarding is not limited to "leaf" components that render DOM nodes. If you write [higher order components](/docs/higher-order-components.html), we recommend using ref forwarding to automatically pass the ref down to the wrapped class component instances.
React's class component API has been around for years with little change. However, as we add support for more advanced features (such as [error boundaries](/docs/react-component.html#componentdidcatch) and the upcoming [async rendering mode](/blog/2018/03/01/sneak-peek-beyond-react-16.html)) we stretch this model in ways that it was not originally intended.
For example, with the current API, it is too easy to block the initial render with non-essential logic. In part this is because there are too many ways to accomplish a given task, and it can be unclear which is best. We've observed that the interrupting behavior of error handling is often not taken into consideration and can result in memory leaks (something that will also impact the upcoming async rendering mode). The current class component API also complicates other efforts, like our work on [prototyping a React compiler](https://twitter.com/trueadm/status/944908776896978946).
Many of these issues are exacerbated by a subset of the component lifecycles (`componentWillMount`, `componentWillReceiveProps`, and `componentWillUpdate`). These also happen to be the lifecycles that cause the most confusion within the React community. For these reasons, we are going to deprecate those methods in favor of better alternatives.
We recognize that this change will impact many existing components. Because of this, the migration path will be as gradual as possible, and will provide escape hatches. (At Facebook, we maintain more than 50,000 React components. We depend on a gradual release cycle too!)
> Even in version 17, it will still be possible to use them, but they will be aliased with an "UNSAFE_" prefix to indicate that they might cause issues. We have also prepared an [automated script to rename them](https://github.com/reactjs/react-codemod#rename-unsafe-lifecycles) in existing code.
* [`getDerivedStateFromProps`](/docs/react-component.html#static-getderivedstatefromprops) is being added as a safer alternative to the legacy `componentWillReceiveProps`. (Note that [in most cases you don't need either of them](/blog/2018/06/07/you-probably-dont-need-derived-state.html).)
* [`getSnapshotBeforeUpdate`](/docs/react-component.html#getsnapshotbeforeupdate) is being added to support safely reading properties from e.g. the DOM before updates are made.
`StrictMode` is a tool for highlighting potential problems in an application. Like `Fragment`, `StrictMode` does not render any visible UI. It activates additional checks and warnings for its descendants.
Although it is not possible for strict mode to catch all problems (e.g. certain types of mutation), it can help with many. If you see warnings in strict mode, those things will likely cause bugs for async rendering.
