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Two-way Binding Helpers

Note: LinkedStateMixin is deprecated as of React v15. The recommendation is to explicitly set the value and change handler, instead of using LinkedStateMixin.

Importing

import LinkedStateMixin from 'react-addons-linked-state-mixin'; // ES6
var LinkedStateMixin = require('react-addons-linked-state-mixin'); // ES5 with npm

Overview

LinkedStateMixin is an easy way to express two-way binding with React.

In React, data flows one way: from owner to child. This is because data only flows one direction in the Von Neumann model of computing. You can think of it as "one-way data binding."

However, there are lots of applications that require you to read some data and flow it back into your program. For example, when developing forms, you'll often want to update some React state when you receive user input. Or perhaps you want to perform layout in JavaScript and react to changes in some DOM element size.

In React, you would implement this by listening to a "change" event, read from your data source (usually the DOM) and call setState() on one of your components. "Closing the data flow loop" explicitly leads to more understandable and easier-to-maintain programs. See our forms documentation for more information.

Two-way binding -- implicitly enforcing that some value in the DOM is always consistent with some React state -- is concise and supports a wide variety of applications. We've provided LinkedStateMixin: syntactic sugar for setting up the common data flow loop pattern described above, or "linking" some data source to React state.

Note:

LinkedStateMixin is just a thin wrapper and convention around the onChange/setState() pattern. It doesn't fundamentally change how data flows in your React application.

LinkedStateMixin: Before and After

Here's a simple form example without using LinkedStateMixin:

var createReactClass = require('create-react-class');

var NoLink = createReactClass({
  getInitialState: function() {
    return {message: 'Hello!'};
  },
  handleChange: function(event) {
    this.setState({message: event.target.value});
  },
  render: function() {
    var message = this.state.message;
    return <input type="text" value={message} onChange={this.handleChange} />;
  }
});

This works really well and it's very clear how data is flowing, however, with a lot of form fields it could get a bit verbose. Let's use LinkedStateMixin to save us some typing:

var createReactClass = require('create-react-class');

var WithLink = createReactClass({
  mixins: [LinkedStateMixin],
  getInitialState: function() {
    return {message: 'Hello!'};
  },
  render: function() {
    return <input type="text" valueLink={this.linkState('message')} />;
  }
});

LinkedStateMixin adds a method to your React component called linkState(). linkState() returns a valueLink object which contains the current value of the React state and a callback to change it.

valueLink objects can be passed up and down the tree as props, so it's easy (and explicit) to set up two-way binding between a component deep in the hierarchy and state that lives higher in the hierarchy.

Note that checkboxes have a special behavior regarding their value attribute, which is the value that will be sent on form submit if the checkbox is checked (defaults to on). The value attribute is not updated when the checkbox is checked or unchecked. For checkboxes, you should use checkedLink instead of valueLink: <input type="checkbox" checkedLink={this.linkState('booleanValue')} />

Under the Hood

There are two sides to LinkedStateMixin: the place where you create the valueLink instance and the place where you use it. To prove how simple LinkedStateMixin is, let's rewrite each side separately to be more explicit.

valueLink Without LinkedStateMixin

var createReactClass = require('create-react-class');

var WithoutMixin = createReactClass({
  getInitialState: function() {
    return {message: 'Hello!'};
  },
  handleChange: function(newValue) {
    this.setState({message: newValue});
  },
  render: function() {
    var valueLink = {
      value: this.state.message,
      requestChange: this.handleChange
    };
    return <input type="text" valueLink={valueLink} />;
  }
});

As you can see, valueLink objects are very simple objects that just have a value and requestChange prop. And LinkedStateMixin is similarly simple: it just populates those fields with a value from this.state and a callback that calls this.setState().

LinkedStateMixin Without valueLink

var LinkedStateMixin = require('react-addons-linked-state-mixin');
var createReactClass = require('create-react-class');

var WithoutLink = createReactClass({
  mixins: [LinkedStateMixin],
  getInitialState: function() {
    return {message: 'Hello!'};
  },
  render: function() {
    var valueLink = this.linkState('message');
    var handleChange = function(e) {
      valueLink.requestChange(e.target.value);
    };
    return <input type="text" value={valueLink.value} onChange={handleChange} />;
  }
});

The valueLink prop is also quite simple. It simply handles the onChange event and calls this.props.valueLink.requestChange() and also uses this.props.valueLink.value instead of this.props.value. That's it!