Multiple Components Back

The aim of building modular components is to reuse, to separate different concerns like divide and conquer.

Let's create a simple Avatar component which shows a Facebook page picture and name using the Facebook Graph API.

var Avatar = React.createClass({
    render: function () {
        return (
            <div>
                <PagePic pagename={this.props.pagename} />
                <PageLink pagename={this.props.pagename} />
            </div>
        );
    }
});

var PagePic = React.createClass({
    render: function() {
        return (
            <img src={'https://graph.facebook.com/' + this.props.pagename + '/picture'} />
        );
    }
});

var PageLink = React.createClass({
    render: function() {
        return (
            <a href={'https://www.facebook.com/' + this.props.pagename}>
                {this.props.pagename}
            </a>
        );
    }
});

ReactDOM.render(
    <Avatar pagename="Engineering" />,
    document.getElementById('example')
);

Ownership

In React, an owner is the component that sets the prop of other components. More formally, if a component X is created in component Y's render() method, it is said that X is owned by Y.

It's important to draw a distinction between the owner-ownee relationship and the parent-child relationship. The owner-ownee relationship is specific to React, while the parent-child relationship is simply the one you know and love from the DOM.

Children

<Parent><Children /></Parent>

In React, Parent can read its children by accessing the special this.props.children prop. This is an opaque data structure(不透明數據結構): use the React.Children utilities to manipulate them.

Children Reconciliation(孩子協調)

Reconciliation is the process by which React updates the DOM with each new render pass.

For example, suppose two render passes generate respective markup:

/** render 1 */
<Card>
    <p>Paragraph 1</p>
    <p>Paragraph 2</p>
</Card>

/** render 2 */
<Card>
    <p>Paragraph 2</p>
</Card>

We can see that render 2 has removed <p>Paragraph 1 </p>, and actually React will reconcile the DOM by changing the text content of the first child to Paragraph 2, and destroying the last child.

Stateful Children

For children which maintains data in this.state, they're not going to be destroied in React. In most case, React will just hide elements:

<Card>
    <p style={ { display: 'none' } }>Paragraph 1</p>
    <p>Paragraph 2</p>
</Card>

Dynamic Chidlren

If the identity and state of each child must be maintained across render passes, you can uniquely identify each child by assigning it a key:

var OrderList = React.createClass({
    render: function () {
        var results = this.props.results;
        return (
            <ol>
                {
                    results.map(function (result) {
                        return <li key={result.id}>{result.text}</li>;
                    });
                }
            </ol>
        );
    }
});

Notice that: the key should always be supplied directly to the components in the array, not to the container HTML child of each component in the array:

/** WRONG !! */
var ListItemWrapper = React.createClass({
    render: function() {
        return <li key={this.props.data.id}>{this.props.data.text}</li>;
    }
});

var MyComponent = React.createClass({
    render: function() {
        return (
            <ul>
                {
                    this.props.results.map(function(result) {
                        return <ListItemWrapper data={result}/>;
                    });
                }
            </ul>
        );
    }
});

/** Correct */
var ListItemWrapper = React.createClass({
    render: function() {
        return <li>{this.props.data.text}</li>;
    }
});

var MyComponent = React.createClass({
    render: function() {
        return (
            <ul>
                {
                    this.props.results.map(function(result) {
                        return <ListItemWrapper key={result.id} data={result}/>;
                    });
                }
            </ul>
        );
    }
});

Data Flow

In React, data flows from owner to owned component through props as discussed above. This is an one-way data binding: owners bind their owned component's props to some value the owner has computed based on its props or state.

A Note on Performance

Whether it's expensive to change data if there are a large number of nodes under an owner? No, because JavaScript is fast and render() tends to be quite simple. The real bottleneck is always the DOM mutation, which React will optimize. - Batching(併發) and Change Detection(增量檢測)

If you want to have control over performance, you can override shouldComponentUpdate() to return false when you want React to skip processing of a subtree.

Note:

If shouldComponentUpdate() returns false when data has actually changed, React can't keep your UI in sync. Be sure you know what you're doing while using it, and only use this function when you have a noticeable performance problem. Don't underestimate(低估) how fast JavaScript is relative to the DOM.

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