Ability to mark subtree in render() for pre-rendering

04 - Layout/05 - Pre Rendering.js

@@ -0,0 +1,68 @@
+import React, { Component } from 'react';
+import ReactDOM from 'react-dom';
+
+let preRenderCache = new WeakMap(); // Could be state instead?
+let measure = document.createElement('div');
+
+export default class WithSize extends Component {
+    componentDidMount () {
+        // If we don't have the item in cache yet, render it.
+        if (!preRenderCache.has(this.props.data)) {
+            // Append a temporary node to the dom to measure it.
+            document.body.appendChild(this.measure);
+
+            // Render the child and append it to the node.
+            let child = this.props.children();
+            let rendered = ReactDOM.render(child, this.measure);
+
+            // Take the measurement and remove the temporary node from the dom.
+            let {clientWidth, clientHeight} = rendered;
+            document.body.removeChild(this.measure);
+
+            // Process layout using the prop function, and force render.
+            let processedLayoutInfo = this.props.processLayout(this.props.data, clientWidth, clientHeight);
+            preRenderCache.set(this.props.data, this.props.children(processedLayoutInfo));
+            this.forceUpdate();
+        }
+    }
+
+    render () {
+        let renderedItem = preRenderCache.get(this.props.data);
+        if (renderedItem) {
+            return renderedItem;
+        }
+
+        // Return nothing until we're ready to render.
+        return null;
+    }
+}
+
+WithSize.propTypes = {
+    children: React.PropTypes.func,
+    data: React.PropTypes.object,
+    processLayout: React.PropTypes.func,
+};
+
+
+// Usage looks like: 
+class DynamicList extends Component {
+    processLayout(width, height) {
+        return {
+            top, left, bottom, right
+        };
+    }
+
+    render () {
+        return (
+            <div>
+                {this.props.items.map((item, idx) =>
+                    <WithSize data={item} key={idx} processLayout={this.processLayout.bind(this)}>
+                    {
+                        (position={left: 0, top: 0}) => <div style={{top: position.top, left: position.left}}>...</div>
+                    }
+                    </WithSize>
+                )}
+            </div>
+        );
+    }
+}