Add dynamically measured images example

Fixes #979

Author: kirill-konshin

docs/Masonry.md

@@ -197,3 +197,104 @@ ReactDOM.render(
   document.getElementById('example')
 );
 ```
+
+### Masonry example with dynamically measured images
+
+Items in the list in basic example can only be measured once, which means client has to know all the
+sizes upfront before displaying. For cases when server cannot deliver such data we need to
+pre-measure images. In order to preserve correct optimized layout images can only be displayed one
+by one in the order they appear in the array, not in the order they are loaded.
+
+These specifics were taken into account in a small library `react-virtualized-image-measurer`,
+here is an example with dynamically measured images:
+
+```js
+import React from "react";
+import { render } from "react-dom";
+import {
+  CellMeasurer,
+  CellMeasurerCache,
+  createMasonryCellPositioner,
+  Masonry
+} from "react-virtualized";
+import ImageMeasurer from "react-virtualized-image-measurer";
+
+// Array of images with captions
+//const list = [{image: 'http://...', title: 'Foo'}];
+
+// We need to make sure images are loaded from scratch every time for this demo
+const noCacheList = list.map(item => ({
+  ...item,
+  image: item.image + "?noCache=" + Math.random()
+}));
+
+const columnWidth = 200;
+const defaultHeight = 250;
+const defaultWidth = columnWidth;
+
+const MasonryComponent = ({ itemsWithSizes }) => {
+  // Default sizes help Masonry decide how many images to batch-measure
+  const cache = new CellMeasurerCache({
+    defaultHeight,
+    defaultWidth,
+    fixedWidth: true
+  });
+
+  // Our masonry layout will use 3 columns with a 10px gutter between
+  const cellPositioner = createMasonryCellPositioner({
+    cellMeasurerCache: cache,
+    columnCount: 3,
+    columnWidth,
+    spacer: 10
+  });
+
+  function cellRenderer({ index, key, parent, style }) {
+    const { item, size } = itemsWithSizes[index];
+    const height = columnWidth * (size.height / size.width) || defaultHeight;
+
+    return (
+      <CellMeasurer cache={cache} index={index} key={key} parent={parent}>
+        <div style={style}>
+          <img
+            src={item.image}
+            alt={item.title}
+            style={{
+              height: height,
+              width: columnWidth
+            }}
+          />
+          <h4>{item.title}</h4>
+        </div>
+      </CellMeasurer>
+    );
+  }
+
+  return (
+    <Masonry
+      cellCount={itemsWithSizes.length}
+      cellMeasurerCache={cache}
+      cellPositioner={cellPositioner}
+      cellRenderer={cellRenderer}
+      height={600}
+      width={800}
+    />
+  );
+};
+
+// Render your grid
+render(
+  <ImageMeasurer
+    items={noCacheList}
+    image={item => item.image}
+    defaultHeight={defaultHeight}
+    defaultWidth={defaultWidth}
+  >
+    {({ itemsWithSizes }) => (
+      <MasonryComponent itemsWithSizes={itemsWithSizes} />
+    )}
+  </ImageMeasurer>,
+  document.getElementById("root")
+);
+```
+
+Live demo: https://codesandbox.io/s/7y66p25qv6