block4.js

render() {
	return (
	  <div
	    className="App"
	    ref="myscroll"
	    style={{ height: "420px", overflow: "auto" }}
	  >
	    <header className="App-header">
	      <h1 className="App-title">Welcome to React</h1>
	    </header>
	    <ul>
	      {this.showItems()}
	    </ul>
	    {this.state.loading
	      ? <p className="App-intro">
		  loading ...
		</p>
	      : ""}
	  </div>
	);
}