Your First Conditional Template with Dart Web Components

As if data binding and custom elements with Dart Web UI isn't enough, you can also conditionally render blocks of elements via simple declarative attributes! The conditionals are "live" and are re-evaluated when data changes, keeping your page dynamic and up-to-date.



(You may want to read Your First Model Driven View with Dart, which contains more information on getting started and configure the necessary tools. This post assumes you've read my previous posts on Web Components and Dart.)

Conditionals templates

With Dart Web UI, you can conditionally display elements. For example, you can declaratively say "If the checkbox is checked, then display the thank you notice." Here's an example:

 <template if="acceptLicense">  
  <h2>Thanks!</h2>  
  <p>We'll send you an activation key to your email.</p>  
 </template>

In the above example, the <h2> and <p> will only be added to the DOM if the acceptLicense variable becomes true. If acceptLicense becomes false, the template contents are removed from the DOM.

A <template> is an inert set of elements, which is parsed but not activated and not immediately added to the DOM. To render the elements in a template, the conditional in the if must become true. You can use any Dart code that results in a boolean for the conditional.

Full example

Here is a complete Dart Web UI page with a conditional template. The below example also demonstrates data binding with a checkbox.

 <!DOCTYPE html>  
   
 <html>  
  <head>  
   <meta charset="utf-8">  
   <title>Simple Conditional</title>  
   <link rel="stylesheet" href="App.css">  
  </head>  
  <body>  
   <h1>Simple Conditional with Data Binding</h1>  
     
   <p>  
    <label>  
     <input type="checkbox" id="accept-license" bind-checked="acceptLicense">  
     I accept the license  
    </label>  
   </p>  
     
   <template if="acceptLicense">  
    <h2>Thanks!</h2>  
    <p>We'll send the activation key to your email.</p>  
   </template>  
   
   <script type="application/dart">  
    bool acceptLicense = false;  
    main() { }  
   </script>  
   <script src="https://dart.googlecode.com/svn/branches/bleeding_edge/dart/client/dart.js"></script>  
  </body>  
 </html>

To compile the above file into vanilla JavaScript and HTML, learn more about the Dart Web Components tools.

Working example

Enjoy this fully working example, compiled to JavaScript.



Summary

Use conditional templates with data binding to control the display of blocks of elements. Use a <template if="some_boolean_expression"> to conditionally render elements to a page. The conditional template is "live" and is re-evaluated when data changes.

You can see the source for this and more Dart Web Components samples on Github. Meanwhile, read more about Dart Web Components. Have fun!

(image courtesy of http://www.flickr.com/photos/84987970@N00/4981201339/)

Popular posts from this blog

Lists and arrays in Dart

(This is Part 3 in a series about Dart. Check out Part 2, Function in Dart .) Warning : We expect the Dart libraries to undergo potentially sweeping changes before Dart goes to alpha. This document is relevant as of 2011-12-22. Intro Dart is a "batteries included" effort to help app developers build modern web apps. An important "battery" is the bundled core Dart libraries, providing common and rich functionality. Dart is building a solution for large, complex web apps, and providing well tested, integrated, and common libraries is key to helping a web app developer be more productive out of the box. The Collection libraries are a crucial set of APIs that Dart developers get for free. Much more than simple arrays and maps, the Collection library includes standard ways to filter, iterate, inspect, compose, and sort your data. This post specifically looks at List<E> , Dart's ordered, indexable collection of objects. Aside: The Dart project is lucky
Read more

Converting Array to List in Scala

Now, this has to have a built-in somewhere in Scala , because it just seems too common. So, how to convert an Array to a List in Scala? Why do I need this? I needed to drop to Java for some functionality, which in this case returns an Array. I wanted to get that Array into a List to practice my functional programming skillz. **Update**: I figured out how to convert Arrays to Lists the Scala way. Turns out it's a piece of cake. val myList = List.fromArray(Array("one", "two", "three")) or val myList = Array("one","two","three").elements.toList The call to elements returns an Iterator , and from there you can convert to a List via toList . Nice. Because my first version wasn't actually tail recursive, what follows is a true tail recursive solution, if I were to implement this by hand. The above, built in mechanism is much better, though. object ArrayUtil { def toList[a](array: Array[a]): List[a] = { d
Read more

Null-aware operators in Dart

Image
Three new language features just landed in the latest dev channel build of the Dart ! Collectively known as null-aware operators , these new features will help you reduce the code required to work with potentially null objects. I'm excited for these new abilities, because typing less is always a good thing. Read on to learn more, and be sure to try these new features on Dart Pad . ?? Use ?? when you want to evaluate and return an expression IFF another expression resolves to null. exp ?? otherExp is similar to ((x) => x == null ? otherExp : x)(exp) ??= Use ??= when you want to assign a value to an object IFF that object is null. Otherwise, return the object. obj ??= value is similar to ((x) => x == null ? obj = value : x)(obj) ?. Use ?. when you want to call a method/getter on an object IFF that object is not null (otherwise, return null). obj?.method() is similar to ((x) => x == null ? null : x.method())(obj) Yo
Read more