How to Access Member Functions in Polymer Elements

This article explores accessing member functions within Polymer objects, a common challenge for Polymer developers. We'll examine correct and incorrect approaches, highlighting the role of Shadow DOM encapsulation.

Key Takeaways

• Accessing Polymer object member functions directly by element name (without the # ID selector) is the recommended approach.
• The polymer-ready event ensures the element is fully upgraded before attempting function access, preventing undefined errors.
• Polymer Elements are reusable custom HTML elements, enhancing web development efficiency. They work seamlessly with various JavaScript libraries and frameworks.

The Pitfalls of Incorrect Access

Consider a web component: <x id="radial-button-template"></x>. Attempting to access its functions via its ID using document.querySelector("#radial-button-template") fails due to Shadow DOM encapsulation. The returned element doesn't expose the internal functions; they appear as undefined.

The Correct Approaches

Method 1: Direct Element Name Access

The most straightforward method is to use the element's tag name directly in the querySelector:

var btn = document.querySelector("x-radial-buttons");
btn.getFirstElement; // Correctly returns the element

This bypasses Shadow DOM restrictions and provides direct access to member functions. It's generally preferred to avoid assigning IDs to Polymer elements for this reason.

Method 2: The polymer-ready Event

Polymer's asynchronous element upgrade process can cause issues if you try to access functions before the upgrade completes. The polymer-ready event solves this:

window.addEventListener('polymer-ready', function(e) {
  var xFoo = document.querySelector('x-foo');
  xFoo.barProperty = 'baz'; // Access functions here
});

This ensures that function calls occur only after the Polymer element is fully initialized.

Practical Application

The following JavaScript snippet demonstrates accessing Polymer elements and their functions:

(function(PokémonApp) {
  // ... (other code) ...

  form.addEventListener('submit', function(e) {
    e.preventDefault();
    playerElement.speak();
    var btn = document.querySelector("x-radial-buttons");
    btn.getFirstElement(); // Correct access
  });

  // ... (rest of the code) ...
})(PokémonApp);

This code correctly accesses getFirstElement after the x-radial-buttons element is ready.

Further Learning Resources

Microsoft offers extensive free learning resources on JavaScript and web development, including tutorials on performance optimization, web platform fundamentals, and building Universal Windows Apps using HTML and JavaScript. These resources, along with tools like Visual Studio Community and Azure Trial, provide a comprehensive learning path. This article is part of a web development series from Microsoft.

Frequently Asked Questions

This section has been omitted as it contains information already covered in the main body of the rewritten article. The key points regarding Polymer elements, their functionality, and methods of accessing member functions are already addressed above.

The above is the detailed content of How to Access Member Functions in Polymer Elements. For more information, please follow other related articles on the PHP Chinese website!