Linking Polymer Model Updates to AngularJS Digest Cycle

A few weeks ago I wrote a post on Using Polymer WebComponents with Angular.js. I had the opportunity to revisit the post and create a component that uses Polymer's property change notification system. This allows us to create a more performant version of the same codebase!

bindPolymer Directive

In the previous post that I wrote regarding using these two frameworks together we utilized the bindPolymer element.

The bindPolymer directive has some pretty cool things going on inside of it:

  • Parsing attributes looking for data-binding
  • Register a MutationObserver for each property that is data-bound (Looking for the {{}} syntax)
  • On a change inside of the MutationObserver push those values to the angular scope and trigger a scope.$apply()
  • Set up listeners to clear out unneeded MutationObservers on element removal

Per a suggestion in the comments of my last post I decided to make some changes.

The new approach

The main thing that I removed when revisiting this was the use of the MutationObserver. Per a suggestion in the comments on my last post, I opted to use the <property>-changed notifications that are built in to Polymer attributes. My revised solution utilizes the attribute parsing from the original bindPolymer element but then utilizes the built in events system to handle scope updates as well as the listener teardown.

The Code

What everyone has been waiting for right?

Attribute Parsing

This portion is reused from the original bindPolymer element. Because we are utilizing the compile directive property we are able to access the raw directive template and tell which values we need to set up bindings for.

...
compile: function bindPolymerCompile(el, attr) {  
  var attrMap = {};
  for (var prop in attr) {
    if (angular.isString(attr[prop])) {
      var _match = attr[prop].match(/\{\{\s*([\.\w]+)\s*\}\}/);
      if (_match) {
        attrMap[prop] = $parse(_match[1]);
      }
    }
  }
  ...
}
...

We parse through all of the attributes looking for those using the binding ({{}}) syntax. For each of those we map the angular scope value to the property value for later usage.

Setting up the Event Listeners

This is where things change. After populating the attrMap (shown above) we then loop through each of the keys and create an event listener looking for key + '-changed' (see the Polymer docs for more on property change notifications).

Each event then enters a scope.$evalAsync block that will trigger an angular $digest after the contents have all been executed (we use scope.$evalAsync to prevent potential errors from being in the middle of a $digest). The scope value is then assigned using the scope mappings we grabbed in the directive compile step.

compile: function bindPolymerCompile(el, attr) {  
  var attrMap = {};
  ...
  return function bindPolymerLink(scope, element, attrs) {
    Object.keys(attrMap).forEach(function(key) {
      element.on(key + '-changed', function(event) {
        scope.$evalAsync(function() {
          if (attrMap[key](scope) === event.detail.value) return;
          attrMap[key].assign(scope, event.detail.value);
        });
      });
    });
  };
}

All together

The final product looks like so:

angular  
.module('app', [])
.directive('bindPolymer', ['$parse', function($parse) {
  return {
    restrict: 'A',
    scope : false,
    compile: function bindPolymerCompile(el, attr) {
      var attrMap = {};
      for (var prop in attr) {
        if (angular.isString(attr[prop])) {
          var _match = attr[prop].match(/\{\{\s*([\.\w]+)\s*\}\}/);
          if (_match) {
            attrMap[prop] = $parse(_match[1]);
          }
        }
      }
      return function bindPolymerLink(scope, element, attrs) {
        Object.keys(attrMap).forEach(function(key) {
          element.on(key + '-changed', function(event) {
            scope.$evalAsync(function() {
              if (attrMap[key](scope) === event.detail.value) return;
              attrMap[key].assign(scope, event.detail.value);
            });
          });
        });
      };
    }
  };
}]);

A working demo of this can be found here: http://jshcrowthe.github.io/polymer-angular-demo/polymer-events.html (NOTE: This is a copy of the example element used in my previous post if you don't understand what's going on head on back and read that post for some context)

Good luck and shoot me any comments/questions you have!

Josh Crowther

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