Is there a way to add a behavior(s) dynamically? - polymer

Question

I want to add a behavior after a component/behavior already loaded or a certain function that will add a behevaior to its components.
Something like this:
<script>
// samplebehavior.html file
// this is the behavior file
samplebehavior = {
testAlert: function(){
alert('test');
}
};
</script>
// my-component.html
<script>
Polymer({
is: "my-component",
test: function() {
url = "samplebehavior.html";
var importHTML = new Promise(function(resolve, reject) {
Polymer.Base.importHref(url, function(e) {
resolve(e.target);
}, reject);
});
importHTML.then(function(element) {
// add a behavior here
// I know this script does not work
this.push('behaviors', samplebehavior);
});
}
});
</script>
So that I can access the testAlert() function.
How to I add a behavior dynamically?

Answer 1

To the best of knowledge it's not possible.
Behaviors are mixed in with the element definition when the prototype is built.
What you could do is generate the behaviors array dynamically
var behavior = {
properties: {
smth: {
value: 'initial value'
}
}
}
Polymer({
is: 'my-elem',
behaviors: getBehaviors()
});
function getBehaviors() {
return [ behavior ];
}
Remember though that getBehaviors will only ever be called once. After that you won't be able to change behaviors of your elements.

Answer 2

It's ugly but you can copy all members of your behavior in your object
<script>
// samplebehavior.html file
// this is the behavior file
samplebehavior = {
testAlert: function(){
alert('test');
}
};
</script>
// my-component.html
<script>
Polymer({
is: "my-component",
test: function() {
url = "samplebehavior.html";
var importHTML = new Promise(function(resolve, reject) {
Polymer.Base.importHref(url, function(e) {
resolve(e.target);
}, reject);
});
importHTML.then(function(element) {
for (let member in samplebehavior) {
this[member] = samplebehavior[member];
}
});
}
});
</script>
Or maybe you can call the internal method _prepBehavior() https://github.com/Polymer/polymer/blob/ff6e884ef4f309d41491333860a8bc9c2f178696/src/micro/behaviors.html#L111
But i don't know if that can do side effects
<script>
// samplebehavior.html file
// this is the behavior file
samplebehavior = {
testAlert: function(){
alert('test');
}
};
</script>
// my-component.html
<script>
Polymer({
is: "my-component",
test: function() {
url = "samplebehavior.html";
var importHTML = new Promise(function(resolve, reject) {
Polymer.Base.importHref(url, function(e) {
resolve(e.target);
}, reject);
});
importHTML.then(function(element) {
this.push('behaviors', samplebehavior);
this._prepBehaviors();
});
}
});
</script>

Answer 3

With introduction to new value for lazyRegister setting this has become partially possible.
By partially i mean you can only do this if you can edit the code of the element in which you want to add the behavior.
Three changes you'll require to add behavior dynamically are
Set lazyRegister setting to max
window.Polymer = {
lazyRegister:"max"
};
Just like #tomasz suggested, in the element you want to add behavior dynamically add behaviors using a function. This is so, because we won't be able to access the element from where we'll try to add new behavior dynamically(atleast i was not able to).
Polymer({
.
.
behaviors: getBehavior(),
.
.
});
function getBehavior(){
var myArr = [myBehavior];
document.addEventListener('add-behavior',function(e){
debugger;
myArr.push(e.detail);
});
return myArr;
}
From the element where you want to dynamically add the behavior use beforeRegister callback to dispatch an event adding the new behavior object
beforeRegister: function(){
var event = new CustomEvent('add-behavior',{
detail:{
func: function(){console.log(this.myProp,"!")}
}
});
document.dispatchEvent(event);
}
Here's a plunkr for working example.