techinplanet staffAsked By: Anonymous
In a normal JavaScript class I can use constructor and setters to ensure that my object is never passed invalid values and thus never enters an invalid state.
How would I do this in Polymer?
Here is why I am asking. I have a bug resulting from one of my component properties having an invalid value. I’m not sure how it became invalid. Normally, my first step would be to ensure that it’s not being passed bad values. I would do this by using constructors and setters.
With Polymer being a layer of magic on top of Plain Old JavaScript, I’m not sure how to approach this problem.
Solution
Answered By: Anonymous
- Option 1: Create a property observer that logs the stacktrace (with
console.trace()) when an invalid value is set. While this option doesn’t prevent the invalid value, it helps you find the bug._x000D__x000D_<head>_x000D_ <base href="https://polygit.org/polymer+1.6.0/components/">_x000D_ <a href="http://webcomponentsjs/webcomponents-lite.min.js">http://webcomponentsjs/webcomponents-lite.min.js</a>_x000D_ <link rel="import" href="polymer/polymer.html">_x000D_ <link rel="import" href="paper-input/paper-input.html">_x000D_ </head>_x000D_ <body>_x000D_ <x-foo></x-foo>_x000D_ _x000D_ <dom-module id="x-foo">_x000D_ <template>_x000D_ <!-- paper-input has better methods of input validation, but this_x000D_ is intended for demonstrating property validation where the_x000D_ value can be set in any manner (not just through inputs) -->_x000D_ <paper-input label="Enter even number" value="{{foo}}"></paper-input>_x000D_ <div>valid: [[_isValid]]</div>_x000D_ </template>_x000D_ <script>_x000D_ // HTMLImports.whenReady() in index.html for demo only_x000D_ HTMLImports.whenReady(function() {_x000D_ "use strict";_x000D_ _x000D_ Polymer({_x000D_ is: 'x-foo',_x000D_ properties : {_x000D_ foo: {_x000D_ type: Number,_x000D_ value: 2,_x000D_ observer: '_fooChanged'_x000D_ },_x000D_ _isValid: {_x000D_ type: Boolean,_x000D_ value: false_x000D_ }_x000D_ },_x000D_ _x000D_ _fooChanged: function(foo) {_x000D_ this._isValid = foo % 2 == 0;_x000D_ if (!this._isValid) {_x000D_ console.trace('invalid value');_x000D_ }_x000D_ }_x000D_ });_x000D_ });_x000D_ </script>_x000D_ </dom-module>_x000D_ </body>_x000D_
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- Option 2: Proxy the property through a validating computed property._x000D__x000D_
<head>_x000D_ <base href="https://polygit.org/polymer+1.6.0/components/">_x000D_ <a href="http://webcomponentsjs/webcomponents-lite.min.js">http://webcomponentsjs/webcomponents-lite.min.js</a>_x000D_ <link rel="import" href="polymer/polymer.html">_x000D_ <link rel="import" href="paper-input/paper-input.html">_x000D_ </head>_x000D_ <body>_x000D_ <x-foo></x-foo>_x000D_ _x000D_ <dom-module id="x-foo">_x000D_ <template>_x000D_ <!-- paper-input has better methods of input validation, but this_x000D_ is intended for demonstrating property validation where the_x000D_ value can be set in any manner (not just through inputs) -->_x000D_ <paper-input label="Enter even number" type="number" value="{{foo}}"></paper-input>_x000D_ <span>nearest even: [[_foo]]</span>_x000D_ </template>_x000D_ <script>_x000D_ HTMLImports.whenReady(() => {_x000D_ "use strict";_x000D_ _x000D_ Polymer({_x000D_ is: 'x-foo',_x000D_ properties : {_x000D_ foo: {_x000D_ type: Number,_x000D_ value: 2_x000D_ },_x000D_ _foo: {_x000D_ computed: '_computeEvenFoo(foo)'_x000D_ }_x000D_ },_x000D_ _x000D_ _computeEvenFoo: function(foo) {_x000D_ const num = Number(foo);_x000D_ const isValid = num % 2 == 0;_x000D_ if (!isValid) {_x000D_ console.trace('overriding invalid value');_x000D_ return num + 1;_x000D_ }_x000D_ return foo;_x000D_ }_x000D_ });_x000D_ });_x000D_ </script>_x000D_ </dom-module>_x000D_ </body>_x000D_
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In addition to (or instead of) logging the stacktrace, you could setup a breakpoint in DevTools (with Chrome, Firefox, Safari, etc.) to catch the bug as it occurs via your observer/computed property.
