Polymer-Dart 1.0 dom-如果不起作用

Question

So I was trying to use the <template is="dom-if"> but I just cannot make it work.

<template is="dom-repeat" items="{{row}}" as="project">
   <template is="dom-if" if="{{project == null}}">
      <span>Untitled project</span>
   </template>
   <template is="dom-if" if="{{project != null}}">
      <span>{{project.name}}</span>
   </template>
 </template>

For some reasons, I never get shown Untitled project although there are 2 elements which are null .

Are the expressions just not yet available in the developer preview of Polymer-Dart?

Answer 1

You can actually just use a simple {{project}} and {{!project}} binding. ! is one of the only expressions allowed in polymer 1.0. It uses JS falsyness so a null value will be "false", but so will an empty list/etc.

You may also need to explicitly initialize the value to null in your ready method as well:

ready() {   if (project == null) set('project', null); }

Basically, for initial values of null we actually send undefined to the js side. However bindings wont run at all for undefined values, so you need to then explicitly assign null to ensure the dom-ifs will evaluate the expression. We do this to avoid an extra change notification for each property when each element starts up.

Answer 2

I don't know about Polymer-Dart, but in Polymer 1.0 expressions are no longer supported ( docs ).

You could use computed bindings to achieve the same goal.