The fromVariable in Application config not work

[nonokangwei]

Output of helm version:

Output of kubectl version:
Client Version: version.Info{Major:"1", Minor:"16", GitVersion:"v1.16.0", GitCommit:"2bd9643cee5b3b3a5ecbd3af49d09018f0773c77", GitTreeState:"clean", BuildDate:"2019-09-18T14:36:53Z", GoVersion:"go1.12.9", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"15", GitVersion:"v1.15.7", GitCommit:"c83d931fb9bece427bc63a02349755e0f8696d3e", GitTreeState:"clean", BuildDate:"2020-01-31T20:09:49Z", GoVersion:"go1.12.12", Compiler:"gc", Platform:"linux/amd64"}
Cloud Provider/Platform (AKS, GKE, Minikube etc.):
AKS

Describe the bug
The fromVariable() in application config not work

OAM yaml files used
apiVersion: core.oam.dev/v1alpha1
kind: ApplicationConfiguration
metadata:
name: example-var-task
spec:
variables:
- name: DEMO
value: HelloWorld
components:
- componentName: helloworld-python-v1
instanceName: one-alpine-var-task
parameterValues:
- name: demo
value: "[fromVariable(DEMO)]"

What happened:
kubectl logs one-alpine-var-task-g4mjf
[fromVariable(DEMO)]

the the variable not take into the pod.

What you expected to happen:
the variable should take into the pod

Relevant screenshots:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know: