ComboBox in UI5 does not display ValueState - sapui5

Question

ComboBox is not showing state like Error, Warning with highlight around the borders. But it does change the state. For example, if it is error state, and if I try to enter new value in combobox, it will show that "invalid Entry" tip near the box. But the box borders are never highlighted in red. Below is the code:
XML.view
<core:FragmentDefinition xmlns="sap.m" xmlns:core="sap.ui.core" xmlns:l="sap.ui.layout">
<Dialog id......>
<ComboBox id="combo1" change="cChanged" items="{path: '/results'}">
<items>
<core:Item key="{ID}" text="{Name}"/>
</items>
</ComboBox>
</Dialog>
Controller.js
cChanged: function(oEvent) {
var newval = oEvent.getParameter("newValue");
var key = oEvent.getSource().getSelectedItem();
if (newval !== "" && key === null) {
sap.ui.getCore().byId("combo1").setValueState("Error");
oEvent.getSource().setValue("");
sap.m.MessageToast.show("Please select from existing IDs")
flag = false;
} else {
oEvent.getSource().setValueState('None');
}

Answer 1

You can also access combo1 control instance by using oEvent.getSource() event OR use byId() from the sap.ui.core.Fragment class and not sap.ui.getCore().byId()
Also, if you are writing a logic only to validate if what the user input in the combobox is a valid item, consider replacing your ComboBox by the sap.m.Select control.
Both ComboBox and Select has same look and feel, but Select does not allow a manual input. It can also have an empty option if you use the property forceSelection