apex:Param not working with CommandButton component

Question

I try to pass a parameter from Visualforce to the controller via apex:param and then upsert the object in the controller. However, the parameter I passed on, "Field__c" did not get written to the database. Here is my code:

<apex:commandButton value="Save" action="{!Save}">
    <apex:param value="Saved" AssignTo="{!item.Field__c }" name="Saved"/>
</apex:commandButton>

Controller:

public PageReference Save() {
    try {
        item.NumberContacts__c = itemOld.NoContacts__c;
        update item;
    }   
    catch(Exception e){
        System.debug('Exception occurred '+String.valueOf(e));
        ApexPages.addMessages(e);
        return null;
    }
    return null;
} 

After reloading, item.Field__c should get displayed on an output field on the Visualforce page, but after hitting the command button, it still displays the old value, not the value "Saved". What do I miss here?

Answer 1

Create a property in your controller, like

public propField {get;set;}

and assign it to the item.field__c like

item.field__c = propField;

Now, in your VF, set the property, like

AssignTo="{! propField }"

Explanation

When your VF page loads, it will feed the property propField with the parameter. Once your object item is ready, you can then set its Field__c value with the property 'propField' inside the controller.

As you might have already guessed now, that using item.Field__c didn't do anything because your object might not had been ready when the VF page loaded.

Update

As per the documentation, apex:param can only be a child of the following items:

• <apex:actionFunction>
• <apex:actionSupport>
• <apex:commandLink>
• <apex:outputLink>
• <apex:outputText>
• <flow:interview>

Try, changing your commandButton to commandLink.

What's wrong with CommandButton?

Apparently, there seems to be bug with setting the property with this component if apex:Param is used. Thanks to Jeff Douglas, there is a workaround to use CommandButton and still be able to set the property value.

<apex:commandButton value="Save" action="{! Save }" rerender="hiddenBlock">
    <apex:param name ="Saved"
                value="Saved"
                assignTo="{! propField }"/>
    </apex:commandButton>

<apex:pageBlock id="hiddenBlock" rendered="false"></apex:pageBlock>

All Hail, Jeff! for the CommandButton workaround.

Answer 2

Looks more like the rerender might have been your issue. For anyone else running into similar issue.

Per, apex:param not assigning value within apex:actionSupport for apex:inputCheckbox