VisualForce AJAX with actionFunction acts in a strange way, any insight?

Question

I have two picklists in a visualforce page with a controller extension:

<apex:actionFunction name="updateModelList" rerender="modelSelect"/>
....
<apex:selectList value="{!selectedBrand}" multiselect="false" size="1" onChange="updateModelList();">
    <apex:selectOptions value="{!brandObjects}"/>
</apex:selectList>
...
<apex:selectList id="modelSelect" value="{!selectedModel}" multiselect="false" size="1">
    <apex:selectOptions value="{!modelObjects}"/>
</apex:selectList> 

Then there is some controller functions code that creates the Options for these selects. This works as long as i don't touch the second picklist. Values are correctly updated and everything is fine.

Basically what I noticed is that whenever I trigger the onChange event on the first picklist and I changed the value on the second picklist, values on the second one are not correctly updated as the controller variable selectedBrand is set to '' (empty string) whereas the variable selectedModel is set to null. This causes the second picklist to be empty after this action.

Any insight?

edit: further info, the controller has two getter methods that return a List of SelectOption one for each picklist.

edit #2: i found out the problem, apparently VisualForce Pages invoke getter methods "randomly" so my bad I had a side effect which initialized the placeholder variable in one of these methods.

Thank You.

Answer 1

Ok after debug logs inspection I found out the main reason and the solution. Visualforce invokes getter methods of your controllers "randomly" so whenever you implement a getter method which is used in a Visualforce page you should always avoid side effects on the controller variables, (i.e. initializations, etc..).

My controller code accidentally had a selectedBrand String initialization to '' (empty string) in the getBrandObjects method, this caused the strange behaviour.