system.xml field dependency

Question

So you can set in the system.xml file of a module a dependency between fields by adding a <depends> tag in one of the fields.

<field1>
    ....
</field1>
<field2>
    ....
    <depends>
       <field1>1</field1>
    </depends>
</field2>

The code above means that field2 will be shown when the value of field1 is 1. I want to know how/if can I tell Magento to show field2 if the value for field1 is 1 OR 2?

Answer 1

Try this:

<depends>
    <field separator="|">
        <value>1|2|3</value>
    </field>
</depends>

Answer 2

[EDIT]

I was wrong in my answer below. I will not delete it (yet) because I got 7 upvotes on this :). But I'm editing it so you all have the chance to retract your vote (even downvote it, because I deserve it).

Original Answer

Ha!..I found it.
Short answer: You cannot!
Long answer: You should be able to do it if someone would have known the difference between an array and an object.
In theory this should work

<field1>
    ....
</field1>
<field2>
    ....
    <depends>
       <field1>
           <value>1|2</value>
           <separator>|</separator>
       </field1>
    </depends>
</field2>

But in the code that handles the dependency, Mage_Adminhtml_Block_System_Config_Form::initFields around line 366 there is this code

if (isset($dependent['separator'])) {
    $dependentValue = explode((string)$dependent['separator'], $dependentValue);
}

$dependent is always an object so $dependent['separator'] is never set.
If I change the code above to

if (isset($dependent->separator)) {
    $dependentValue = explode((string)$dependent->separator, $dependentValue);
}

everything works smoothly.
I guess I cannot change the core just for the sake of an extension so I have to create 2 fields instead of 1, one for each value from field1 or create a custom js that handles this and add it to the config page.

Answer 3

I know this was asked under Magento 1 but since I landed here looking for an example for Magento 2 and the answers here pointed me in the right direction, here is a working example tested in Magento 2.4.4 in case it helps anyone else:

<field id="time" translate="label comment" type="time" sortOrder="2" showInDefault="1" showInWebsite="1" showInStore="1" canRestore="1">
<label>Start Time</label>
<depends>
    <field id="frequency" separator="|">D|W|M</field>
</depends>

There is also a negative parameter that can be used to tailor the condition even more.