Rewrite Block Class that is rewriting Mage Core Class

Question

I'm trying to override a Block class called Smartwave_Megamenu_Block_Topmenu, which is declared like the following:

    <blocks>
        <catalog>
            <rewrite>
                <navigation>Smartwave_Megamenu_Block_Navigation</navigation>
                <!-- v1.4-1.6 -->
            </rewrite>
        </catalog>
        <page>
            <rewrite>
                <html_topmenu>Smartwave_Megamenu_Block_Topmenu</html_topmenu>
                <!-- v1.7 -->
            </rewrite>
        </page>
        <megamenu>
            <class>Smartwave_Megamenu_Block</class>
        </megamenu>
    </blocks>

As you can see this config.xml-Block node is rewriting html_topmenu of Mage_Core.

However, if I try to rewrite megamenu like the following:

    <blocks>
        <vendor_template>
            <class>Vendor_Template_Block</class>
        </vendor_template>
        <megamenu>
            <rewrite>
                <topmenu>Vendor_Template_Block_Topmenu</topmenu>
            </rewrite>
        </megamenu>
    </blocks> 

The same old Class is still loaded. Why is this happening and how can I rewrite this Block Class properly?

I've cleared caches, of course.

Answer 1

This way not working. You need to overwrite the core class and extend Smartwave_Megamenu_Block_Topmenu class.

Ex: First comment out the following line from Smartwave_Megamenu module config.xml

    <blocks>
        <catalog>
            <rewrite>
                <navigation>Smartwave_Megamenu_Block_Navigation</navigation>
                <!-- v1.4-1.6 -->
            </rewrite>
        </catalog>
        <!--
        <page>
            <rewrite>
                <html_topmenu>Smartwave_Megamenu_Block_Topmenu</html_topmenu>
            </rewrite>
        </page>
        -->
        <megamenu>
            <class>Smartwave_Megamenu_Block</class>
        </megamenu>
    </blocks>

Now overwrite your module config:

    <blocks>
        <vendor_template>
            <class>Vendor_Template_Block</class>
        </vendor_template>
        <page>
            <rewrite>
                <html_topmenu>Vendor_Template_Block_Topmenu</html_topmenu>
            </rewrite>
        </page>
    </blocks> 

After that :

class Vendor_Template_Block_Topmenu extends Smartwave_Megamenu_Block_Topmenu
{

}