Can anyone explain me how apex:actionfunction works? with simple example?

A component that provides support for invoking controller action methods directly from JavaScript code using an AJAX request. An <apex:actionFunction> component must be a child of an <apex:form> component.

<apex:page controller="exampleCon">
<!-- Add the onclick event listener to a panel. When clicked, the panel triggers
    the methodOneInJavascript actionFunction with a param -->
    <apex:outputPanel onclick="methodOneInJavascript('Yes!')" styleClass="btn"> 
        Click Me 
    </apex:outputPanel>
    <apex:form>
    <apex:actionFunction action="{!methodOne}" name="methodOneInJavascript" rerender="showstate">
        <apex:param name="firstParam" assignTo="{!state}" value="" />
    </apex:actionFunction>
    </apex:form>
</apex:page>

In the above visualforce page javascript method in the onclickevent will call the apex controller method with the help of actionfunction.

Apex controller. With the help of actionfunction we can able to make a call to the apex controller class

/*** Controller ***/
public class exampleCon {
    String uname;
    public String getUsername() {
        return uname;
    }
            
    public PageReference sayHello() {
        uname = UserInfo.getName();
        return null;
    }
            
    public void setState(String n) {
        state = n;
    }
            
    public String getState() {
        return state;
    }
            
    public PageReference methodOne() {
        return null;
    }
            
    private String state = 'no';
}

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