How can I build a complex Visualforce Dynamic Binding dynamically?

Question

How can I build a complex Visualforce Dynamic Binding dynamically? Yeah, I realize the question seems a bit redundant, but I couldn't think of a better way to describe what I'm trying to do. The short version of my question is that I need to build a dynamic binding for both lookup (relationship) fields and standard fields in a Visualforce page, but I'm stuck on an error; how can I accomplish this?

Background

I have a few different sObjects to display, and I have a controlling Custom Setting object with records that indicates which fields should be displayed. The reason for this is to satisfy a requirement to display multiple fields from separate sObjects on a single Visualforce page with an administrative interface to update or add to those fields.

Here's some more detail. The sObjects are contained in a map using the sObject API Name as the key (Map<String, sObject> sObjectsByType). Next, I have a list of Custom Setting records that contain the sObject API Name, the API Name of a Lookup Relationship (if applicable), and the API Name of the field (List<Field__c>).

Issue

I ran into an issue with lookup fields and dynamic bindings while implementing this. Sometimes the f.Lookup_sObject__c field is null, and that causes a Visualforce error (Error evaluating dynamic reference '').

<apex:repeat value="{!Fields}" var="f">
    <apex:inputfield value="{!sObjectsByType[f.sObject__c][f.Lookup_sObject__c][f.Field_API_Name__c]}" />
</apex:repeat>

What I've tried

I tried to dynamically render the inputfields, but even when I do that, I receive the same error because Visualforce cannot resolve the dynamic reference.

<apex:repeat value="{!Fields}" var="f">
    <apex:inputfield value="{!sObjectsByType[f.sObject__c][f.Lookup_sObject__c][f.Field_API_Name__c]}" rendered="{!f.Lookup_sObject__c != null}" />
    <apex:inputfield value="{!sObjectsByType[f.sObject__c][f.Field_API_Name__c]}" rendered="{!f.Lookup_sObject__c == null}" />
</apex:repeat>

Question

How can I use dynamic bindings on an <apex:inputField> to display fields that are lookups to fields on other objects as well as fields that are not? Is this even possible?

Answer 1

The expression value can be set in an Apex controller using a dynamiccomponent. Because the expression is built outside of Visualforce, it can be dynamically set based on a Custom Setting. Here's what I ended up with:

Visualforce

<apex:dynamicComponent componentValue="{!SectionsAndFields}"/>

Apex

public Component.Apex.OutputPanel getSectionsAndFields() 
{
    Component.Apex.PageBlockSection PageBlockSection 
        = new Component.Apex.PageBlockSection();

    PageBlockSection.title = 'Section';
    PageBlockSection.collapsible = false;

    for(Field__c f : Fields)
    {
        Component.Apex.InputField InputField = new Component.Apex.InputField();

        if(f.Lookup_sObject__c != null 
        && sObjectsByType.get(f.sObject__c).getSObject(f.Lookup_sObject__c) != null) 
        {
            // use dot notation to reference fields on an sObject
            // use brackets and escaped quotes around a key 
            //   to get a value from a map (sObjectsByType in this example)
            InputField.expressions.value = '{!sObjectsByType' 
                + '[\'' + f.sObject__c + '\']' 
                + '.' + f.Lookup_sObject__c + '.' + f.Field_API_Name__c + '}';

            PageBlockSection.childComponents.add(InputField);
        }

        if(f.Lookup_sObject__c == null) 
        {
            InputField.expressions.value = '{!sObjectsByType' 
                + '[\'' + f.sObject__c + '\']' 
                + '.' + f.Field_API_Name__c + '}';

            PageBlockSection.childComponents.add(InputField);
        }
    }

    return PageBlockSection;
}