[[where-am-i]]
= Where am I? (Navigable trees)

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The Wicket viewer provides a secondary navigation scheme that allows users to quickly access domain objects, that are organized in a hierarchical manner.

This hierarchy can be understood as a navigable tree-structure, where the navigable tree-nodes are either domain objects or domain views.
Domain objects that take part in such a navigable-tree-structure need to declare their actual navigable parent within this hierarchy using the xref:refguide:applib:index/annotation/PropertyLayout.adoc#navigable[@PropertyLayout#navigable()] annotation.


== Screenshots

The following screenshot (from the link:https://demo-wicket-jpa-snapshot.causeway.dev/[demo app]) shows the navigation links below the top menu bar.

image::where-am-i/hello_grey_bg.png[width="720px"]


== Domain Code

To declare a domain object's (or view's) navigable parent, add a `@PropertyLayout(navigable=Navigable.PARENT)` annotation to a field (that has an associated getter) or a no-arg method, that returns the parent object:

[source,java]
.Company.java
----
@DomainObject
public class Company { /* ... */ }
----

[source,java]
.Employee.java
----
@DomainObject
public class Employee {

    @PropertyLayout(navigable=Navigable.PARENT) // annotated field with e.g. lombok provided getter
    @Getter
    Company myCompany;
}
----

[source,java]
.PhoneNumber.java
----
@DomainObject
public class PhoneNumber {

    @PropertyLayout(navigable=Navigable.PARENT)  // annotated no-arg supplier
    Employee employee(){
        return ...
    }
}
----

This results in a navigable tree-structure ...

 Company > Employee > PhoneNumber

Notes:

. Any use of `@PropertyLayout(navigable=Navigable.PARENT)` with Java interfaces is simply ignored.
These do not contribute to the domain meta model.

. Any class (abstract or concrete) may at most have one `@PropertyLayout` annotation, having `navigable=Navigable.PARENT` 'flag' set (on either a method or a field); otherwise meta-model validation will fail.

. The annotated member (method or field), when ...

.. \... a method: then must be a no-arg method returning a non-primitive type (e.g. a getter)

.. \... a field: then the field must be of non-primitive type and must also have a getter (as specified by the Java Beans Standard, particularly to allow `@PropertyLayout(navigable=Navigable.PARENT)` annotations on fields that use the lombok `@Getter` annotation)

. Starting from the current domain-object, we search the object's class inheritance hierarchy (super class, super super class, ...), until we find the first class that has a `@PropertyLayout(navigable=Navigable.PARENT)` annotation.
That's the one we use to resolve the current domain-object's navigable parent.


== User Experience

When viewing a domain object that is part of a hierarchical structure, one can easily navigate to any parent of this object.
Horizontally arranged text links separated by the 'greater than' symbol (>) are provided below the main menu.
(Traditionally called breadcrumbs.)



== Related functionality

The navigable tree-structure, as provided by the "where am i" feature, is declared at compile-time (predefined by the programmer), whereas xref:vw:ROOT:features.adoc#bookmarked-pages[Bookmarked Pages]) are populated at runtime only after user interaction.




