/*******************************************************************************
 * Copyright (c) 2010, 2011 Sonatype, Inc.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *    Sonatype, Inc. - initial API and implementation
 *******************************************************************************/
package org.eclipse.aether;

/**
 * A trace of nested requests that are performed by the repository system. This trace information can be used to
 * correlate repository events with higher level operations in the application code that eventually caused the events. A
 * single trace can carry an arbitrary object as data which is meant to describe a request/operation that is currently
 * executed. For call hierarchies within the repository system itself, this data will usually be the {@code *Request}
 * object that is currently processed. When invoking methods on the repository system, client code may provide a request
 * trace that has been prepopulated with whatever data is useful for the application to indicate its state for later
 * evaluation when processing the repository events.
 * 
 * @see RepositoryEvent#getTrace()
 */
public class RequestTrace
{

    private final RequestTrace parent;

    private final Object data;

    /**
     * Creates a child of the specified request trace. This method is basically a convenience that will invoke
     * {@link RequestTrace#newChild(Object) parent.newChild()} when the specified parent trace is not {@code null} or
     * otherwise instantiante a new root trace.
     * 
     * @param parent The parent request trace, may be {@code null}.
     * @param data The data to associate with the child trace, may be {@code null}.
     * @return The child trace, never {@code null}.
     */
    public static RequestTrace newChild( RequestTrace parent, Object data )
    {
        if ( parent == null )
        {
            return new RequestTrace( data );
        }
        return parent.newChild( data );
    }

    /**
     * Creates a new root trace with the specified data.
     * 
     * @param data The data to associate with the trace, may be {@code null}.
     */
    public RequestTrace( Object data )
    {
        this( null, data );
    }

    /**
     * Creates a new trace with the specified data and parent
     * 
     * @param parent The parent trace, may be {@code null} for a root trace.
     * @param data The data to associate with the trace, may be {@code null}.
     */
    protected RequestTrace( RequestTrace parent, Object data )
    {
        this.parent = parent;
        this.data = data;
    }

    /**
     * Gets the data associated with this trace.
     * 
     * @return The data associated with this trace or {@code null} if none.
     */
    public final Object getData()
    {
        return data;
    }

    /**
     * Gets the parent of this trace.
     * 
     * @return The parent of this trace or {@code null} if this is the root of the trace stack.
     */
    public final RequestTrace getParent()
    {
        return parent;
    }

    /**
     * Creates a new child of this trace.
     * 
     * @param data The data to associate with the child, may be {@code null}.
     * @return The child trace, never {@code null}.
     */
    public RequestTrace newChild( Object data )
    {
        return new RequestTrace( this, data );
    }

    @Override
    public String toString()
    {
        return String.valueOf( getData() );
    }

}