//  Copyright 2004 The Apache Software Foundation
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package org.apache.tapestry.pageload;

import ognl.Ognl;

import org.apache.tapestry.ApplicationRuntimeException;
import org.apache.tapestry.IComponent;
import org.apache.tapestry.ILocation;
import org.apache.tapestry.IResourceResolver;
import org.apache.tapestry.Tapestry;
import org.apache.tapestry.event.PageDetachListener;
import org.apache.tapestry.event.PageEvent;
import org.apache.tapestry.util.prop.OgnlUtils;

/**
 *  Given a component, a property and a value, this object will
 *  reset the property to the value whenever the page
 *  (containing the component) is detached.  This is related
 *  to support for {@link org.apache.tapestry.spec.IPropertySpecification}s.
 *
 *  @author Howard Lewis Ship
 *  @version $Id$
 *  @since 3.0
 **/

public class PropertyInitializer implements PageDetachListener
{
    private IResourceResolver _resolver;
    private IComponent _component;
    private String _propertyName;
    private String _expression;
    private boolean _invariant;
    private Object _value;
    private ILocation _location;

    public PropertyInitializer(
        IResourceResolver resolver,
        IComponent component,
        String propertyName,
        String expression,
        ILocation location)
    {
        _resolver = resolver;
        _component = component;
        _propertyName = propertyName;
        _expression = expression;
        _location = location;

        prepareInvariant();
    }

    public void prepareInvariant()
    {
        _invariant = false;

        try
        {
            // If no initial value expression is provided, then read the current
            // property of the expression.  This may be null, or may be
            // a value set in finishLoad() (via an abstract accessor).

            if (Tapestry.isBlank(_expression))
            {
                _invariant = true;
                _value = OgnlUtils.get(_propertyName, _resolver, _component);
            }
            else
                if (Ognl.isConstant(_expression, Ognl.createDefaultContext(_component, _resolver)))
                {
                    // If the expression is a constant, evaluate it and remember the value 
                    _invariant = true;
                    _value = OgnlUtils.get(_expression, _resolver, _component);
                }
        }
        catch (Exception ex)
        {
            throw new ApplicationRuntimeException(
                Tapestry.format(
                    "PageLoader.unable-to-initialize-property",
                    _propertyName,
                    _component,
                    ex.getMessage()),
                _location,
                ex);
        }
    }

    public void pageDetached(PageEvent event)
    {
        try
        {
            if (_invariant)
                OgnlUtils.set(_propertyName, _resolver, _component, _value);
            else
            {
                Object value = OgnlUtils.get(_expression, _resolver, _component);
                OgnlUtils.set(_propertyName, _resolver, _component, value);
            }
        }
        catch (Exception ex)
        {
            throw new ApplicationRuntimeException(
                Tapestry.format(
                    "PageLoader.unable-to-initialize-property",
                    _propertyName,
                    _component,
                    ex.getMessage()),
                _location,
                ex);
        }
    }

}