# ValueBuilderWithState.java

    /*
     * Copyright (c) 2012, Kent Sølvsten. All Rights Reserved.
     *
     * 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.qi4j.runtime.value;

    import org.qi4j.api.association.AssociationStateHolder;
    import org.qi4j.api.common.ConstructionException;
    import org.qi4j.api.value.ValueBuilder;
    import org.qi4j.runtime.composite.StateResolver;
    import org.qi4j.spi.module.ModelModule;
    import org.qi4j.runtime.structure.ModuleInstance;

    public class ValueBuilderWithState<T> implements ValueBuilder<T>
    {
        private final ModelModule<ValueModel> model;
        private ValueInstance prototypeInstance;

        public ValueBuilderWithState( ModelModule<ValueModel> compositeModelModule,
                                      ModuleInstance currentModule,
                                      StateResolver stateResolver )
        {
            ValueStateInstance state = new ValueStateInstance( compositeModelModule, currentModule, stateResolver );
            ValueInstance instance = compositeModelModule.model().newValueInstance( compositeModelModule.module(), state );
            instance.prepareToBuild();
            this.model = compositeModelModule;
            this.prototypeInstance = instance;
        }

        @Override
        public T prototype()
        {
            verifyUnderConstruction();
            return prototypeInstance.<T>proxy();
        }

        @Override
        public AssociationStateHolder state()
        {
            verifyUnderConstruction();
            return prototypeInstance.state();
        }

        @Override
        public <K> K prototypeFor( Class<K> mixinType )
        {
            verifyUnderConstruction();

            return prototypeInstance.newProxy( mixinType );
        }

        @Override
        public T newInstance()
            throws ConstructionException
        {
            verifyUnderConstruction();

            // Set correct info's (immutable) on the state
            prototypeInstance.prepareBuilderState();

            // Check that it is valid
            model.model().checkConstraints( prototypeInstance.state() );

            try
            {
                return prototypeInstance.<T>proxy();
            }
            finally
            {
                // Invalidate builder
                prototypeInstance = null;
            }
        }

        private void verifyUnderConstruction()
        {
            if( prototypeInstance == null )
            {
                throw new IllegalStateException( "ValueBuilder instances cannot be reused" );
            }
        }
    }