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The Alexa Skills Kit MVC Framework extends the existing ASK SDK, adding features for mapping requests to methods and rendering responses via view scripts and templates such as Nashorn Javascript and Freemarker. It also integrates with the ASK SDK Interaction Model Mapper framework to create a single environment where both the interaction model and business logic of skills can be managed as code packages.
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/*
    Copyright 2018 Amazon.com, Inc. or its affiliates. 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. A copy of the License is located at

        http://aws.amazon.com/apache2.0/

    or in the "license" file accompanying this file. This file 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 com.amazon.ask.mvc.view.resolver;

import com.amazon.ask.model.RequestEnvelope;
import com.amazon.ask.model.Response;
import com.amazon.ask.mvc.plugin.ViewResolver;
import com.amazon.ask.mvc.view.ModelAndView;
import com.amazon.ask.mvc.view.View;
import com.amazon.ask.mvc.view.resolver.cache.SingleThreadedViewCache;
import com.amazon.ask.mvc.view.resolver.cache.ViewCache;
import com.amazon.ask.mvc.view.resolver.candidate.LocaleViewCandidateEnumerator;
import com.amazon.ask.mvc.view.resolver.candidate.ViewCandidateEnumerator;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.*;
import java.util.regex.Pattern;
import java.util.stream.Stream;

import static com.amazon.ask.util.ValidationUtils.assertNotNull;

/**
 * Implements default view resolution logic:
 * - Enumerate and filter through an ordered set of candidates for the request
 * - (Optional) Complete partial view names by appending a prefix and suffix
 * - (Optional) Exclude views if they don't match regular expression
 * - Load view and cache it for re-use
 */
public abstract class BaseViewResolver implements ViewResolver {
    /** Object mapper for parsing {@link Response} json from views **/
    private static final ObjectMapper RESPONSE_MAPPER = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    protected final Logger logger = LoggerFactory.getLogger(getClass());

    protected final ObjectMapper mapper;
    protected final ViewCache cache;
    protected final List viewCandidateEnumerators;
    protected final Collection viewNamePatterns;
    protected final String prefix;
    protected final String suffix;

    protected BaseViewResolver(ObjectMapper mapper, ViewCache cache, List viewCandidateEnumerators,
                               Collection viewNamePatterns, String prefix, String suffix) {
        this.mapper = assertNotNull(mapper, "mapper");
        this.cache = cache == null ? SingleThreadedViewCache.defaultCache() : cache;
        this.viewCandidateEnumerators = viewCandidateEnumerators == null ? Collections.singletonList(new LocaleViewCandidateEnumerator()) : viewCandidateEnumerators;
        this.viewNamePatterns = viewNamePatterns == null ? Collections.emptyList() : viewNamePatterns;
        this.prefix = prefix == null ? "" : prefix;
        this.suffix = suffix == null ? "" : suffix;
    }

    @Override
    public Optional resolve(Object handlerOutput, RequestEnvelope requestEnvelope) throws Exception {
        if (handlerOutput instanceof ModelAndView) {
            ModelAndView modelAndView = (ModelAndView) handlerOutput;
            if (supports(modelAndView)) {
                return resolve(modelAndView, requestEnvelope);
            }
        }
        return Optional.empty();
    }

    protected Optional resolve(ModelAndView modelAndView, RequestEnvelope requestEnvelope) throws Exception {
        for (ViewCandidateEnumerator enumerator: this.viewCandidateEnumerators) {
            Iterator candidates = enumerator.enumerate(modelAndView.getViewName(), requestEnvelope).iterator();
            while (candidates.hasNext()) {
                String candidate = buildCompleteViewName(candidates.next());
                logger.trace("Attempting to resolve view candidate: {}", candidate);

                if (candidateExists(candidate)) {
                    View view = cache.getOrCreate(candidate, () -> resolveCandidate(candidate));
                    logger.trace("Resolved view candidate: {}", candidate);
                    return Optional.of(view);
                }
            }
        }
        return Optional.empty();
    }

    protected abstract boolean candidateExists(String candidate);

    /**
     * Resolve a view candidate.
     *
     * @param candidate view name prefixed and suffixed with
     * @return the view if it exists
     * @throws Exception if there was an error loading or parsing the view
     */
    protected abstract View resolveCandidate(String candidate) throws Exception;

    /**
     * Check if this view resolver supports this view
     *
     * @param modelAndView model and view name
     * @return true if this resolver supports the view
     */
    protected boolean supports(ModelAndView modelAndView) {
        return viewNamePatterns.isEmpty() || viewNamePatterns.stream().anyMatch(pattern -> pattern.matcher(modelAndView.getViewName()).matches());
    }

    /**
     * Builds a view value by concatenating the prefix, view name and suffix
     *
     * @param viewName partial view name
     * @return full view name
     */
    protected String buildCompleteViewName(String viewName) {
        return prefix + viewName + suffix;
    }

    /**
     * Generic builder to support polymorphic constructors of view resolvers.
     *
     * @param  type of this builder
     */
    public static class Builder> {
        protected ObjectMapper mapper = RESPONSE_MAPPER;
        protected ViewCache cache;
        protected List viewCandidateEnumerators;
        protected Collection viewNamePatterns;
        protected String prefix;
        protected String suffix;

        public Self withObjectMapper(ObjectMapper mapper) {
            this.mapper = mapper;
            return getThis();
        }

        public Self withCache(ViewCache cache) {
            this.cache = cache;
            return getThis();
        }

        public Self withViewCandidateEnumerator(List viewCandidateEnumerators) {
            this.viewCandidateEnumerators = viewCandidateEnumerators;
            return getThis();
        }

        public Self addViewCandidateEnumerator(ViewCandidateEnumerator viewCandidateEnumerator) {
            return addViewCandidateEnumerators(viewCandidateEnumerator);
        }

        public Self addViewCandidateEnumerators(ViewCandidateEnumerator... viewCandidateEnumerators) {
            return addViewCandidateEnumerator(Arrays.asList(viewCandidateEnumerators));
        }

        public Self addViewCandidateEnumerator(Collection viewCandidateEnumerators) {
            if (this.viewCandidateEnumerators == null) {
                this.viewCandidateEnumerators = new ArrayList<>();
            }
            this.viewCandidateEnumerators.addAll(viewCandidateEnumerators);
            return getThis();
        }

        public Self withViewNamePatterns(Collection viewNamePatterns) {
            this.viewNamePatterns = viewNamePatterns;
            return getThis();
        }

        public Self withPrefix(String prefix) {
            this.prefix = prefix;
            return getThis();
        }

        public Self withSuffix(String suffix) {
            this.suffix = suffix;
            return getThis();
        }

        @SuppressWarnings("unchecked")
        protected Self getThis() {
            return (Self) this;
        }
    }
}