    001    // Copyright 2010 The Apache Software Foundation
    002    //
    003    // Licensed under the Apache License, Version 2.0 (the "License");
    004    // you may not use this file except in compliance with the License.
    005    // You may obtain a copy of the License at
    006    //
    007    // http://www.apache.org/licenses/LICENSE-2.0
    008    //
    009    // Unless required by applicable law or agreed to in writing, software
    010    // distributed under the License is distributed on an "AS IS" BASIS,
    011    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    012    // See the License for the specific language governing permissions and
    013    // limitations under the License.
    014
    015    package org.apache.tapestry5.func;
    016
    017    /**
    018     * A lazy function is used to populate a {@link Flow} incrementally. Lazy functions allow calculations to be
    019     * deferred. They also support infinitely large Flows, where successive values are computed just as needed.
    020     *
    021     * @since 5.2.0
    022     */
    023    public interface LazyFunction<T>
    024    {
    025        /**
    026         * Calculates the next value for the function. The return value is tricky: it combines the next value in the
    027         * {@link Flow} with a function to compute the value after that as a {@link LazyContinuation}. Alternately, a
    028         * LazyFunction can return null to indicate that it has returned all the values it can.
    029         *
    030         * @return continuation containing next value and next function, or null when no more values can be produced
    031         */
    032        LazyContinuation<T> next();
    033    }