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 015package 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 */ 023public 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}