PageableIterable.java

import lombok.Setter;
import lombok.experimental.Accessors;
import org.jetbrains.annotations.NotNull;

import java.util.Iterator;
import java.util.function.BiFunction;

/**
 * 支持翻页遍历的Iterable
 *
 * @author cqy
 * @date 2018/06/11
 */
public class PageableIterable<T> implements Iterable<T> {

    @java.beans.ConstructorProperties({"pageSize", "pageFunc"})
    private PageableIterable(int pageSize,
                             BiFunction<Integer, Integer, Page<T>> pageFunc) {
        this.pageSize = pageSize;
        this.pageFunc = pageFunc;
    }

    public static <T> PageableIterable<T> of(int pageSize,
                                             BiFunction<Integer, Integer, Page<T>> func) {
        return new PageableIterable<T>(pageSize, func);
    }

    public interface Page<T> {
        boolean hasNextPage();

        Iterator<T> iterator();
    }

    public static <T> Page<T> page(boolean hasNextPage, Iterator<T> iterator) {
        return new Page<T>() {
            @Override
            public boolean hasNextPage() {
                return hasNextPage;
            }

            @Override
            public Iterator<T> iterator() {
                return iterator;
            }
        };
    }

    final int pageSize;
    final BiFunction<Integer, Integer, Page<T>> pageFunc;

    @Setter
    @Accessors(fluent = true)
    private int minPageIndex = 0;
    @Setter
    @Accessors(fluent = true)
    private int maxPageIndex = Integer.MAX_VALUE;

    @NotNull
    @Override
    public Iterator<T> iterator() {
        return new Iterator<T>() {
            private int pageIndex = PageableIterable.this.minPageIndex;

            Page<T> target = null;

            @Override
            public boolean hasNext() {
                if (target == null || (!target.iterator().hasNext() && target.hasNextPage())) {
                    if (pageIndex > maxPageIndex) {
                        return false;
                    }
                    target = pageFunc.apply(pageIndex++, pageSize);
                }
                return target.iterator().hasNext();
            }

            @Override
            public T next() {
                return target == null ? null : target.iterator().next();
            }
        };
    }
}