Async task helper, help you the async tasks one by one.

Async.java

package com.example.lib;

import java.util.ArrayList;
import java.util.List;

/**
 * Async task helper
 * Help you the async tasks one by one
 *
 * @author 19Site
 * @version 1
 */
public class Async {

    /**
     * run task one by one
     */
    public static void waterfall(Tasks tasks, OnWaterfallCompleted completed) {

        if (tasks == null) {

            waterfall(new ArrayList<Task>(), completed);
        } else {

            waterfall(tasks.getAll(), completed);
        }
    }

    /**
     * run task one by one
     */
    public static void waterfall(Task[] tasks, OnWaterfallCompleted completed) {

        if (tasks == null) {

            waterfall(new ArrayList<Task>(), completed);
        } else {

            ArrayList<Task> aTasks = new ArrayList<Task>();

            for (Task task : tasks) {

                aTasks.add(task);
            }

            waterfall(aTasks, completed);
        }
    }

    /**
     * run task one by one
     */
    public static void waterfall(final List<Task> tasks, OnWaterfallCompleted completed) {

        // init complete
        final OnWaterfallCompleted mCompleted = completed != null ? completed : new OnWaterfallCompleted() {

            @Override
            public void run(Exception ex) {

            }
        };

        // no tasks
        if (tasks == null || tasks.size() == 0) {

            mCompleted.run(null);

            return;
        }

        // next
        Next next = new Next() {

            @Override
            public void run(Exception ex) {

                if (ex != null) {

                    mCompleted.run(ex);
                } else if (tasks.size() == 0) {

                    mCompleted.run(null);
                } else {

                    tasks.remove(0).run(this);
                }
            }
        };

        // run
        next.run(null);
    }

    /**
     * task
     */
    public interface Task {

        void run(Next next);
    }

    /**
     * next
     */
    public interface Next {

        void run(Exception ex);
    }

    /**
     * on waterfall task completed
     */
    public interface OnWaterfallCompleted {

        void run(Exception ex);
    }

    /**
     * tasks collection
     */
    public static class Tasks {

        // tasks container
        private List<Task> tasks;

        /**
         * constructor
         */
        public Tasks() {

            tasks = new ArrayList<Task>();
        }

        /**
         * add a task
         */
        public Tasks put(Task task) {

            tasks.add(task);

            return this;
        }

        /**
         * get task by index
         */
        public Task get(int index) {

            return tasks.get(index);
        }

        /**
         * get task by index
         */
        public List<Task> getAll() {

            return tasks;
        }
    }
}