Add simpler API for using the scheduler. Adds BUKKIT-836

The new methods return the actual task that gets created from the
scheduler. They are also named such that auto-complete puts the
asynchronous methods after the normal ones. These two additions are
simply semantic.

Tasks now have a method to cancel themselves using their task id. This
is provided as a convenience.

A new class called SimpleRunnable was added. It is an abstract Runnable
such that anonymous classes may subclass it. It provides six convenience
methods for scheduling as appropriate. It also provides a cancel method
for convenience. The functionality of SimpleRunnable only stores an
integer representing the task id. A SimpleRunnable can only be scheduled
once; attempting to reschedule results in IllegalStateException.

src/main/java/org/bukkit/scheduler/BukkitRunnable.java

@@ -0,0 +1,143 @@
+package org.bukkit.scheduler;
+
+import org.bukkit.Bukkit;
+import org.bukkit.plugin.Plugin;
+
+/**
+ * This class is provided as an easy way to handle scheduling tasks.
+ */
+public abstract class BukkitRunnable implements Runnable {
+    private int taskId = -1;
+
+    /**
+     * Attempts to cancel this task
+     * @throws IllegalStateException if task was not scheduled yet
+     */
+    public synchronized void cancel() throws IllegalStateException {
+        Bukkit.getScheduler().cancelTask(getTaskId());
+    }
+
+    /**
+     * Schedules this in the Bukkit scheduler to run on next tick
+     *
+     * @param plugin the reference to the plugin scheduling task
+     * @return a BukkitTask that contains the id number
+     * @throws IllegalArgumentException if plugin is null
+     * @throws IllegalStateException if this was already scheduled
+     * @see BukkitScheduler#runTask(Plugin, Runnable)
+     */
+    public synchronized BukkitTask runTask(Plugin plugin) throws IllegalArgumentException, IllegalStateException {
+        checkState();
+        return setupId(Bukkit.getScheduler().runTask(plugin, this));
+    }
+
+    /**
+     * <b>Asynchronous tasks should never access any API in Bukkit.
+     *    Great care should be taken to assure the thread-safety of asynchronous tasks.</b>
+     * <br>
+     * <br>Schedules this in the Bukkit scheduler to run asynchronously.
+     *
+     * @param plugin the reference to the plugin scheduling task
+     * @return a BukkitTask that contains the id number
+     * @throws IllegalArgumentException if plugin is null
+     * @throws IllegalStateException if this was already scheduled
+     * @see BukkitScheduler#runTaskAsynchronously(Plugin, Runnable, long, long)
+     */
+    public synchronized BukkitTask runTaskAsynchronously(Plugin plugin) throws IllegalArgumentException, IllegalStateException  {
+        checkState();
+        return setupId(Bukkit.getScheduler().runTaskAsynchronously(plugin, this));
+    }
+
+    /**
+     * Schedules this to run after the specified number of server ticks.
+     *
+     * @param plugin the reference to the plugin scheduling task
+     * @param task the task to be run
+     * @param delay the ticks to wait before running the task
+     * @return a BukkitTask that contains the id number
+     * @throws IllegalArgumentException if plugin is null
+     * @throws IllegalStateException if this was already scheduled
+     * @see BukkitScheduler#runTaskLater(Plugin, Runnable, long, long)
+     */
+    public synchronized BukkitTask runTaskLater(Plugin plugin, long delay) throws IllegalArgumentException, IllegalStateException  {
+        checkState();
+        return setupId(Bukkit.getScheduler().runTaskLater(plugin, this, delay));
+    }
+
+    /**
+     * <b>Asynchronous tasks should never access any API in Bukkit.
+     *    Great care should be taken to assure the thread-safety of asynchronous tasks.</b>
+     * <br>
+     * <br>Schedules this to run asynchronously after the specified number of server ticks.
+     *
+     * @param plugin the reference to the plugin scheduling task
+     * @param delay the ticks to wait before running the task
+     * @return a BukkitTask that contains the id number
+     * @throws IllegalArgumentException if plugin is null
+     * @throws IllegalStateException if this was already scheduled
+     * @see BukkitScheduler#runTaskLaterAsynchronously(Plugin, Runnable, long, long)
+     */
+    public synchronized BukkitTask runTaskLaterAsynchronously(Plugin plugin, long delay) throws IllegalArgumentException, IllegalStateException  {
+        checkState();
+        return setupId(Bukkit.getScheduler().runTaskLaterAsynchronously(plugin, this, delay));
+    }
+
+    /**
+     * Schedules this to repeatedly run until cancelled, starting after the specified number of server ticks
+     *
+     * @param plugin the reference to the plugin scheduling task
+     * @param delay the ticks to wait before running the task
+     * @param period the ticks to wait between runs
+     * @return a BukkitTask that contains the id number
+     * @throws IllegalArgumentException if plugin is null
+     * @throws IllegalStateException if this was already scheduled
+     * @see BukkitScheduler#runTaskTimer(Plugin, Runnable, long, long)
+     */
+    public synchronized BukkitTask runTaskTimer(Plugin plugin, long delay, long period) throws IllegalArgumentException, IllegalStateException  {
+        checkState();
+        return setupId(Bukkit.getScheduler().runTaskTimer(plugin, this, delay, period));
+    }
+
+    /**
+     * <b>Asynchronous tasks should never access any API in Bukkit.
+     *    Great care should be taken to assure the thread-safety of asynchronous tasks.</b>
+     * <br>
+     * <br>Schedules this to repeatedly run asynchronously until cancelled, starting after the specified number of server ticks.
+     *
+     * @param plugin the reference to the plugin scheduling task
+     * @param delay the ticks to wait before running the task for the first time
+     * @param period the ticks to wait between runs
+     * @return a BukkitTask that contains the id number
+     * @throws IllegalArgumentException if plugin is null
+     * @throws IllegalStateException if this was already scheduled
+     * @see BukkitScheduler#runTaskTimerAsynchronously(Plugin, Runnable, long, long)
+     */
+    public synchronized BukkitTask runTaskTimerAsynchronously(Plugin plugin, long delay, long period) throws IllegalArgumentException, IllegalStateException  {
+        checkState();
+        return setupId(Bukkit.getScheduler().runTaskTimerAsynchronously(plugin, this, delay, period));
+    }
+
+    /**
+     * Gets the task id for this runnable
+     * @return the task id that this runnable was scheduled as
+     * @throws IllegalStateException if task was not scheduled yet
+     */
+    public synchronized int getTaskId() throws IllegalStateException {
+        final int id = taskId;
+        if (id == -1) {
+            throw new IllegalStateException("Not scheduled yet");
+        }
+        return id;
+    }
+
+    private void checkState() {
+        if (taskId != -1) {
+            throw new IllegalStateException("Already scheduled as " + taskId);
+        }
+    }
+
+    private BukkitTask setupId(final BukkitTask task) {
+        this.taskId = task.getTaskId();
+        return task;
+    }
+}