@PublicEvolving
public abstract class Trigger<T, W extends Window> implements Serializable {

    private static final long serialVersionUID = -4104633972991191369L;

    /**
     * Called for every element that gets added to a pane. The result of this will determine whether
     * the pane is evaluated to emit results.
     *
     * @param element The element that arrived.
     * @param timestamp The timestamp of the element that arrived.
     * @param window The window to which the element is being added.
     * @param ctx A context object that can be used to register timer callbacks.
     */
    public abstract TriggerResult onElement(T element, long timestamp, W window, TriggerContext ctx)
            throws Exception;

    /**
     * Called when a processing-time timer that was set using the trigger context fires.
     *
     * @param time The timestamp at which the timer fired.
     * @param window The window for which the timer fired.
     * @param ctx A context object that can be used to register timer callbacks.
     */
    public abstract TriggerResult onProcessingTime(long time, W window, TriggerContext ctx)
            throws Exception;

    /**
     * Called when an event-time timer that was set using the trigger context fires.
     *
     * @param time The timestamp at which the timer fired.
     * @param window The window for which the timer fired.
     * @param ctx A context object that can be used to register timer callbacks.
     */
    public abstract TriggerResult onEventTime(long time, W window, TriggerContext ctx)
            throws Exception;

    /**
     * Returns true if this trigger supports merging of trigger state and can therefore be used with
     * a {@link org.apache.flink.streaming.api.windowing.assigners.MergingWindowAssigner}.
     *
     * <p>If this returns {@code true} you must properly implement {@link #onMerge(Window,
     * OnMergeContext)}
     */
    public boolean canMerge() {
        return false;
    }

    /**
     * Called when several windows have been merged into one window by the {@link
     * org.apache.flink.streaming.api.windowing.assigners.WindowAssigner}.
     *
     * @param window The new window that results from the merge.
     * @param ctx A context object that can be used to register timer callbacks and access state.
     */
    public void onMerge(W window, OnMergeContext ctx) throws Exception {
        throw new UnsupportedOperationException("This trigger does not support merging.");
    }

    /**
     * Clears any state that the trigger might still hold for the given window. This is called when
     * a window is purged. Timers set using {@link TriggerContext#registerEventTimeTimer(long)} and
     * {@link TriggerContext#registerProcessingTimeTimer(long)} should be deleted here as well as
     * state acquired using {@link TriggerContext#getPartitionedState(StateDescriptor)}.
     */
    public abstract void clear(W window, TriggerContext ctx) throws Exception;

    // ------------------------------------------------------------------------

    /**
     * A context object that is given to {@link Trigger} methods to allow them to register timer
     * callbacks and deal with state.
     */
    public interface TriggerContext {
        // ...
    }

    /**
     * Extension of {@link TriggerContext} that is given to {@link Trigger#onMerge(Window,
     * OnMergeContext)}.
     */
    public interface OnMergeContext extends TriggerContext {
        <S extends MergingState<?, ?>> void mergePartitionedState(
                StateDescriptor<S, ?> stateDescriptor);
    }
}

public enum TriggerResult {

    // 不觸發(fā)，也不刪除元素
    CONTINUE(false, false),

    // 觸發(fā)窗口，窗口出發(fā)后刪除窗口中的元素
    FIRE_AND_PURGE(true, true),

    // 觸發(fā)窗口，但是保留窗口元素
    FIRE(true, false),

    // 不觸發(fā)窗口，丟棄窗口，并且刪除窗口的元素
    PURGE(false, true);

    // ------------------------------------------------------------------------

    private final boolean fire;
    private final boolean purge;

    TriggerResult(boolean fire, boolean purge) {
        this.purge = purge;
        this.fire = fire;
    }

    public boolean isFire() {
        return fire;
    }

    public boolean isPurge() {
        return purge;
    }
}

@PublicEvolving
public class ProcessingTimeTrigger extends Trigger<Object, TimeWindow> {
    private static final long serialVersionUID = 1L;

    private ProcessingTimeTrigger() {}

    @Override
    public TriggerResult onElement(
            Object element, long timestamp, TimeWindow window, TriggerContext ctx) {
        ctx.registerProcessingTimeTimer(window.maxTimestamp());
        return TriggerResult.CONTINUE;
    }

    @Override
    public TriggerResult onEventTime(long time, TimeWindow window, TriggerContext ctx)
            throws Exception {
        return TriggerResult.CONTINUE;
    }

    @Override
    public TriggerResult onProcessingTime(long time, TimeWindow window, TriggerContext ctx) {
        return TriggerResult.FIRE;
    }

    @Override
    public void clear(TimeWindow window, TriggerContext ctx) throws Exception {
        ctx.deleteProcessingTimeTimer(window.maxTimestamp());
    }

    @Override
    public boolean canMerge() {
        return true;
    }

    @Override
    public void onMerge(TimeWindow window, OnMergeContext ctx) {
        // only register a timer if the time is not yet past the end of the merged window
        // this is in line with the logic in onElement(). If the time is past the end of
        // the window onElement() will fire and setting a timer here would fire the window twice.
        long windowMaxTimestamp = window.maxTimestamp();
        if (windowMaxTimestamp > ctx.getCurrentProcessingTime()) {
            ctx.registerProcessingTimeTimer(windowMaxTimestamp);
        }
    }

    @Override
    public String toString() {
        return "ProcessingTimeTrigger()";
    }

    /** Creates a new trigger that fires once system time passes the end of the window. */
    public static ProcessingTimeTrigger create() {
        return new ProcessingTimeTrigger();
    }
}

private void evict(Iterable<TimestampedValue<Object>> elements, int size, EvictorContext ctx) {
    if (size <= maxCount) {
        // 小于最大數(shù)量，不做處理
        return;
    } else {
        int evictedCount = 0;
        for (Iterator<TimestampedValue<Object>> iterator = elements.iterator(); iterator.hasNext();){
            iterator.next();
            evictedCount++;
            if (evictedCount > size - maxCount) {
                break;
            } else {
                // 移除前size - maxCount個(gè)元素，只剩下最后maxCount個(gè)元素
                iterator.remove();
            }
        }
    }
}

private void evict(Iterable<TimestampedValue<T>> elements, int size, EvictorContext ctx) {

    // 獲取最后一個(gè)元素
    TimestampedValue<T> lastElement = Iterables.getLast(elements);

    for (Iterator<TimestampedValue<T>> iterator = elements.iterator(); iterator.hasNext();){
        TimestampedValue<T> element = iterator.next();
        // 依次計(jì)算每個(gè)元素和最后一個(gè)元素的delta值，同時(shí)和threshold的值進(jìn)行比較
        // 若計(jì)算結(jié)果大于threshold值或者是相等，則該元素會(huì)被移除
        if (deltaFunction.getDelta(element.getValue(), lastElement.getValue()) >= this.threshold) {
            iterator.remove();
        }
    }
}

private void evict(Iterable<TimestampedValue<Object>> elements, int size, EvictorContext ctx) {

    // 如果element沒(méi)有timestamp，直接返回
    if (!hasTimestamp(elements)) {
        return;
    }

    // 獲取elements中最大的時(shí)間戳（到來(lái)最晚的元素的時(shí)間）
    long currentTime = getMaxTimestamp(elements);
    // 截止時(shí)間為： 到來(lái)最晚的元素的時(shí)間 - 窗口大小（可以理解為保留最近的多久的元素）
    long evictCutoff = currentTime - windowSize;

    for (Iterator<TimestampedValue<Object>> iterator = elements.iterator(); iterator.hasNext(); ) {
        TimestampedValue<Object> record = iterator.next();

        // 清除所有時(shí)間戳小于截止時(shí)間的元素
        if (record.getTimestamp() <= evictCutoff) {
            iterator.remove();
        }
    }
}