How can custom triggers be defined in Flink?

To customize triggers in Flink, you need to implement the Trigger interface, which is defined as follows:

public interface Trigger<T, W extends Window> extends Serializable {

    // 初始化触发器
    void open(TriggerContext ctx) throws Exception;

    // 每次元素到来时都会调用此方法，决定是否触发窗口计算
    TriggerResult onElement(T element, long timestamp, W window, TriggerContext ctx) throws Exception;

    // 每次处理时间定时器到来时都会调用此方法，决定是否触发窗口计算
    TriggerResult onProcessingTime(long time, W window, TriggerContext ctx) throws Exception;

    // 每次事件时间定时器到来时都会调用此方法，决定是否触发窗口计算
    TriggerResult onEventTime(long time, W window, TriggerContext ctx) throws Exception;

    // 当窗口计算完成时会调用此方法
    void clear(W window, TriggerContext ctx) throws Exception;

    // 序列化
    default void write(DataOutputView out) throws IOException {}

    // 反序列化
    default void read(DataInputView in) throws IOException {}
}

Custom triggers need to implement the methods onElement, onProcessingTime, onEventTime, and clear, and initialize the trigger in the open method. In addition, TriggerContext provides some context information that can be used in the trigger. By implementing the Trigger interface, the triggering logic can be defined according to business requirements, achieving a more flexible window calculation method.