這兩天在使用Asp.net MVC 4開發COMET消息通知機制，在后端使用異步線程對消息進行訂閱，客戶端通過AJAX長連接請求MVC中的ACTION，如：，即表示獲取ImageSet對象的變更消息（新增，更新和刪除消息)。

1.事件消息的類IEventEntity<TEntity>類的定義

復制代碼 代碼如下:

public interface IEntityEvent<TEntity>
    {
        //變更的實體類對象
        TEntity[] Entities
        {
            get;
        }

        //操作類型
        EntityEventType Type
        {
            get;
        }
    }

    public enum EntityEventType : int
    {
        Create = 0,
        Update = 1,
        Removed = 2
    }

復制代碼 代碼如下:

[SessionState(SessionStateBehavior.ReadOnly)]
    public class EntityEventController : Controller
    {
        //異步獲取對ImageSet對象操作的變更事件Action，millsecondsTimeout為超時時間。
        public async Task<ActionResult> ImageSet(int millisecondsTimeout = 10000)
        {
            return await this.EventAsync<ImageSetData>(millisecondsTimeout);
        }

        private async Task<ActionResult> EventAsync<TEntity>(int millisecondsTimeout)
        {
            IEntityEvent<TEntity> entityEvent = await EntityEventSubcriber.Instance.WaitForEntityEvent<TEntity>(millisecondsTimeout);

            return this.Json(new
            {
                HasEvent = null != entityEvent,
                EntityEvent = entityEvent

            }, JsonRequestBehavior.AllowGet);
        }
    }

(2) 如果Controller層面應用或者Filter中操作過Session，那么為了避免長鏈接時不會導至同一Session在其他調用中Session Block的情況，需要在Controller頭上加入[SessionState(SessionStateBehavior.ReadOnly)] 的Attribute，來表示當前Controller對Session為只讀操作，這樣就其他操作就不會被阻塞了。

3.這里就不具體寫EntityEventSubscriber消息訂閱器的代碼了，以后將在“消息訂閱與發布”的文章中詳細描述。

4.jQuery AJAX客戶端代碼

復制代碼 代碼如下:

$(document).ready(function () {

        var $hoverList = $("#imageSets").hoverList({title:"圖片集列表", selectedIndex: 1 });

        var getEvent = function(){

            var getPattern = "/EasyshirtBackend/imageSet/0";

            $.getJSON("/EasyshirtBackend/event/imageSet/100000" , function(data){

                if(data.HasEvent){

                    //Create
                    if(data.EntityEvent.Type == 0){

                        $.each(data.EntityEvent.Entities, function(i, entity){

                //TODO: 處理實體類新增

                            if(i == data.EntityEvent.Entities.length - 1){

                                getEvent();
                            }
                        });

                        return;
                    }

                    //Update
                    if(data.EntityEvent.Type == 1){

                        $.each(data.EntityEvent.Entities, function(i, entity){

                            //TODO: 處理實體類更新

                            if(i == data.EntityEvent.Entities.length - 1){

                                getEvent();
                            }
                        });

                        return;
                    }

                    //Delete
                    if(data.EntityEvent.Type == 2){

                        $.each(data.EntityEvent.Entities, function(i, entity){

                            //TODO: 處理實體類刪除

                            if(i == data.EntityEvent.Entities.length - 1){

                                getEvent();
                            }
                        });
                    }

                }else{

                    $("#imageSets").hoverList("add", data);
                    getEvent();
                }
            });
        };

        getEvent();
    });