工作流，并不像我原本想象的那样，仅仅是个什么工作的流程。我的理解，工作流是一个对程序流程逻辑与具体操作的分离，程序的流程和逻辑可以在工作流图上清晰的展现，图中每个步骤或状态中的操作就不在掺杂着太繁杂的逻辑结构了。这么做的好处呢，就是从复杂的程序逻辑，抽出一个大的轮廓，通过一个工作流图，把大的问题切割成一个个小块儿了。其实，所有的架构设计不都是为了分解复杂的问题吗。

VS2008里的工作流编辑器，让创建工作流的工作变得相当简单和直观，简单了解了一些常用的Activity的用途和使用之后，半天时间就足够学会用工作流图表达程序逻辑了。

可能是因为我对C#的委托和事件不够熟悉，在工作流和Host的通信上花费了不少时间，才理清了这个过程。因为Workflow和Host程序是在不同线程中运行的，所以相互之间的调用只能通过delegation和event。Host通过触发ICustomeService接口中的事件，调用注册在这个事件上的HandleExternalEventActivity。另一个方向上，Workflow调用ICustomService接口中的方法，方法里触发一个CustomService中定义的公开事件，而Host程序把需要被调用的代码，通过委托注册到这个事件上，从而被Workflow调用。

先不写了，明天有空了补上个例子和图片吧。

数据库只有三个字段：

id     varchar(36)   Workflow Guid
state   blob              Workflow State Serialized
status    tinyint       Workflow  Status

代码如下：

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Workflow.ComponentModel;
using System.Workflow.Runtime;
using System.Workflow.Runtime.Hosting;
using Microsoft.Practices.EnterpriseLibrary.Data;

namespace Ganji.OA.Workflow
{
    public class MysqlWorkflowPersistenceService : WorkflowPersistenceService
    {
        public MysqlWorkflowPersistenceService()
            : base()
        {}

        public List GetAllWorkflows()
        {
            List result = new List();
            Database db = DatabaseFactory.CreateDatabase("OAData");
            DbCommand command = db.GetSqlStringCommand("SELECT id FROM workflow_state");
            DbDataReader reader = command.ExecuteReader();
            while (reader.Read())
            {
                result.Add(new Guid((string)reader["id"]));
            }
            reader.Close();
            return result;
        }

        protected override Activity LoadCompletedContextActivity(Guid scopeId, Activity outerActivity)
        {
            Guid instanceId = WorkflowEnvironment.WorkflowInstanceId;
            Activity activity = Deserialize(instanceId, scopeId, outerActivity);
            return activity;
        }

        protected override Activity LoadWorkflowInstanceState(Guid instanceId)
        {
            Activity activity = Deserialize(instanceId, Guid.Empty, null);
            return activity;
        }

        protected override void SaveCompletedContextActivity(Activity activity)
        {
            Guid instanceId = WorkflowEnvironment.WorkflowInstanceId;
            Guid contextId = (Guid)activity.GetValue(Activity.ActivityContextGuidProperty);
            Serialize(instanceId, contextId, activity);
        }

        protected override void SaveWorkflowInstanceState(Activity rootActivity, bool unlock)
        {
            WorkflowStatus status = GetWorkflowStatus(rootActivity);
            Guid instanceId = WorkflowEnvironment.WorkflowInstanceId;

            if (status == WorkflowStatus.Terminated || status == WorkflowStatus.Completed)
            {
                DeleteWorkflow(instanceId);
            }
            else
            {
                Serialize(instanceId, Guid.Empty, rootActivity);
            }
        }

        protected override bool UnloadOnIdle(Activity activity)
        {
            return true;
        }

        protected override void UnlockWorkflowInstanceState(Activity rootActivity)
        {
        }

        private void Serialize(Guid instanceId, Guid contextId, Activity activity)
        {
            byte[] state = WorkflowPersistenceService.GetDefaultSerializedForm(activity);
            Database db = DatabaseFactory.CreateDatabase("OAData");
            String sql_del = "DELETE FROM workflow_state WHERE id=@id";
            DbCommand command_del = db.GetSqlStringCommand(sql_del);
            db.AddInParameter(command_del, "@id", DbType.String, instanceId.ToString());

            String sql = "INSERT INTO workflow_state (`id`, `state`, `status`) VALUES (@id, @state, @status)";
            DbCommand command = db.GetSqlStringCommand(sql);
            db.AddInParameter(command, "@id", DbType.String, instanceId.ToString());
            db.AddInParameter(command, "@state", DbType.Binary, state);
            db.AddInParameter(command, "@status", DbType.Byte, 0);

            using (DbConnection conn = db.CreateConnection())
            {
                conn.Open();
                db.ExecuteNonQuery(command_del);
                db.ExecuteNonQuery(command);
                conn.Close();
            }
        }

        private Activity Deserialize(Guid instanceId, Guid contextId, Activity outerActivity)
        {
            Database db = DatabaseFactory.CreateDatabase("OAData");
            string sql = "SELECT * FROM workflow_state WHERE id='" + instanceId.ToString() + "'";
            Activity activity = null;
            using (DbDataReader reader = db.ExecuteReader(CommandType.Text, sql) as DbDataReader)
            {
                if (reader.HasRows)
                {
                    reader.Read();
                    byte[] state = (byte[])reader["state"];
                    reader.Close();
                    activity = WorkflowPersistenceService.RestoreFromDefaultSerializedForm(state, outerActivity);
                }
            }
            return activity;
        }

        private void DeleteWorkflow(Guid instanceId)
        {
            Database db = DatabaseFactory.CreateDatabase("OAData");
            string sql = "DELETE FROM workflow_state WHERE id=@id";
            DbCommand command = db.GetSqlStringCommand(sql);
            db.AddInParameter(command, "@id", DbType.String, instanceId.ToString());
            using (DbConnection conn = db.CreateConnection())
            {
                conn.Open();
                command.ExecuteNonQuery();
                conn.Close();
            }
        }
    }
}

重要更正: 在Pro WF书里说保存和恢复Activity的状态是用的Activity对象的Save和Load方法，这样保存的对象信息有六十多KB，还只是对于一个只有5个状态的小工作流，这个尺寸放在MySQL的blob里都放不下，造成了运行时一开始可以正常的存入工作流状态，但状态变更后，工作流序列化后的对象增大，保存进数据库的时候被截断了，造成了下次从数据库取得状态时报一个“在分析完成之前就遇到流结尾。”的错误，浪费了近一天的时间去解决这个问题。最终在MSDN的一篇文章的一小段例子里，看到它恢复Activity状态用的是WorkflowPersistenceService对象的这个方法

protected static Activity RestoreFromDefaultSerializedForm(
	byte[] activityBytes,
	Activity outerActivity
)

相应的保存Activity状态的方法是：
protected static byte[] GetDefaultSerializedForm(
	Activity activity
)
这一对方法对序列化的状态用GZIP进行了压缩，上面六十多KB的数据现在只有13K左右了。