efcore 新特性 SaveChanges Events

Intro

昨天早上看到之前关注的一个 efcore 的 issue 被 closed ，于是看了一眼， ef core 新合并了一个 PR，在 DbContext 中增加了 SaveChanges 相关的几个事件，具体的变更可以参数 PR https://github.com/dotnet/efcore/pull/21862

Events

之前写过两篇关于 EF Core 做自动审计的文章

第一次的实现需要显式继承一个 AuditDbContext ，在有些需要没办法修改 DbContext 或者原有 DbContext 已经有继承某一个类，就没有办法用了，可以参考 https://www.cnblogs.com/weihanli/p/auto-audit-for-entity-framework.html

后面结合 AOP 改进了一版，通过一个审计切面逻辑完成自动审计，但是需要引入 AOP 组件支持，对于不想引入额外组件的项目来说也并非特别友好，可以参考 https://www.cnblogs.com/weihanli/p/12819634.html

在这个 PR 合并之后，我们可以通过 SavingChanges 事件获取保存之前 DbContext 的状态，通过 SavedChanges 事件来获取保存成功后的 DbContext 信息，SaveChangesFailed 事件获取保存失败信息

事件定义如下：

 /// <summary> ///     An event fired at the beginning of a call to <see cref="M:SaveChanges"/> or <see cref="M:SaveChangesAsync"/> /// </summary> public event EventHandler<SavingChangesEventArgs> SavingChanges;  /// <summary> ///     An event fired at the end of a call to <see cref="M:SaveChanges"/> or <see cref="M:SaveChangesAsync"/> /// </summary> public event EventHandler<SavedChangesEventArgs> SavedChanges;  /// <summary> ///     An event fired if a call to <see cref="M:SaveChanges"/> or <see cref="M:SaveChangesAsync"/> fails with an exception. /// </summary> public event EventHandler<SaveChangesFailedEventArgs> SaveChangesFailed; 

事件参数定义如下：

/// <summary> ///     Base event arguments for the <see cref="M:DbContext.SaveChanges" /> and <see cref="M:DbContext.SaveChangesAsync" /> events. /// </summary> public abstract class SaveChangesEventArgs : EventArgs {     /// <summary>     ///     Creates a base event arguments instance for <see cref="M:DbContext.SaveChanges" />     ///     or <see cref="M:DbContext.SaveChangesAsync" /> events.     /// </summary>     /// <param name="acceptAllChangesOnSuccess"> The value passed to SaveChanges. </param>     protected SaveChangesEventArgs(bool acceptAllChangesOnSuccess)     {         AcceptAllChangesOnSuccess = acceptAllChangesOnSuccess;     }      /// <summary>     ///     The value passed to <see cref="M:DbContext.SaveChanges" /> or <see cref="M:DbContext.SaveChangesAsync" />.     /// </summary>     public virtual bool AcceptAllChangesOnSuccess { get; } }  /// <summary> ///     Event arguments for the <see cref="DbContext.SavingChanges" /> event. /// </summary> public class SavingChangesEventArgs : SaveChangesEventArgs {     /// <summary>     ///     Creates event arguments for the <see cref="M:DbContext.SavingChanges" /> event.     /// </summary>     /// <param name="acceptAllChangesOnSuccess"> The value passed to SaveChanges. </param>     public SavingChangesEventArgs(bool acceptAllChangesOnSuccess)         : base(acceptAllChangesOnSuccess)     {     } }  /// <summary> ///     Event arguments for the <see cref="DbContext.SavedChanges" /> event. /// </summary> public class SavedChangesEventArgs : SaveChangesEventArgs {     /// <summary>     ///     Creates a new <see cref="SavedChangesEventArgs" /> instance with the given number of entities saved.     /// </summary>     /// <param name="acceptAllChangesOnSuccess"> The value passed to SaveChanges. </param>     /// <param name="entitiesSavedCount"> The number of entities saved. </param>     public SavedChangesEventArgs(bool acceptAllChangesOnSuccess, int entitiesSavedCount) : base(acceptAllChangesOnSuccess)     {         EntitiesSavedCount = entitiesSavedCount;     }      /// <summary>     ///     The number of entities saved.     /// </summary>     public virtual int EntitiesSavedCount { get; } }  /// <summary> ///     Event arguments for the <see cref="DbContext.SaveChangesFailed" /> event. /// </summary> public class SaveChangesFailedEventArgs : SaveChangesEventArgs {     /// <summary>     /// Creates a new <see cref="SaveChangesFailedEventArgs"/> instance with the exception that was thrown.     /// </summary>     /// <param name="acceptAllChangesOnSuccess"> The value passed to SaveChanges. </param>     /// <param name="exception"> The exception thrown. </param>     public SaveChangesFailedEventArgs(bool acceptAllChangesOnSuccess, [NotNull] Exception exception)         : base(acceptAllChangesOnSuccess)     {         Exception = exception;     }      /// <summary>     /// The exception thrown during<see cref="M:DbContext.SaveChanges"/> or <see cref="M:DbContext.SaveChangesAsync"/>.     /// </summary>     public virtual Exception Exception { get; } } 

More

除了上面的审计，你也可以使用通过这些事件，实现保存之前的自动更新数据库字段的值，比如 Add 或 Update 操作数据时自动设置更新时间等信息

本文提到的特性还未正式发布，预计会在 .net5 下一个预览版中发布，如果想现在要尝试，请使用 efcore 的 daily build 的包，可以参考 https://github.com/dotnet/aspnetcore/blob/master/docs/DailyBuilds.md

Reference

• https://github.com/dotnet/efcore/issues/15910
• https://github.com/dotnet/efcore/pull/21862