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
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