gorm 源码阅读之 callback

//main.go 创建db db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
//gorm.go 这里Dialector是驱动 config.Dialector.Initialize(db)
//mysql.go 这里我用的mysql的驱动，包是gorm.io/driver/mysqlfunc (dialector Dialector) Initialize(db *gorm.DB) (err error)
//注册db的callbacksfunc RegisterDefaultCallbacks(db *gorm.DB, config *Config) {  //...  queryCallback := db.Callback().Query()  queryCallback.Register("gorm:query", Query)  queryCallback.Register("gorm:preload", Preload)  queryCallback.Register("gorm:after_query", AfterQuery)  //...}

//callbacks就是一个map，map里面是一个processortype callbacks struct {  processors map[string]*processor}
//这里要区分db.callbacks和processor.callbacks，两个是不同的东西type processor struct {  db        *DB  fns       []func(*DB)  callbacks []*callback}
//这个是processor里面存的callbacktype callback struct {  name      string  before    string  after     string  remove    bool  replace   bool  match     func(*DB) bool  handler   func(*DB)  processor *processor}
//初始化的时候，就是为curd等几个操作分别创建一个processorfunc initializeCallbacks(db *DB) *callbacks {  return &callbacks{    processors: map[string]*processor{      "create": {db: db},      "query":  {db: db},      "update": {db: db},      "delete": {db: db},      "row":    {db: db},      "raw":    {db: db},    },  }}

//参考 <1、调用堆栈>，注册db默认的callbacksfunc RegisterDefaultCallbacks(db *gorm.DB, config *Config) {  //...    //先取出db.callbacks这个map里query的processor  queryCallback := db.Callback().Query()  queryCallback.Register("gorm:query", Query)  queryCallback.Register("gorm:preload", Preload)  queryCallback.Register("gorm:after_query", AfterQuery)  //...}
//往processor里注册回调函数fnfunc (p *processor) Register(name string, fn func(*DB)) error {    //这里是新建了一个临时的callback，然后将这个callback保存到processor里面    //这里的callback和db.callbacks不是同一个东西哦  return (&callback{processor: p}).Register(name, fn)}
//将name和回调函数存在新建的临时callback里，同时将这个callback保存到processor里面func (c *callback) Register(name string, fn func(*DB)) error {  c.name = name  c.handler = fn  c.processor.callbacks = append(c.processor.callbacks, c)  return c.processor.compile()}
//将processor里面的callback进行优先级排序，同时将排序后的回调保存在p.fns里func (p *processor) compile() (err error) {  var callbacks []*callback  for _, callback := range p.callbacks {    if callback.match == nil || callback.match(p.db) {      callbacks = append(callbacks, callback)    }  }  p.callbacks = callbacks
    //对processor.callbacks进行排序  if p.fns, err = sortCallbacks(p.callbacks); err != nil {    p.db.Logger.Error(context.Background(), "Got error when compile callbacks, got %v", err)  }  return}
//如何自定义注册带顺序的回调db.Callback().Create().Before("gorm:create").Register("update_created_at", updateCreated)db.Callback().Create().After("gorm:create").Register("update_created_at", updateCreated)db.Callback().Query().After("gorm:query").Register("my_plugin:after_query", afterQuery)db.Callback().Delete().After("gorm:delete").Register("my_plugin:after_delete", afterDelete)db.Callback().Update().Before("gorm:update").Register("my_plugin:before_update", beforeUpdate)

//以下面这个查询为例var user Userif err := db.Preload("Clazz").Where("id = ?", 1).First(&user).Error; nil != err {  fmt.Printf("err:%v\n", err)}
//db.First()才开始真正的执行查询func (db *DB) First(dest interface{}, conds ...interface{}) (tx *DB) {  //...    //这里是真正的执行查询  tx.callbacks.Query().Execute(tx)  //...}
//tx.callbacks.Query()就是取出query对应的processorfunc (cs *callbacks) Query() *processor {  return cs.processors["query"]}
//执行processor的Execute()func (p *processor) Execute(db *DB) {    //...    //这里调用p.fns，也就是排过序的callbacks  for _, f := range p.fns {    f(db)  }}

//上面的堆栈执行到p.fns，在一开始的时候，db就注册了到fns了，//就是下面这个Query函数func RegisterDefaultCallbacks(db *gorm.DB, config *Config) {  //...    //先取出db.callbacks这个map里query的processor  queryCallback := db.Callback().Query()  queryCallback.Register("gorm:query", Query)  queryCallback.Register("gorm:preload", Preload)  queryCallback.Register("gorm:after_query", AfterQuery)  //...}
//callbacks/query.go里面定义了真正的查询的地方func Query(db *gorm.DB) {  if db.Error == nil {    BuildQuerySQL(db)
    if !db.DryRun && db.Error == nil {      rows, err := db.Statement.ConnPool.QueryContext(db.Statement.Context, db.Statement.SQL.String(), db.Statement.Vars...)      if err != nil {        db.AddError(err)        return      }      defer rows.Close()
      gorm.Scan(rows, db, false)    }  }}