自定义回调处理是最常见的接口开发实现，我们往往只需要对接口中的部分实现进行替换修改，以在原有的实现逻辑中注入自定义的逻辑实现。我们来看一个自定义回调处理的示例，我们需要将所有执行的 SQL 语句记录到 monitor 表中，以方便于进行 SQL 审计。

为简化示例编写，我们这里实现了一个自定义的 MySQL 驱动，该驱动继承于 gdb 模块中已经实现的 DriverMysql，并按照需要修改覆盖相应的接口方法。由于所有的 SQL 语句执行必定会通过 DoQuery 或者 DoExec 接口，因此我们在自定义的驱动中实现并覆盖这两个接口方法即可。

package driver

import (
    "context"
    "database/sql"
    "github.com/gogf/gf/database/gdb"
    "github.com/gogf/gf/os/gtime"
)

// MyDriver is a custom database driver, which is used for testing only.
// For simplifying the unit testing case purpose, MyDriver struct inherits the mysql driver
// gdb.DriverMysql and overwrites its functions DoQuery and DoExec.
// So if there's any sql execution, it goes through MyDriver.DoQuery/MyDriver.DoExec firstly
// and then gdb.DriverMysql.DoQuery/gdb.DriverMysql.DoExec.
// You can call it sql "HOOK" or "HiJack" as your will.
type MyDriver struct {
    *gdb.DriverMysql
}

var (
    // customDriverName is my driver name, which is used for registering.
    customDriverName = "MyDriver"
)

func init() {
    // It here registers my custom driver in package initialization function "init".
    // You can later use this type in the database configuration.
    if err := gdb.Register(customDriverName, &MyDriver{}); err != nil {
        panic(err)
    }
}

// New creates and returns a database object for mysql.
// It implements the interface of gdb.Driver for extra database driver installation.
func (d *MyDriver) New(core *gdb.Core, node *gdb.ConfigNode) (gdb.DB, error) {
    return &MyDriver{
        &gdb.DriverMysql{
            Core: core,
        },
    }, nil
}

// DoQuery commits the sql string and its arguments to underlying driver
// through given link object and returns the execution result.
func (d *MyDriver) DoQuery(ctx context.Context, link gdb.Link, sql string, args ...interface{}) (rows *sql.Rows, err error) {
    tsMilli := gtime.TimestampMilli()
    rows, err = d.DriverMysql.DoQuery(ctx, link, sql, args...)
    link.Exec(
        "INSERT INTO `monitor`(`sql`,`cost`,`time`,`error`) VALUES(?,?,?,?)",
        gdb.FormatSqlWithArgs(sql, args),
        gtime.TimestampMilli()-tsMilli,
        gtime.Now(),
        err,
    )
    return
}

// DoExec commits the query string and its arguments to underlying driver
// through given link object and returns the execution result.
func (d *MyDriver) DoExec(ctx context.Context, link gdb.Link, sql string, args ...interface{}) (result sql.Result, err error) {
    tsMilli := gtime.TimestampMilli()
    result, err = d.DriverMysql.DoExec(ctx, link, sql, args...)
    link.Exec(
        "INSERT INTO `monitor`(`sql`,`cost`,`time`,`error`) VALUES(?,?,?,?)",
        gdb.FormatSqlWithArgs(sql, args),
        gtime.TimestampMilli()-tsMilli,
        gtime.Now(),
        err,
    )
    return
}

我们看到，这里在包初始化方法 init 中使用了 gdb.Register("MyDriver", &MyDriver{}) 来注册了了一个自定义名称的驱动。我们也可以通过 gdb.Register("mysql", &MyDriver{}) 来覆盖已有的框架 mysql 驱动为自己的驱动。

驱动名称 mysql 为框架默认的 DriverMysql 驱动的名称。

由于这里我们使用了一个新的驱动名称 MyDriver，因此在 gdb 配置中的 type 数据库类型时，需要填写该驱动名称。以下是一个使用配置的示例：

[database]
    type = "MyDriver"
    link = "root:12345678@tcp(127.0.0.1:3306)/test"