<?php namespace Laravel\Database;

use Laravel\Fluent;
use Laravel\Database as DB;

class Schema {

	/**
	 * Begin a fluent schema operation on a database table.
	 * 在数据库表上开始流畅的模式操作。
	 * @param  string   $table
	 * @param  Closure  $callback
	 * @return void
	 */
	public static function table($table, $callback)
	{
		call_user_func($callback, $table = new Schema\Table($table));

		return static::execute($table);
	}

	/**
	 * Create a new database table schema.
	 * 创建一个新的数据库表架构。
	 * @param  string   $table
	 * @param  Closure  $callback
	 * @return void
	 */
	public static function create($table, $callback)
	{
		$table = new Schema\Table($table);

		// To indicate that the table is new and needs to be created, we'll run
		// the "create" command on the table instance. This tells schema it is
		// not simply a column modification operation.
        // 为了表明表是新的并且需要创建，我们将在表实例上运行“create”命令。 这告诉模式它不仅仅是一个列修改操作。
		$table->create();

		call_user_func($callback, $table);

		return static::execute($table);
	}

	/**
	 * Drop a database table from the schema.
	 * 从架构中删除数据库表。
	 * @param  string  $table
	 * @return void
	 */
	public static function drop($table)
	{
		$table = new Schema\Table($table);

		// To indicate that the table needs to be dropped, we will run the
		// "drop" command on the table instance and pass the instance to
		// the execute method as calling a Closure isn't needed.
        // 为了指示需要删除表，我们将在表实例上运行“drop”命令并将实例传递给 execute 方法，因为不需要调用闭包。
		$table->drop();

		return static::execute($table);
	}

	/**
	 * Execute the given schema operation against the database.
	 * 对数据库执行给定的模式操作。
	 * @param  Schema\Table  $table
	 * @return void
	 */
	public static function execute($table)
	{
		// The implications method is responsible for finding any fluently
		// defined indexes on the schema table and adding the explicit
		// commands that are needed to tbe schema instance.
        // 暗示方法负责在模式表上找到任何流畅定义的索引，并将需要的显式命令添加到模式实例。
		static::implications($table);

		foreach ($table->commands as $command)
		{
			$connection = DB::connection($table->connection);

			$grammar = static::grammar($connection);

			// Each grammar has a function that corresponds to the command type and
			// is for building that command's SQL. This lets the SQL syntax builds
			// stay granular across various database systems.
            // 每个语法都有一个与命令类型相对应的函数，用于构建该命令的 SQL。 这让 SQL 语法构建在各种数据库系统中保持细粒度。
			if (method_exists($grammar, $method = $command->type))
			{
				$statements = $grammar->$method($table, $command);

				// Once we have the statements, we will cast them to an array even
				// though not all of the commands return an array just in case it
				// needs multiple queries to complete.
                // 一旦我们有了这些语句，我们就会将它们转换为一个数组，即使不是所有的命令都返回一个数组，以防它需要多个查询才能完成。
				foreach ((array) $statements as $statement)
				{
					$connection->query($statement);
				}
			}
		}
	}

	/**
	 * Add any implicit commands to the schema table operation.
	 * 向模式表操作添加任何隐式命令。
	 * @param   Schema\Table  $table
	 * @return  void
	 */
	protected static function implications($table)
	{
		// If the developer has specified columns for the table and the table is
		// not being created, we'll assume they simply want to add the columns
		// to the table and generate the add command.
        // 如果开发人员已为表指定了列，但未创建表，我们将假设他们只是想将列添加到表中并生成添加命令。
		if (count($table->columns) > 0 and ! $table->creating())
		{
			$command = new Fluent(array('type' => 'add'));

			array_unshift($table->commands, $command);
		}

		// For some extra syntax sugar, we'll check for any implicit indexes
		// on the table since the developer may specify the index type on
		// the fluent column declaration for convenience.
        // 对于一些额外的语法糖，我们将检查表上的任何隐式索引，因为开发人员可以为方便起见在 fluent 列声明中指定索引类型。
		foreach ($table->columns as $column)
		{
			foreach (array('primary', 'unique', 'fulltext', 'index') as $key)
			{
				if (isset($column->attributes[$key]))
				{
					$table->$key($column->name);
				}
			}
		}
	}

	/**
	 * Create the appropriate schema grammar for the driver.
	 * 为驱动程序创建适当的架构语法。
	 * @param  Connection  $connection
	 * @return Grammar
	 */
	public static function grammar(Connection $connection)
	{
		$driver = $connection->driver();

		switch ($driver)
		{
			case 'mysql':
				return new Schema\Grammars\MySQL($connection);

			case 'pgsql':
				return new Schema\Grammars\Postgres($connection);

			case 'sqlsrv':
				return new Schema\Grammars\SQLServer($connection);

			case 'sqlite':
				return new Schema\Grammars\SQLite($connection);
		}

		throw new \Exception("Schema operations not supported for [$driver].");
	}

}

github地址： https://github.com/liu-shilong/laravel3-scr