建構者模式 (Builder)

建構者模式，主要透過條件堆疊取得產物。

每一個堆疊的步驟，都是一個 Builder，例如透過 set 的方式來傳入 Builder。

實際用途，可運用在設計資料庫查詢器的功能，例如，要查詢一個用戶，名字為 john

例如，以下擷取 Laravel 的查詢器部分程式碼，應該就可清楚了解 Builder 用途：

$name = DB::table('users')->where('name', 'John')->pluck('name');

這裡先列出一些 Builder 程式片段：

<?php
...

class Builder
{
    use BuildsQueries, ForwardsCalls, Macroable {
        __call as macroCall;
    }

    /**
     * The current query value bindings.
     *
     * @var array
     */
    public $bindings = [
        'select' => [],
        'from'   => [],
        'join'   => [],
        'where'  => [],
        'having' => [],
        'order'  => [],
        'union'  => [],
    ];
 
    /**
     * Set the columns to be selected.
     *
     * @param  array|mixed  $columns
     * @return $this
     */
    public function select($columns = ['*'])
    {
        $this->columns = is_array($columns) ? $columns : func_get_args();

        return $this;
    }
    ...

    /**
     * Create a new query builder instance.
     *
     * @param  \Illuminate\Database\ConnectionInterface  $connection
     * @param  \Illuminate\Database\Query\Grammars\Grammar|null  $grammar
     * @param  \Illuminate\Database\Query\Processors\Processor|null  $processor
     * @return void
     */
    public function __construct(ConnectionInterface $connection,
                                Grammar $grammar = null,
                                Processor $processor = null)
    {
        $this->connection = $connection;
        $this->grammar = $grammar ?: $connection->getQueryGrammar();
        $this->processor = $processor ?: $connection->getPostProcessor();
    }
    ...

    /**
     * Add a join clause to the query.
     *
     * @param  string  $table
     * @param  \Closure|string  $first
     * @param  string|null  $operator
     * @param  string|null  $second
     * @param  string  $type
     * @param  bool    $where
     * @return $this
     */
    public function join($table, $first, $operator = null, $second = null, $type = 'inner', $where = false)
    {
        $join = $this->newJoinClause($this, $type, $table);

        // If the first "column" of the join is really a Closure instance the developer
        // is trying to build a join with a complex "on" clause containing more than
        // one condition, so we'll add the join and call a Closure with the query.
        if ($first instanceof Closure) {
            call_user_func($first, $join);

            $this->joins[] = $join;

            $this->addBinding($join->getBindings(), 'join');
        }

        // If the column is simply a string, we can assume the join simply has a basic
        // "on" clause with a single condition. So we will just build the join with
        // this simple join clauses attached to it. There is not a join callback.
        else {
            $method = $where ? 'where' : 'on';

            $this->joins[] = $join->$method($first, $operator, $second);

            $this->addBinding($join->getBindings(), 'join');
        }

        return $this;
    }

    ...

    /**
     * Get the SQL representation of the query.
     *
     * @return string
     */
    public function toSql()
    {
        return $this->grammar->compileSelect($this);
    }

    /**
     * Execute a query for a single record by ID.
     *
     * @param  int|string  $id
     * @param  array  $columns
     * @return mixed|static
     */
    public function find($id, $columns = ['*'])
    {
        return $this->where('id', '=', $id)->first($columns);
    }

    ...

    /**
     * Set the table which the query is targeting.
     *
     * @param  string  $table
     * @return $this
     */
    public function from($table)
    {
        $this->from = $table;

        return $this;
    }

    /**
     * Handle dynamic method calls into the method.
     *
     * @param  string  $method
     * @param  array   $parameters
     * @return mixed
     *
     * @throws \BadMethodCallException
     */
    public function __call($method, $parameters)
    {
        if (static::hasMacro($method)) {
            return $this->macroCall($method, $parameters);
        }

        if (Str::startsWith($method, 'where')) {
            return $this->dynamicWhere($method, $parameters);
        }

        static::throwBadMethodCallException($method);
    }
}