Creating A Nested To-Do List in PHP

To-Do lists are a great organizational tool that can be made even better when you introduce some categorization abilities. In this example, we'll be taking advantage of the Composite pattern to form a To-Do list that can contain sub-lists and sub-categories!

TodoList Component:

abstract class TodoList
{
   public abstract function Read(int $indent = 0);

   protected function Tab(int $indent)
   {
       return str_repeat(" ", $indent * 4);
   }
}

ListItem Leaf:

class ListItem extends TodoList
{
   private $taskDescription;

   function __construct(string $description)
   {
       $this->taskDescription = $description;
   }

   public function Read(int $indent = 0)
   {
       echo $this->Tab($indent) . $this->taskDescription . "\n";
   }
}

TaskList Composite:

class TaskList extends TodoList
{
   private $taskListName;
   private $todoList;

   function __construct(string $name)
   {
       $this->taskListName = $name;
       $this->todoList = array();
   }

   public function Read(int $indent = 0)
   {
       echo $this->Tab($indent) . $this->taskListName . "\n";
       foreach($this->todoList as $item)
       {
           $item->Read($indent + 1);
       }
   }

   public function Add(TodoList $task)
   {
       $this->todoList[] = $task;
   }

   public function Remove(TodoList $task)
   {
       if (($key = array_search($task, $this->todoList)) !== false) {
           unset($this->todoList[$key]);
       }
   }
}

Demo of the Composite Pattern:

// Top level todo list:
$todo = new TaskList("To-Do for today:");

// List for groceries:
$groceries = new TaskList("Go to the store:");
$groceries->Add(new ListItem("Apples"));
$groceries->Add(new ListItem("Bread"));
$groceries->Add(new ListItem("Yogurt"));
$groceries->Add(new ListItem("Popcorn"));

// Add list to the top level:
$todo->Add($groceries);

// Homework list:
$homework = new TaskList("Do my homework:");
$homework->Add(new ListItem("Math paper"));
$homework->Add(new ListItem("Physics quiz"));
// CS sub-list:
$csHomework = new TaskList("CSC:");
$csHomework->Add(new ListItem("Lab"));
$csHomework->Add(new ListItem("Assignment"));
$csHomework->Add(new ListItem("Make flashcards"));
// Add CS list to homework list:
$homework->Add($csHomework);
// Add homework list to top level:
$todo->Add($homework);

// Read off the list:
$todo->Read();

// Output:
/*
   To-Do for today:
   Go to the store:
       Apples
       Bread
       Yogurt
       Popcorn
   Do my homework:
       Math paper
       Physics quiz
       CSC:
           Lab
           Assignment
           Make flashcards
*/