家庭任務平台在紀錄活動時，活動會隨著計畫或是任務的變化而一起被記錄下來，如下圖左邊的部分，紀錄了：

• @ jhao has updated the note of the project: 1 second ago
• @ jhao has created a task '確定行程' : 1 minute ago
• @ jhao has created a project: 2 minutes ago

為了紀錄活動，可以把紀錄不同活動的程式碼寫在Controller的各個對應方法中，例如紀錄更新計畫的活動時，則把該部分的程式碼寫在Controller的update方法中，但是這樣的壞處有：

1. Controller會越來越膨脹。
2. 紀錄活動的程式碼會四散，很難管理。

為了處理上面提到的壞處，可以使用Laravel所提供的模型觀察者（Model Observer），下面是ProjectObserver/TaskObserver的程式碼，可以看到其中提供對應的Model活動，如created等等：

1. ProjectObserver

class ProjectObserver
{
    use ActivityChange;
    /**
     * Handle the project "created" event.
     *
     * @param  \App\Project  $project
     * @return void
     */
    public function created(Project $project)
    {
       
        $project->recordActivity('created');

    }

    /**
     * Handle the project "updated" event.
     *
     * @param  \App\Project  $project
     * @return void
     */
    public function updated(Project $project)
    {
        $changes =$this->getChanges($project);
        $project->recordActivity('updated',$changes);
    }

}

2. TaskObserver

class TaskObserver
{
    use ActivityChange;
    /**
     * Handle the task "created" event.
     *
     * @param  \App\Task  $task
     * @return void
     */
    public function created(Task $task)
    {
        $task->recordActivity('created_task');
        
    }

    /**
     * Handle the task "updated" event.
     *
     * @param  \App\Task  $task
     * @return void
     */
    public function updated(Task $task)
    {
        $changes = $this->getChanges($task);
        if ($this->isCompleted($task)) {
            $task->recordActivity('completed_task', $changes);
        } else if ($this->isUncompleted($task)) {
            $task->recordActivity('uncompleted_task', $changes);
        } else {
            $task->recordActivity('updated_task', $changes);
        }

    }

    /**
     * Handle the task "deleted" event.
     *
     * @param  \App\Task  $task
     * @return void
     */
    public function deleted(Task $task)
    {
        $task->recordActivity('deleted_task');
    }

    protected function isCompleted(Task $task)
    {
        return $task->completed && !$task->getOriginal('completed');
    }
    protected function isUncompleted(Task $task)
    {
        return !$task->completed && $task->getOriginal('completed');
    }

trait ActivityChange
{
    protected function getChanges($recordable){
        $after = Arr::except($recordable->getChanges(),'updated_at');
        $original= $recordable->getOriginal();
        $before = array_intersect_key($original,$after);
        $changes = 
        ['before'=>$before,
        'after'=>$after];
        return $changes;
    }
}

所以我們要來認識一下Laravel所提供的Observor。