Add a basic example

Author: Intrepidd

examples/basic.md

@@ -0,0 +1,85 @@
+# Basic form example
+
+## Form
+```ruby
+class ContactForm < ApplicationForm
+  proxy_for Contact, :@contact
+
+  attribute :first_name
+  attribute :last_name
+  attribute :birth_date, :date
+
+  validates :first_name, presence: true
+  validates :last_name, presence: true
+  validates :birth_date, presence: true
+
+
+  def setup(contact)
+    @contact = contact
+    self.first_name = @contact.first_name
+    self.last_name = @contact.last_name
+    self.birth_date = @contact.birth_date
+    ## You could also do :
+    # self.attributes = @contact.attributes.slice(:first_name, :last_name, birth_date)
+  end
+
+  def perform
+    @contact.update!(
+      first_name:,
+      last_name:,
+      birth_date:
+    )
+  end
+end
+```
+
+## Controller
+```ruby
+class ContactsController
+  def new
+    @form = ContactForm.new(Contact.new)
+  end
+
+  def create
+    @form = ContactForm.new(Contact.new)
+
+    if @form.submit(params[:contact])
+      redirect_to root_path, notice: "Contact created"
+    else
+      render :new, status: :unprocessable_entity
+    end
+  end
+
+  def edit
+    @contact = Contact.find(params[:id])
+    @form = ContactForm.new(@contact)
+  end
+
+  def update
+    @contact = Contact.find(params[:id])
+    @form = ContactForm.new(@contact)
+
+    if @form.submit(params[:contact])
+      redirect_to root_path, notice: "Contact updated"
+    else
+      render :edit, status: :unprocessable_entity
+    end
+  end
+end
+```
+
+## Views
+### new.html.erb and edit.html.erb
+```erb
+<%= render "form", form: @form %>
+```
+
+### _form.html.erb
+```erb
+<%= form_with(model: form) do |f| %>
+  <%= f.text_field :first_name, placeholder: "First name" %>
+  <%= f.text_field :last_name, placeholder: "Last name" %>
+  <%= f.date_field :birth_date %>
+  <%= f.submit "Save" %>
+<% end %>
+```