Add default_form_class and allow form class overrides

This is especially useful with bootstrap forms where you might want to
switch to a `form-inline` layout even though the default is a
`form-horizontal`.

This deprecates SimpleForm.form_class in favor of
SimpleForm.default_form_class.

e.g.
config.default_form_class = "form-horizontal"
simple_form_for @user, html: { class: "form-inline" }

lib/generators/simple_form/templates/config/initializers/simple_form.rb

@@ -103,8 +103,9 @@ SimpleForm.setup do |config|
   # You can define the class to use on all labels. Default is nil.
   # config.label_class = nil
 
-  # You can define the class to use on all forms. Default is simple_form.
-  # config.form_class = :simple_form
+  # You can define the default class to be used on forms. Can be overriden
+  # with `html: { :class }`. Defaulting to none.
+  # config.default_form_class = nil
 
   # You can define which elements should obtain additional classes
   # config.generate_additional_classes_for = [:wrapper, :label, :input]

lib/simple_form.rb

@@ -96,10 +96,16 @@ See https://github.com/plataformatec/simple_form/pull/997 for more information.
   mattr_accessor :boolean_style
   @@boolean_style = :inline
 
-  # You can define the class to be used on all forms. Default is simple_form.
+  # DEPRECATED: You can define the class to be used on all forms. Default is
+  # simple_form.
   mattr_accessor :form_class
   @@form_class = :simple_form
 
+  # You can define the default class to be used on all forms. Can be overriden
+  # with `html: { :class }`. Defaults to none.
+  mattr_accessor :default_form_class
+  @@default_form_class = nil
+
   # You can define which elements should obtain additional classes.
   mattr_accessor :generate_additional_classes_for
   @@generate_additional_classes_for = [:wrapper, :label, :input]
@@ -249,6 +255,11 @@ See https://github.com/plataformatec/simple_form/pull/997 for more information.
     ActiveSupport::Deprecation.warn "[SIMPLE_FORM] SimpleForm.default_input_size= is deprecated and has no effect", caller
   end
 
+  def self.form_class=(value)
+    ActiveSupport::Deprecation.warn "[SIMPLE_FORM] SimpleForm.form_class= is deprecated and will be removed in 4.x. Use SimpleForm.default_form_class= instead", caller
+    @@form_class = value
+  end
+
   # Default way to setup Simple Form. Run rails generate simple_form:install
   # to create a fresh initializer with all configuration values.
   def self.setup

lib/simple_form/action_view_extensions/form_helper.rb

@@ -16,7 +16,11 @@ module SimpleForm
         unless options[:html].key?(:novalidate)
           options[:html][:novalidate] = !SimpleForm.browser_validations
         end
-        options[:html][:class] = [SimpleForm.form_class, simple_form_css_class(record, options)].compact.join(" ")
+        if options[:html].key?(:class)
+          options[:html][:class] = [SimpleForm.form_class, options[:html][:class]].compact
+        else
+          options[:html][:class] = [SimpleForm.form_class, SimpleForm.default_form_class, simple_form_css_class(record, options)].compact
+        end
 
         with_simple_form_field_error_proc do
           form_for(record, options, &block)

test/action_view_extensions/form_helper_test.rb

@@ -13,6 +13,25 @@ class FormHelperTest < ActionView::TestCase
     assert_select 'form.simple_form'
   end
 
+  test 'SimpleForm allows overriding default form class' do
+    swap SimpleForm, default_form_class: "my_custom_class" do
+      with_concat_form_for :user, html: { class: "override_class" }
+      assert_no_select 'form.my_custom_class'
+      assert_select 'form.override_class'
+    end
+  end
+
+  # Remove this test when SimpleForm.form_class is removed in 4.x
+  test 'SimpleForm allows overriding default form class, but not form class' do
+    ActiveSupport::Deprecation.silence do
+      swap SimpleForm, form_class: "fixed_class", default_form_class: "my_custom_class" do
+        with_concat_form_for :user, html: { class: "override_class" }
+        assert_no_select 'form.my_custom_class'
+        assert_select 'form.fixed_class.override_class'
+      end
+    end
+  end
+
   test 'SimpleForm uses default browser validations by default' do
     with_concat_form_for(:user)
     assert_no_select 'form[novalidate]'

test/form_builder/general_test.rb

@@ -217,10 +217,30 @@ class FormBuilderTest < ActionView::TestCase
   end
 
   # COMMON OPTIONS
+  # Remove this test when SimpleForm.form_class is removed in 4.x
   test 'builder adds chosen form class' do
-    swap SimpleForm, form_class: :my_custom_class do
+    ActiveSupport::Deprecation.silence do
+      swap SimpleForm, form_class: :my_custom_class do
+        with_form_for @user, :name
+        assert_select 'form.my_custom_class'
+      end
+    end
+  end
+
+  # Remove this test when SimpleForm.form_class is removed in 4.x
+  test 'builder adds chosen form class and default form class' do
+    ActiveSupport::Deprecation.silence do
+      swap SimpleForm, form_class: "my_custom_class", default_form_class: "my_default_class" do
+        with_form_for @user, :name
+        assert_select 'form.my_custom_class.my_default_class'
+      end
+    end
+  end
+
+  test 'builder adds default form class' do
+    swap SimpleForm, default_form_class: "default_class" do
       with_form_for @user, :name
-      assert_select 'form.my_custom_class'
+      assert_select 'form.default_class'
     end
   end