Better isolation of controllers testing without using instance variables and checking assigns in specs

application_controller.rb

class ApplicationController < ActionController::Base
  private

  def on_error_render(action, assigns = nil)
    yield
  rescue StandardError
    render action, assigns:, status: :internal_server_error
  end
end

home_controller.rb

class HomeController < ApplicationController
  def new
    render :new, assigns: { user: User.new }
  end

  def create
    user = User.new(name: params[:user][:name])

    on_error_render(:new, user: user) do
      user.save!

      render :new, assigns: { user: user }
    end
  end
end

home_controller_spec.rb

require 'rails_helper'

describe HomeController, type: :controller do
  describe "GET new" do
    it 'renders the new view' do
      @user = double("user", name: "Foo")
        
      expect(User).to receive(:new).with(no_args).and_return(@user)
      expect(controller).to render_view_with(:new, assigns: {user: @user})

      get :new
    end
  end

  describe "POST create" do
    it 'renders creates a new user and renderes the form' do
      @user = double("user", name: "Foo")

      expect(User).to receive(:new).with(name: "Foo").and_return(@user)
      expect(@user).to receive(:save!)

      expect(controller).to handle_error_with(:new, user: @user)
      expect(controller).to render_view_with(:new, assigns: { user: @user })

      post :create, params: {user: {name: "Foo"}}
    end
  end
end

new.html.erb

<%= tag.mark(flash[:success]) if flash[:success] %>

<%= form_for @user, url: {action: "create"} do |f| %>
  <%= f.text_field :name, placeholder: "what's your name" %>

  <%= tag.mark(@user.errors.full_messages.join) if @user.errors.any? %>

  <div><%= f.submit "Create" %></div>
<% end %>

rails_helper.rb

RSpec.configure do |c|
  c.include Module.new {
    def render_view_with(*args)
      receive(:render).with(*args).and_call_original
    end

    def handle_error_with(*args)
      receive(:on_error_render).with(*args).and_call_original
    end
  }, type: :controller
end