在spec_helper.rb中打包以下内容：

config.before(:each, type: :system, js: true) do
  desired_capabilities = Selenium::WebDriver::Remote::Capabilities.chrome(
    'chromeOptions' => {
      'prefs' => {
        'download.default_directory' => Rails.root.join('spec/downloads'),
        'download.prompt_for_download' => false,
        'plugins.plugins_disabled' => ["Chrome PDF Viewer"]
      }
    }
  )
  driven_by :selenium, using: :chrome, options: { desired_capabilities: desired_capabilities }
end

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瞧！

最新的Chrome Ruby已经更新到这样

prefs = {
  prompt_for_download: false, 
  default_directory: "/path/to/dir"
}
options = Selenium::WebDriver::Chrome::Options.new
options.add_preference(:download, prefs)
driver = Selenium::WebDriver.for :chrome, options: options

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来源