add before_install and before_uninstall

lib/cask/artifact.rb

@@ -7,6 +7,7 @@ module Cask::Artifact; end
 require 'cask/artifact/app'
 require 'cask/artifact/binary'
 require 'cask/artifact/after_block'
+require 'cask/artifact/before_block'
 require 'cask/artifact/colorpicker'
 require 'cask/artifact/font'
 require 'cask/artifact/nested_container'
@@ -27,6 +28,7 @@ module Cask::Artifact
   #
   def self.artifacts
     [
+      Cask::Artifact::BeforeBlock,
       Cask::Artifact::NestedContainer,
       Cask::Artifact::App,
       Cask::Artifact::Colorpicker,

lib/cask/artifact/before_block.rb

@@ -0,0 +1,14 @@
+class Cask::Artifact::BeforeBlock < Cask::Artifact::Base
+  def self.me?(cask)
+    cask.artifacts[:before_install].any? ||
+      cask.artifacts[:before_uninstall].any?
+  end
+
+  def install
+    @cask.artifacts[:before_install].each { |block| @cask.instance_eval &block }
+  end
+
+  def uninstall
+    @cask.artifacts[:before_uninstall].each { |block| @cask.instance_eval &block }
+  end
+end

lib/cask/dsl.rb

@@ -89,7 +89,9 @@ def caveats(*string, &block)
 
     ARTIFACT_BLOCK_TYPES = [
       :after_install,
-      :after_uninstall
+      :after_uninstall,
+      :before_install,
+      :before_uninstall,
     ]
 
     ARTIFACT_BLOCK_TYPES.each do |type|

test/cask/artifact/before_block_test.rb

@@ -0,0 +1,45 @@
+require 'test_helper'
+
+describe Cask::Artifact::BeforeBlock do
+  describe 'install' do
+    it 'calls the specified block before installing' do
+      called      = false
+      yielded_arg = nil
+
+      CaskWithBeforeInstall = Class.new(Cask)
+      CaskWithBeforeInstall.class_eval do
+        before_install do |c|
+          called = true
+          yielded_arg = c
+        end
+      end
+
+      cask = CaskWithBeforeInstall.new
+      Cask::Artifact::BeforeBlock.new(cask).install
+
+      called.must_equal true
+      yielded_arg.must_equal cask
+    end
+  end
+
+  describe 'uninstall' do
+    it 'calls the specified block before uninstalling, passing the cask' do
+      called      = false
+      yielded_arg = nil
+
+      CaskWithBeforeUninstall = Class.new(Cask)
+      CaskWithBeforeUninstall.class_eval do
+        before_uninstall do |c|
+          called = true
+          yielded_arg = c
+        end
+      end
+
+      cask = CaskWithBeforeUninstall.new
+      Cask::Artifact::BeforeBlock.new(cask).uninstall
+
+      called.must_equal true
+      yielded_arg.must_equal cask
+    end
+  end
+end