Add @import UIKit; or @import Cocoa; for generated main.m

lib/cocoapods/validator.rb

@@ -421,7 +421,12 @@ module Pod
                                ''
                              end
         end
-        source_file.open('w') { |f| f << "@import Foundation;\n#{import_statement}int main() {}\n" }
+        source_file.open('w') do |f|
+          f << "@import Foundation;\n"
+          f << "@import UIKit;\n" if consumer.platform_name == :ios || consumer.platform_name == :tvos
+          f << "@import Cocoa;\n" if consumer.platform_name == :osx
+          f << "#{import_statement}int main() {}\n"
+        end
       end
       source_file
     end

spec/unit/validator_spec.rb

@@ -524,6 +524,7 @@ module Pod
               file.basename.to_s.should == 'main.m'
               file.read.should == <<-OBJC.strip_heredoc
                 @import Foundation;
+                @import UIKit;
                 @import ModuleName;
                 int main() {}
               OBJC
@@ -547,6 +548,7 @@ module Pod
               file.basename.to_s.should == 'main.m'
               file.read.should == <<-OBJC.strip_heredoc
                 @import Foundation;
+                @import UIKit;
                 #import <ModuleName/ModuleName.h>
                 int main() {}
               OBJC
@@ -559,6 +561,7 @@ module Pod
               file.basename.to_s.should == 'main.m'
               file.read.should == <<-OBJC.strip_heredoc
                 @import Foundation;
+                @import UIKit;
                 int main() {}
               OBJC
             end