Fixed iOS Simulator trouble from Xcode 3.X to 4.X

!!Notice do not use "root" login

#Xcode 3 ver in Xcode 4 project modify
cd into the PROJECT_NAME.xcodeproj directory
delete all previous [LoginUserName] files and directories
Reopen Xcode4 and build

#solve simulator-> error: failed to attach to process ID 0
Basically it means Xcode is not able to attach a build of your app to the simulator.
To solve this problem, simply do the following:

1.Click on Simulator and reset the simulator by going to the > OS Simulator > main menu choose > Reset Content and Settings

2."Product" > "Edit Scheme". On the left menu panel, click on "Run Project_Name.app".
The info display on the right should have the "Debug Process As" radio button picked as "Me (user-name)".
If not, make sure you do not login as "root".

3.Xcode, go to the menu "Window" > "Organizer". On the top menu, click on "Projects". Go ahead and delete "Derived Data".

4.delete "~/Library/Application Support/iPhone Simulator/6.0/Applications"