We have built a tip calculator. We have a UlTextField where the user can input an amount We have a UIButton, which the user can press to calculate the tip We have a UlLabel, which displays the result of our calculation We have a UIViewController named TipCalculatorViewController and a model named TipCalculation Model 's The 's responsibility is to contain the amount" TextField, the "output" label and the "calculate" button. When the user enters the amount and presses the button, it is the responsibility to recognize this user interaction, and notify the , using the communication pattern The 's job is now to coordinate between the It converts the user input from a String to a Double and calls a method "calculateTip" on the . This direction of communication is + The 's responsibilities are to keep track of the tip amount, and to perform the The 's responsibilities are to keep track of the tip amount, and to perform the calculation. Once we have the result, it is now the 's responsibility to pass the result of our calculation from the to the . This direction of communication is The 's responsibility is to show the result to the user, using the UiLabel View Model Controller M We have built a tip calculator We have a UlTextField where the user can input an amount We have a UIButton, which the user can press to calculate the tip We have a Ullabel, which displays the result of our calculation We have a UIViewController named TipCalculatorViewController and a model named TipCalculation Model The View 's responsibility is to contain the amount TextField, the output label and the calculate button. When the user enters the amount and presses the button, it is the Controller: 's responsibility to recognize this user interaction, and notify the Model using the unrestricted communication pattern Callbacks The Controller's job is now to coordinate between the Controller and the Model It converts the user input from a String to a Double and calls a method calculateTip" on the View This direction of communication is restricted The Model : responsibilities are to keep track of the tip amount, and to perform the calculation. Once we have the result, it is now the Controller responsibility to pass the result of our calculation from the Model communication is restricted . + to the View This direction of The Vow . 's responsibility is to show the result to the user, using the Vilabel