Viewing the app's code structure

…Before we start writing any code, let's take a look at what…our app's purpose is and how our code is going to be organized.…So first, the app plan.…We want to be able to tap numbers on the…calculator, and see them at the top of the screen.…Then you could tap the Plus or Minus button.…After you tap the Plus or Minus button, you can tap more…numeric buttons, and then press the Equals button to see the result.…In addition, you want to be able to tap the C…button the clear the screen, and reset the calculator's value to 0.…

To accomplish that, we're going to use several instance variables.…We're going to have one named Total that's…an integer, that will hold the numeric values entered.…We'll have one called mode, that will also be an…integer that holds whether the user is adding or subtracting.…We'll have another one that's Boolean, called lastButtonWasMode.…And it stores a true or false value that…stores if the mode button was the last button pressed.…Now the mode button is a plus or minus button.…

Next we'll have a variable called value string that is…