Friday, 11 March 2016

Android Development for Everyday People - Diving In Part II

Welcome to a new week. Last week, we created the user interface of the Three Number Average application. By now, I expect that you have installed BlueStacks based on the instructions given on this blog or that you have followed the instructions on the App Inventor website and configured to run your mobile device to run applications.

This week we will be looking how to add events to our mobile application. This will ensure that when we click the Compute Average button, the button computes the average of the 3 numbers and displays the result in our 5th label.

Last week, we gave two conditions that had to be met in order for our application to work properly. For the sake of review, they are:
  1. The textboxes should only allow numbers as input
  2. The program should only compute the average when all the 3 textboxes are filled
We have ensured the first condition by ensuring that our textboxes only takes numbers in. To do this we used the NumbersOnly property for a textbox in App Inventor.

To meet the second condition, we will have to check that the values in each textbox is not empty. To do this, we first have to learn about variables, procedures and functions.

Variables, Procedures and Functions

If you were a librarian and you had a book in a library and you were searching for a book in the library, the easiest way would be to check the library reference. All things being equal, a check at the library reference would reveal the location of your book. Once you have the location of the book, you can then go to its location to retrieve it.

From the above example, the book you want to retrieve is a variable. The library reference is a function and you are a procedure. In computer science terms, a variable is a store of value, a procedure carries out an action and a function returns a result.

Once you understand how to think in this manner, you are well on your way to becoming a programmer. Getting this concept is the hardest step in programming. However, once you do, you can learn how to program in any language of your choice.

Enough talk. Let’s get started. Open your browser and visit the App Inventor website (Put link). You will see a list of projects. My projects list is shown below:

Click on the ThreeNumberAverage on your list. If you haven’t created any other project, you should have only 1 project listed. However, if you haven’t created any other project, I must state that I am disappointed. I told you to create the user interface for your own project. The best way to learn is by doing.

Once you click on the project, it loads up in the browser window. My screen is shown below:

To calculate the average of the three numbers, we have to get the values from their respective text boxes. To do this, we will need to access the Text property for each Textbox. This is shown below:

We will need four variables to calculate the average of the three numbers. Remember what I said about naming, our variable names must make sense to us. Remember that programming isn’t just giving instructions, it is communication.

Our variable names are:
  1. firstNumber
  2. secondNumber
  3. thirdNumber
  4. average
We need just 1 procedure to display the average in the 5th label. However we will need 2 functions. Our first function called checkInput will check the textboxes to ensure that all of them have a number. Since it is a function, it will return either true of false. Once it is true, we will calculate the average otherwise, we will clear the textboxes and notify the user of the need to fill all the textboxes.

Programming Events

The events in App Inventor are found in by clicking Blocks in our menu. This is shown below:

Right now our Designer window is active. Clicking on Blocks takes us to our Blocks menu which is shown below:

The Blocks Viewer is empty. This is because we haven’t done any programming. Do note the dustbin at the corner. It is used to discard blocks we are no longer using. On the left, you should see a list of all your components. Look through this list carefully. You will realize that this is the same list we saw in the designer view.

To create a variable, go to Blocks. Scroll up until you see the Built-in menu. It is shown below:

Under Built-in, you see a list of stuff you can do. But where are the functions? In classical computer science, procedures are different from functions. However, App Inventor considers them the same. They only vary in usage.

So it begins. In App Inventor, all our programming is drag and drop. To accomplish this, click on the Variables link. It will bring up a pop out as shown below:

Now drag out the initialize global block to the white space. Your Blocks area should now look like this:

Next go to Math under the Built-in menu and click on it. You should see a pop out as shown below:

Drag out the empty block and snap it to the initialize global name block. Drag out the block with 0 and snap it to the initialize global name block.

You Block area should be as shown below:

Now click inside the area with name and change the name to firstNumber. Your Block area will be as shown below:

Do this for your other variables. Your Block area should be as shown below:

Now we can start. I won’t go over this again. Just use matching colours to follow along and ask me any questions if you need help. I can’t promise an immediate response as I work a full time job but I promise to answer as soon as I am able.

To handle our events, click on Button1, you will see a pop out as shown below:

Drag out the when block into your code area. This block is called an event listener. Event listeners respond to an action. The action we want is that when Button1 is clicked, the checkInput function should first check if the text areas are empty. This function acts like a sentinel and protects our program.

Creating the checkInput Function

Our first function works by checking if the contents of the three textboxes have been filled. If they have been filled, we simply need to go ahead to calculate the average. Otherwise, we have to clear the text boxes and display a message to the user.

The code that does this is shown below:

To create this code, you need to use blocks from the Procedures, Control, Logic and Text under the Built-in drawer.

I won’t be showing you how to do this. Did you think this teacher would let you off so easy. Trust me on this. It isn’t hard to do. However, if you have any problems, Google “how to create a procedure in App Inventor”. This course is meant to teach you how to think like a programmer and part of that thought process is the ability to research. If you find it too hard to do, let me hear from you in the comments.


Debugging is the act of getting our programs to work as expected. Whenever we create programs, it is rare to have them work exactly as we want them to work. To this end, we need a way to understand what is going on in our program.

In App Inventor, we debug by using the Notifier component. It is found under the user interface drawer in App Inventor. Add it to your Screen. Since it is a non visible component, it will not show on the screen.
Switch to the block area and drag click on your Notifier object. Next drag out the ShowAlert block for the Notifier object.

This is shown below:

Note the pouch near notice. It is where we will place the call to the checkInput function in this pouch.
Click on the Procedures drawer. Now you will see the call checkInput block.

Drag it out and place it on the pouch of the ShowAlert block. You code will be as shown below:

To call this block, click on Button1 and drag out its Click event listener. An event listener waits for an event to occur. Drag it and place the completed Notifier block inside it. Your code should be as shown below:

Build your app in App Inventor and test it on BlueStacks. Your application should look as shown below:

Note that the textboxes have their hints on them thus showing that they are empty. When you click the Compute Average button, you should see a popup saying false. When you fill just 1 of the textboxes, the false still pops up. However, once you fill all 3 textboxes and click on the Compute Average button.

As always, if you need help, fill them in the comments and I will get back to you when I can. But search before you ask. That is what I would do as a programmer.

Creating the calculateAverage Function

This function takes the values entered by the user and returns the average. The average is the sum of the numbers divided by 3.

The algorithm for this function is shown below:
  1. Assign the values in the textboxes to their respective variables
  2. Add the variables together and divide them by 3
  3. Assign the value obtained from 2 above to the sum variable

When you drag out the procedure block from Procedure, to add parameters to it, you need to click on the blue notch on the function body it will bring out the following:

Now drag the input x block into the inputs block. Doing this will add your first parameter. This is shown below:

Do this 3 times to and rename your parameters to firstNumber, secondNumber and thirdNumber. Also rename your procedure to calculateAverage. Your function should look like below:

Now create the part that will compute our average. Drag out the get block from the Variables drawer and select firstNumber. Do this for secondNumber and thirdNumber.

Next go to the Maths drawer and drag out the addition block. Click on its notch. It will give you this diagram.

The code for the entire function is shown below:

Now add a 3 parameter and place your variables inside them. Now your function should look like this:

Next in Maths, find a division block. Place this block into your result and let the above block be on its left and the number 3 on its right. Your final function is shown below:

As always, let me have your questions in the comments area. The 3 was placed by using the number block found in the Math drawer.

Your function is complete.

Creating the displayAverage Procedure

A procedure carries out an action. Our procedure takes the average as a parameter and displays the result in the 5th label. This procedure is easy so I will just show the code.

Putting it all together

Now that we have completed our functions and procedure, we can create the code for our button. This is shown below:

Build your application. Now you have successfully created your first application using App Inventor.

Adding an Application Icon

To customize your application, you need to add an icon. To do this, go Designer view and click on Screen1.
Browse down to Icon. This is shown below:

When you click inside it, you get a popup as shown below:

Click on the Upload File button to choose an icon of your choice. I will use my company logo. So I will navigate to it and load it up. This is shown below:

Click on the OK button. This will now load our icon. When we now build our app in BlueStacks or our mobile device, our app icon should have changed. Try it out yourself.


Congratulations on making it to the end of the second section of this series. I would be lying to you if I said it wasn’t long. Take heart in the fact that I wrote it.

I took my time with this section because I wanted to be sure that we went together. At the end of this section, I expect that the Three Number Average is running on your setup. Whether you are using BlueStacks or your mobile device.

What a long post. I now assume you know what to do with App Inventor. If you need help, leave a comment or do a Google Search. Have a great weekend.


  1. Hello,
    Cant find call checkpoint block on app inventor, Somebody help me out.

  2. In my own opinion, the Call CheckPoint Block does not exist in App Inventor.

    What exactly do you think it does. Have you checked online if such a block exists?

    1. I was following the ThreeNumberAverage tutorial and the tutorial says i should click and drag the call checkpoint block but all i saw were to procedure do...,to procedure result... and call procedure.I have been stuck here for over 24hrs. Pls what's the way out?Thanks.

    2. Really appreciate your great works, pls kindly drop your email or whatsapp. Mine my whatsapp, 080 3922 4004.Thanks