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MIT App Inventor: Events and Decisions | ICSE Class 8

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Computer ApplicationsICSE Class 8 · App Development

MIT App Inventor: Components, Events, Properties and Decisions (ICSE Class 8)

Updated 17 October 2026 · Fact-checked

MIT App Inventor is a free, browser-based tool for making Android apps by arranging components in the Designer and snapping blocks together in the Blocks editor. An event such as Button1.Click runs blocks that read or set properties like Label1.Text, and an if-then-else block makes a decision. No typed code is needed.

Clicking the button (an event) runs the blocks that add the two TextBox values and set the Label's Text property.

Illustration drawn for QuestionBuddy.

Understand MIT App Inventor: Components, Events, Properties and Decisions

Two parts of App Inventor.

  • Designer: build the screen. The Palette lists components; drag them into the Viewer; change Properties (text, colour, size) on the right.
  • Blocks editor: build the logic from puzzle-piece blocks.

Useful components. Button, Label (shows text), TextBox (the user types), Image, HorizontalArrangement (items side by side).

Event, property, decision.

  • Event: something that happens, for example Button1.Click, Screen1.Initialize (the app opens), TextBox1.GotFocus.
  • Property: a feature that can be read or changed, for example Label1.Text, Button1.BackgroundColor, Image1.Visible.
  • Decision: the if-then-else block chooses what to do, just like an algorithm decision.

Example: Adder app. Designer: TextBox1 (hint First number), TextBox2 (hint Second number), Button1 (text Add), Label1 for the answer; set the TextBoxes' NumbersOnly property to true. Blocks: when Button1.Click do set Label1.Text to (TextBox1.Text + TextBox2.Text). Tests: 12 and 30 give 42; 2.5 and 4.25 give 6.75.

Example: BMI checker. BMI = weight in kg divided by (height in metres) squared. When ButtonCheck.Click: set bmi to weight / (height x height); if bmi < 18.5 show Underweight; else if bmi < 25 show Normal; else show Overweight. Test: 45 kg, 1.5 m gives 45 / 2.25 = 20, so Normal.

Testing and sharing. Connect a phone with the AI2 Companion app by scanning the QR code, or use the emulator. To share, build an Android app (.apk) from the Build menu.

Good design. Large labelled buttons, readable colours, hints in text boxes and a message when input is blank.

MIT App Inventor: Components, Events, Properties and Decisions: key facts

Two views
Designer: screen and components | Blocks editor: logic
Palette, Viewer and Properties are in the Designer.
Event, property, decision
EventButton1.Click
PropertyLabel1.Text
Decisionif then else
Events start the blocks; properties are read or set.
BMI decision
bmi below 18.5Underweight
bmi below 25Normal
OtherwiseOverweight
45 kg and 1.5 m give 20: Normal.

How to answer MIT App Inventor: Components, Events, Properties and Decisions questions

Use this for building and tracing an App Inventor app.

  1. 1List the components needed and the properties to set in the Designer.
  2. 2Choose the event that starts the action.
  3. 3Write the blocks inside the event in order: calculate, then decide, then set the output property.
  4. 4Trace with two test values, one for each path.
  5. 5Add a message for blank input.

MIT App Inventor: Components, Events, Properties and Decisions in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

A hypothetical tip calculator

Suppose a student builds an app where the user types a bill and taps Calculate to see 10 percent as a tip.

List components and blocks and give the output for a bill of 450.

Try it first, then show the solutionSolution
  1. Components: TextBox1 (bill), Button1 (Calculate), Label1 (result).
  2. Event: when Button1.Click.
  3. Block: set Label1.Text to TextBox1.Text x 10 / 100.
  4. 450 x 10 / 100 = 45.

Answer: The Label shows 45.

Memory hook: Click event, then set the label.

Tricks to remember MIT App Inventor: Components, Events, Properties and Decisions

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • Designer draws, Blocks think

    The Designer builds what you see; Blocks decide what happens.

    Use it when: Two views.

  • E-P-D

    Event starts, Property changes, Decision chooses.

    Use it when: App logic.

MIT App Inventor: Components, Events, Properties and Decisions: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Reacting to a tap · 1 mark · MCQBoard level

In MIT App Inventor, which block starts the action when a user taps a button?
(a) set Label1.Text (b) when Button1.Click do (c) TextBox1.Text (d) Image1.Visible

Try it first, then show the solutionSolution
  1. An event block waits for the tap.
  2. The other options are property blocks.

Answer: (b) when Button1.Click do.

Example 2: Event or property? · 2 marks · Short answerBoard level

Classify each as an event or a property: (i) Button1.Click (ii) Label1.Text (iii) Screen1.Initialize (iv) Image1.Visible. (2 marks)

Try it first, then show the solutionSolution
  1. Events: Button1.Click and Screen1.Initialize (1).
  2. Properties: Label1.Text and Image1.Visible (1).

Answer: Events: (i) and (iii). Properties: (ii) and (iv).

Example 3: Trace the Adder app · 3 marks · Short answerHarder board level

The Adder app sets Label1.Text to TextBox1.Text + TextBox2.Text when Button1 is clicked. What does it show for 12 and 30, and for 2.5 and 4.25? (3 marks)

Try it first, then show the solutionSolution
  1. 12 + 30 = 42 (1).
  2. 2.5 + 4.25 = 6.75 (1).
  3. The event is Button1.Click (1).

Answer: 42 for 12 and 30, and 6.75 for 2.5 and 4.25.

Example 4: Trace the BMI checker · 4 marks · Long answerHarder board level

With the BMI rule below 18.5 Underweight, below 25 Normal, otherwise Overweight, find the result for 45 kg at 1.5 m and for 80 kg at 1.6 m. (4 marks)

Try it first, then show the solutionSolution
  1. 45 / (1.5 x 1.5) = 45 / 2.25 = 20 (1).
  2. 20 is not below 18.5 but is below 25: Normal (1).
  3. 80 / (1.6 x 1.6) = 80 / 2.56 = 31.25 (1).
  4. 31.25 is not below 25: Overweight (1).

Answer: 45 kg at 1.5 m gives BMI 20, Normal. 80 kg at 1.6 m gives BMI 31.25, Overweight.

Example 5: Handling blank input · 4 marks · Long answerHOTS / competency

The Adder app crashes when a TextBox is left blank. Suggest a fix using a decision, and state a test to show it works. (4 marks)

Try it first, then show the solutionSolution
  1. Add an if-then-else inside Button1.Click (1).
  2. Condition: either TextBox is empty (1).
  3. Then set Label1.Text to a message such as Please enter both numbers (1).
  4. Test with one box blank, then with 12 and 30 giving 42 (1).

Answer: Inside Button1.Click use an if block that checks whether either TextBox is empty; if yes, set Label1.Text to "Please enter both numbers", else do the addition. Test with one blank box (message appears) and with 12 and 30 (42).

Where students lose marks in MIT App Inventor: Components, Events, Properties and Decisions

The slips examiners see most often, and what to write instead.

  • Mistake: Confusing event and property

    An event happens; a property is a feature.

    Wrong working

    Label1.Text is an event.

    Right working

    Label1.Text is a property; Button1.Click is an event.

    Do this: Events usually contain Click, Initialize or GotFocus.

    Usually costs: 1 mark

  • Mistake: Forgetting to square the height

    BMI uses height squared.

    Wrong working

    bmi = 45 / 1.5 = 30.

    Right working

    bmi = 45 / (1.5 x 1.5) = 20.

    Do this: Multiply the height by itself first.

    Usually costs: 1 to 2 marks

  • Mistake: Describing blocks in the Designer

    Logic is built in the Blocks editor.

    Wrong working

    Place the if block in the Designer.

    Right working

    Place the if block in the Blocks editor.

    Do this: Designer is for what you see.

    Usually costs: 1 mark

Exam tips

  • Always name the component with the property: Label1.Text, not just Text.
  • Show a trace with real numbers for decisions.
  • Mention the AI2 Companion or emulator for testing.
  • State that App Inventor is free and browser-based when asked about it.

Practice questions on MIT App Inventor: Components, Events, Properties and Decisions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    Which component lets the user type a number?
    (a) Label (b) Button (c) TextBox (d) Image

    Hint

    The user types in it.

    Answer

    (c) TextBox.

  2. Q2 · 2 marks · Short answerBoard level

    Name the two main parts of MIT App Inventor and state what each is used for. (2 marks)

    Hint

    Screen and logic.

    Answer

    The Designer is used to build the screen with components and properties; the Blocks editor is used to program the logic with blocks.

  3. Q3 · 4 marks · Long answerHarder board level

    Describe the components and blocks of an app that doubles a number typed by the user, and give the output for 8.5. (4 marks)

    Hint

    TextBox, Button, Label.

    Answer

    Components: a TextBox for the number, a Button named Double and a Label for the answer. Blocks: when Button1.Click do set Label1.Text to TextBox1.Text x 2. For 8.5 the Label shows 17.

  4. Q4 · 4 marks · Long answerHarder board level

    Write the blocks, in words, of an app that shows Pass if marks typed are 40 or more and Fail otherwise. Trace for 40 and 39. (4 marks)

    Hint

    Use if then else.

    Answer

    When Button1.Click: if TextMarks.Text >= 40 then set LabelResult.Text to Pass else set LabelResult.Text to Fail. For 40 the result is Pass; for 39 it is Fail.

  5. Q5 · 4 marks · Long answerHOTS / competency

    A student uses a bright yellow label on a white background with tiny buttons. Suggest three design improvements and one test to run. (4 marks)

    Hint

    Think of readability and touch.

    Answer

    Use a dark colour on the yellow or white background so the text is readable; make the buttons large and clearly labelled; add hints in the text boxes. Test the app on a real phone through the AI2 Companion to check that a finger can tap each button.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.What is MIT App Inventor?Show answer

    A free, browser-based tool for building Android apps with blocks.

  2. 2.What does the Palette list?Show answer

    The components you can drag into the Viewer.

  3. 3.Which block makes a decision?Show answer

    The if-then-else block.

  4. 4.What does NumbersOnly do?Show answer

    It lets a TextBox accept only numbers.

  5. 5.What file makes an installable app?Show answer

    The Android app (.apk).

  6. 6.How do you test on a phone?Show answer

    With the AI2 Companion app by scanning the QR code.

MIT App Inventor: Components, Events, Properties and Decisions: frequently asked questions

Do I need to type code in App Inventor?

No, you snap together blocks.

Does App Inventor make iPhone apps?

The chapter covers Android apps; use only what your school software supports.

Why use a Label?

It shows text such as the answer.