Mobile apps with MIT App Inventor

Key points

• An app has a visible side (the screen) and a behaviour side (what happens on events).
• App Inventor has a Designer view for the screen and a Blocks view for the logic.
• Components have properties that can be set in the Designer or changed by blocks.
• The Companion app shows your app live on a phone; the emulator is a pretend phone on the computer.

Key points

• Talk To Me uses a Button, a Label and an invisible TextToSpeech component.
• when Button.Click is an event handler that runs when the button is tapped.
• call TextToSpeech.Speak needs a message input, which can be fixed text or TextBox.Text.
• Properties can be read and set with blocks while the app runs.

Key points

• Horizontal, Vertical and Table arrangements control where components sit.
• Percent and Fill parent sizes adapt to different phone sizes.
• Upload small images and set a component's Picture or Image property.
• Extra screens are added with Add Screen and opened with the open another screen block.

Key points

• All logic sits inside event handlers; the app waits for events to happen.
• Global variables are created with initialize global and used with get and set.
• Math blocks nest, so (bill × percent) / 100 works as one expression.
• Use NumbersOnly, starting values and rounding to keep calculations tidy.

Key points

• if then else runs different blocks depending on a true or false condition.
• Branches of else if are tested in order and the first true one wins.
• A procedure is a named, reusable group of blocks; result procedures give back a value.
• Screen1.Initialize runs once when the screen opens, a good place to set things up.

Key points

• A list stores many values in order and positions start at 1.
• add items to list, select list item, remove list item and length of list are the core blocks.
• A ListPicker shows a list to choose from; AfterPicking gives the Selection.
• After changing a list, set the ListView Elements again so the screen updates.

Key points

• The Canvas has (0, 0) at the top-left and y grows downwards.
• Dragged gives previous and current positions, which is how a paint app draws lines.
• ImageSprite and Ball live inside a Canvas and have X, Y, Heading and Speed.
• A Clock's Timer event can move a sprite at regular intervals to make a game.

Key points

• The AccelerometerSensor gives Shaking and AccelerationChanged events for tilt and shake apps.
• The LocationSensor gives latitude and longitude; location is private and needs permission.
• The Clock fires a Timer event every TimerInterval milliseconds and also reads date and time.
• Turn sensors off when not needed to save battery.

Key points

• Variables are lost when the app closes; TinyDB keeps values on the phone.
• TinyDB stores tag and value pairs with StoreValue and GetValue (with a default).
• Load data in Screen1.Initialize and save after every change.
• The Sharing component lets the user send text or files through apps they choose.

Key points

• Error pop-ups, the warnings counter, Do It and the Notifier help find bugs.
• Test with empty inputs, rotation, long text, no internet and other people's phones.
• Build an APK or AAB from the Build menu after setting the icon, name and version.
• Ask only for permissions you need, respect privacy and credit any material you use.