Understanding an App’s Architecture App Inventor – Ch 14 Understanding an App’s Architecture

App Inventor – Ch14 Apps are composed of: Components Behaviors Event Handlers Types of Events

App Architecture

Components Visible Invisible or Non-visible Buttons, Textboxes, Labels, Images, Canvases Make up the User Interface of the App Invisible or Non-visible Texting, Sound, SMS, LocationSensor, Accelerometer, Timer, TextToSpeech Provide access to built-in technology of the phone

Component Properties All components, visible and invisible, have properties Properties are memory slots for storing information about components Visible: width, height, alignment Non-visible: sound-source, image-source, timer- interval Initially set in Designer, can be modified in Blocks

Behavior Components are usually straightforward; behaviors can be quite complex. Thinks of: An App As a Recipe An App As a Set of Event Handlers App as multiple recipes hooked to events Internal and External Events

App as Recipe (old-school) Start a bank transaction Perform computation that modifies customer’s account Print new balance on screen

App as Set of Event Handlers

Behavior There are several Event Types: User-initiated – button-click Initialization – when app launches, do … Timer – when 20 milliseconds passes, do … Animation event – when two objects collide, do … External event – when text comes in, do …

Behavior – Event Types (visually)

Examples of Events

All Programming Languages share: Sequence – recipe Selection – branching (Ch. 18) if-then-do (all languages incl. AppInventor) other languages: switch, select-case Repetition – repeating blocks foreach, while do (AppInventor & most langs) other languages: for-next

Sequence Default processing in any computer program Work step-wise through set of instructions Example: get up in the morning 1. Respond to alarm 2. Get out of bed 3. Get dressed 4. Make coffee 5. Get paper 6. Pour coffee 6. Drink coffee/read paper

Selection Apps can ask questions

Behavior – if-then-do Event Handlers can ask questions Is a certain condition true? Yes, branch one way No, branch another way Conditionals can be more complex than yes/no If-then-do in App Inventor Conditionals handled in detail in Ch 18

Selection Can choose alternate paths through instructions Example: Eat breakfast Check for milk in fridge, cereal in cabinet If both are there: Get bowl and spoon Pour cereal in bowl, pour milk in Eat cereal Brush teeth What are the two possible paths through these instructions?

Example of If-Then Block

Repetition Can choose to repeat instructions Example: Brushing teeth Apply toothpaste to toothbrush. Brush up and down 10 times on outside of teeth on left side of mouth Repeat Step 2 on outside of teeth on right side of mouth Repeat Step 2 on outside of teeth in front of mouth Repeat Steps 2-4 on inside of teeth In terms of how the human mouth is constructed, what’s wrong with Step 5?

Behavior – foreach & while do Event handlers can repeat blocks (iteration) foreach in App Inventor while do in App Inventor Ch 20 discusses iteration in detail

Example of Repeated Block

Behavior – variables & storage Event handlers can remember things Temporary memory (RAM): variables Persistent data (Flash drive or HD): database Variables are the subject matter of Ch 16 Data storage is detailed in Ch 22

Behavior – web apps Event handlers can talk to the web Web Service APIs – example: App Inventor can communicate with Twitter accounts Mashups: apps that can talk to more than one web service Web-enabled apps: subject matter of Ch 24

Summary As app creator, you view apps from the end-user’s perspective AND also the designer’s perspective Use Designer first to create UI . . . Components – visible Components – non-visible . . . Then program behaviors in Blocks Event Handlers – when-do Types of Events – user-initiated, initialization, timer, animation, external Characteristics – selection, repetition, store in memory, database storage, talk to web Test and deploy your app