Fixed vs Responsive Sizing

App Inventor allows apps to be built with either Fixed or Responsive sizing, controlled by the Sizing parameter of Screen1. This parameter is available only in the Designer, and cannot be changed dynamically.

Fixed Sizing
Apps are built with a fixed width of 320 pixels in portrait mode, or a fixed height of 320 pixels in landscape mode. The longer dimension varies depending on the device; designing for 320x480 pixels works in most cases. The majority of apps in App Inventor tutorials and in the DCU Access courses use Fixed Sizing for consistency.

Responsive Sizing
Apps are built to use the full screen size of the device, which varies considerably across devices. This means that scaling of design elements becomes a factor, especially when apps may be used on both phones and tablets. Read more here. Responsive sizing appears to be necessary for well-behaved video playback.

Note that most Android devices use 'soft buttons' at the bottom of the screen, which reduces the available vertical screen size; this is not the case with the Emulator or ARC Welder. The table below shows the screen size available to App Inventor for a selection of devices using each Sizing option.

Fixed Sizing	Portrait	Landscape
Emulator	320 x 480	480 x 320
ARC Welder, Phone	320 x 569	569 x 320
ARC Welder, Tablet	320 x 512	512 x 320
Fire 7" tablet	320 x 520	546 x 294
Nexus 7 tablet	319 x 482	512 x 294
Responsive Sizing	Portrait	Landscape
Emulator	320 x 480	480 x 320
ARC Welder, Phone	360 x 640	640 x 360
ARC Welder, Tablet	800 x 1280	1280 x 800
Fire 7" tablet	600 x 976	1024 x 552
Nexus 7 tablet	600 x 905	961 x 552

Core 2: Brainstormer Updates

The videos for Core 2: The Brainstormer have been edited to remove 'first takes' as needed. There is no change to the instructional content, so there is no need to revisit videos that students have already viewed. The old versions of the videos are now marked private.

The downloadable AIA file WhoIsQuiz_example.aia has been updated to prevent the app from crashing after the last question is answered. It differs from the code built up in videos 4 and 5 when dealing with user-typed answers, and shown again in video 8 when dealing with the drop-down list, as follows:

• Incrementing the list index CurrentQuestion has been removed from the SubmitButton event handler in the first version of the quiz, and from the AnswerPicker event handler in the second version.
• In the NextButton event handler, the test for the end of the list has been changed to a test for equality, as the change above means the list index is not incremented before it is tested.
• An else clause has been added to the conditional block, in which the list index is incremented and the next image is displayed only if the game is not over.

Before: the index is incremented when the third question is answered correctly, and the NextButton handler then attempts to display the fourth image in a list of three, crashing the app before the check for the end of the game

After: when the Next button is clicked, we check if the game is over; if it's not, the index is incremented and the next image is displayed

It would have been possible to fix the crash without moving the CurrentQuestion increment into the NextButton handler, but the program flow is easier to follow with the solution above. It also means that the AnswerPicker handler (or the SubmitButton handler in the first version) deals only with whether or not the user gave the correct answer, and the NextButton handler deals only with determining how to move the game forward. This is an example of the programming principles of Single Responsibility and Separation of Concerns.

Student Resource Centre

Building on the shortcut URLs for course material, a new Student Resource Centre page has been created, with links to key course material from Google Drive and YouTube:

http://unitydcu.ie/smart-skills-student-resource-centre/

There's nothing on this page that's not already publicly accessible and linked from the lesson plans; the idea is to provide a convenient starting point for students to find the material without them needing access to the lesson plans. It's intended for reference and revision, not as a substitute for students working through the lesson plans under a teacher's guidance. The page can also be reached via the shortcut http://unitydcu.ie/ss-src.

App Library

A library of apps is being assembled on Google Drive at http://unitydcu.ie/ss-app, and will hold sample apps used in the Smart Skills course as well as other apps of interest. For each app, there are four files in the library:

• appname.aia - project source code to import into App Inventor
• appname.apk - compiled app to load into ARC Welder or install on a device
• appname.png - screenshot of code blocks
• appname_QR.png - QR code to download apk file directly onto a tablet or phone

At the time of writing, the first two Smart Skills apps (WhackAMole and WhoIsQuiz) are available, along with apps from the APPlied App Development course for First Years.

App Inventor Coordinate System

For handling moving objects ("sprites"), App Inventor uses an X/Y coordinate system for position on the screen, Heading for direction of movement, and Speed and Interval for velocity.

X and Y measure horizontal and vertical distance from the top left of the screen in units of pixels. By default, the Screen1 Sizing parameter is set to Fixed, which generates apps with a horizontal dimension of 320 pixels in portrait orientation. The vertical dimension varies with the device; designing for 320x480 pixels should work on most devices. Changing the Sizing parameter to Responsive results in apps that use the full screen resolution, which varies considerably from one device to another. The apps in this course use Fixed sizing for predictable behaviour.

The Z value of a sprite determines the stacking order when sprites overlap, with higher numbered sprites rendered in front of lower numbered sprites.

Heading is expressed as an angle between 0 and 359 degrees, with 0 representing movement to the right, and proceeding anti-clockwise from there.

Interval determines the time in milliseconds between updates to a sprite's position, while Speed determines how many pixels the sprite moves per update. Setting the Speed to 5 and the Interval to 10 (i.e. 5 pixels every 10 ms) is a reasonable starting point for a rolling ball with smooth animation.

There are built-in procedures to detect when a sprite reaches the edge of the screen, with the value of Edge set to 1, 3, -1, -3 for top, right, bottom, left respectively. Edge values are also used for the corners, where 2, 4, -2, -4 represent top right, bottom right, bottom left, top left respectively.

This document contains a reference diagram showing the X/Y, Heading and Edge schemes, and would be useful to have to hand when working on arcade style games: App Inventor Coordinate System

Shortened URLs

Short URLs have been created for the Smart Skills video playlists and course materials. These can be shared with students as needed, so that they can access the material without needing to enter unwieldy YouTube or Google Drive URLs. The shortcuts are listed below, and can also be found by hovering over the corresponding links in the Resources section of each lesson page.

Reference Material
http://unitydcu.ie/ss-ytb	Smart Skills YouTube Channel
http://unitydcu.ie/ss-inv	Introductory Videos ("Week 0")
http://unitydcu.ie/ss-ref	Reference Library
http://unitydcu.ie/ss-sta	Starter Apps
http://unitydcu.ie/ss-tst	Testing Your Apps
Project Tools
http://unitydcu.ie/ss-dev	Development Cycle
http://unitydcu.ie/ss-bst	Brainstormer
http://unitydcu.ie/ss-pjm	Project Manager
Core 1: Coding
http://unitydcu.ie/ss-c1v	Video Playlist
http://unitydcu.ie/ss-c1b	Project Brief
http://unitydcu.ie/ss-c1d	Downloads
http://unitydcu.ie/ss-c1n	Project Notes
http://unitydcu.ie/ss-c1c	Extra Challenges

Additional shortcuts will be created as new materials are added to the course.

Supporting documents

Three new documents have been added to the Week 0 - Introduction to Smart Skills resources section:

• Reference Materials links to key reference pages on the App Inventor website
• Starter Apps suggests some small apps that can be built quickly by new users
• Testing Your Apps outlines several test setups, and the pros and cons of each

Two new documents have been added to the Core 1: Coding resources section:

• Project Notes includes additional information to complement the course videos
• Extra Challenges suggests some additional features that students who progress quickly may like to try

If you or your students have built additional features into one of the course apps, we'd love to hear from you: it would be great to showcase apps designed and coded in our partner schools.