Using Lists in App Inventor

A frequent search topic on this blog is how to use lists. “Lists” are a way of storing a collection of data in App Inventor programs. Many other programming languages provide a number of methods for storing collections of data including arrays, records, even lists, and even something called “collections” (imagine that!)

App Inventor provides Lists, plus you can also use components like the TinyDB.

For a detailed description of using Lists, please refer to Chapter 8 – Introduction to Lists in my e-book “App Inventor 2: Tutorial”, available for $2.99 from Amazon. I hope to make this text available in additional e-reader formats during November.

Using Images and the Camera in App Inventor

In an earlier post, I mentioned that ImageSprites use images – and suggested I do a tutorial on images so here is a first look at using images in App Inventor 2.

For this tutorial, we create a very simple app that uses the camera to take a picture, or we can fetch a selected photo from the phone’s photo gallery. Either way, the image that is obtained is displayed on the screen.

First, create a simple user interface. To set this up, I used the VerticalArrangement layout and then added a button, an ImagePicker and an Image on to the Viewer:

Image1-UserInterface

The Image component is located in the User Interface section of the Palette, in the Designer.

The ImagePicker is located in the Media section of the Palette. Drag each to the Viewer window.

You also need to drag a Camera component to the Viewer; the Camera is also located in the Media section. Note that the Camera is an invisible component and will appear below your app’s user interface screen, as shown above.

After renaming the components, we should have a set of components like this:

Image1-Components

For the Image1 component, the Properties look like:

Image1-Properties1

Note that the text box field named Picture has the name of a .jpg image file entered into, in this example. Sorry for the ugly gibberish filename – I just grabbed a photo off of my own Flickr page, saved to my hard drive and then uploaded to App Inventor.

To upload a photo from your computer to the Image1 control, click on the Picture field – this displays a prompt:

Image1-Properties2

The prompt shows the full name of the image I have already uploaded, and this file is already highlighted. Or, click on Upload File … at bottom to select and upload your own .jpg image file.

This uploaded .jpg becomes the default image initially displayed in the program. If you want, you could leave this blank but having something to show on screen makes the program a bit more interesting and helps the user to see where photos will be displayed.

The Blocks

Taking a photo is absurdly easy. When the button for taking a photo (btnTakePhoto) is clicked on (or rather, pressed with your finger), the Click event is raised – and all this event handled need do is call the camera’s TakePicture method:

Image1-TakingPhoto

Once the camera completes taking the picture, the AfterPicture event occurs. This is where we fetch the image (hover your mouse over the pink “image” within the AfterPicture block and select “get image”). Assign the image that was just taken (the get image) to the Picture property of the Image1 control. This copies the photo that was just taken, and displays it on screen.

We could stop here, but let’s take a look at grabbing a picture from the photo gallery on the phone. To implement this, we use ImagePicker. The ImagePicker is a control that displays the photo gallery, let’s you select an image from the photo gallery, and then returns that selected image to the app.

The ImagePicker displayed in the Designer is effectively a button that contains its own event handler for the Click event. Pressing the ImagePicker button on the main screen automatically enters the gallery where you select your photo. When the selection is complete, the ImagePicker’s AfterPicking event is raised and we add a tiny bit of code to process the image, merely copying the “Selection” property of the ImagePicker control (renamed here as impgSelectPhoto) to the Image control on screen to display the selected photo.

Image1-ImagePickerAfter

Running the App

When activating the camera, take a photo and wait a moment – you should see a “checkmark” at the bottom of the screen after the photo is processed.  Tap the checkmark icon to signify you are done and you should be returned to the app and the photo you took should be displayed.

When selecting a photo from the gallery, wait for the gallery to appear, scroll through and select a photo. Tap the photo and wait for a moment – and control should return to the app and the selected photo will appear on the screen.

Note – if you have more than one photo gallery app registered on your phone (I have 3 such apps), Android will prompt you as to which of the gallery apps you wish to use.

There are additional features that are not shown here. For example, if your phone has a front facing and back facing camera, its possible to switch between the cameras.

SOURCE CODE

Source code is available in the MIT App Inventor Gallery –> here

MIT App Inventor Source Code

This is for advanced software developers only: MIT App Inventor Sources. That’s the actual source code that makes App Inventor work behind the scenes – from the browser based editor to the server side support to the AI Companion. It’s all there.

MIT has made the source code for App Inventor publicly available. At a minimum, you need to be proficient in Java, Javascript, and have familiarity with various web techniques, such as JSON. Exploring this source code is not for the neophyte.

With access to the source code it is possible to host App Inventor on your own server and it is possible to develop your own App Inventor components (this also requires making a custom version of AI Companion).

Again, this is for advanced software developers.

How to create a bouncing ball animation in App Inventor 2

App Inventor 2 provides easy to use features for creating games. These features have proven popular for introducing programming concepts to students in K-12, especially at the elementary and middle school level. While our focus is oriented towards practical applications rather than games, we can have a little fun too!

The purpose of the sample app in this blog post is to illustrate the basic programming concepts of a bouncing ball animation.

The “Game”

Well, its not much of a game! To keep the programming simple, all this does is enable a ball to bounce around on the screen, to change its heading or trajectory, and to make it travel faster or slower. When the ball hits the edge of the screen, it rebounds in an appropriate direction. Here’s the user interface as seen on the smart phone:

Screenshot_2014-10-13-11-40-46

Press the Start button to make the ball start moving; press Stop to end the movement.

To change the speed of movement press the Faster or Slower buttons. Pretty easy?

To change the trajectory, you may enter a heading value in degrees, in the field next to the Set heading button, and then press the Set heading button.

Continue reading How to create a bouncing ball animation in App Inventor 2

Using ListPicker for displaying and selecting from on screen lists

What is ListPicker?

ListPicker is a user interface component that makes it easy to display a list of items and have the user make a selection from the list. For example, let’s say we’d like to display six things on the screen and then have the user select an item from the list.

For simplicity, here is a sample ListPicker list showing six items labeled “A”, “B”, “C”, “D”, “E” and “F”.  Those items are arbitrary – your list could contain “Oranges”, “Bananas”, “Apples”, “Grapes” or whatever other text descriptions you would like to have. (The default color is white text on black but the color properties, as well as the text size, may be changed in the App Inventor Designer.)

Screenshot_2014-10-10-12-03-13

 

When this screen is displayed the user may touch an item in the list to select it.

Use ListPicker whenever you have a set of items from which the user is expected to make a selection. Example applications could include selecting a product from a list of inventory items, select a food item from a menu, select a name from a display of names and addresses and many more.

Learning how to use ListPicker is helpful for new App Inventor programmers as many online tutorials and example programs use ListPicker – but without explaining what it is or how to use it!  Becoming familiar with ListPicker will make those other tutorials easier to understand!

Example

When an item is selected an event is created, and your event handler can then identify which item was selected in this list.  To illustrate, let’s start with a simple app that displays a list of items when the “Click me to display a short list” is pressed:

Screenshot_2014-10-10-12-03-24

ListPicker does all the hard work for us – display the items on screen (as shown in the previous screen snap shot) and processing the selected element.

In the example above, “C” was selected.

How to Build the ListPicker Demonstration Application

Step 1 is to drag a ListPicker component from the Palette to the Viewer. Rename the control if you wish and change the text of the ListPicker control button to something useful 🙂 Here is the app after dragging the ListPicker component and changing the text (in the ListPicker properties) to “Click me to display a short list”:

LP-Designer

Step 2 is to place some items into the list. We do that by typing a comma separated list in the ElementsFromString property of ListPicker, as shown here:

LP-Properties

 

We have now added six items – A, B, C, D, E and F. We could enter longer text descriptions if we wished – for example “Oranges, Bananas,Apples” and so on:

LP-Properties2

Step 3 is to add an event handler for the list to process the list selection – this turns out to be really super hard – not! It is actually super easy! Click on the ListPicker component in the Blocks Editor and then drag the .AfterPicking event into your program:

LP-EventBlockListPicker handles all the details of the on screen selection. All that is needed is to fetch the item that was selected, store that in a variable, or, as in this example, display the selection in a label field on the app’s screen.

That’s it for the basic operation of the ListPicker. If you have many elements – too many to fit on one screen – the list automatically handles scrolling. Just use your finger to drag up or down the list. You can also change ListPicker properties such as the color of the list and more.

Of course there are many ListPicker features, but this is intended as a brief introduction.

But one feature we wish to mention is that you can add items to the list to display from within your program by putting your items in a list and assigning them as shown here:

LP-BlocksAddingThe ListPicker1.Elements property contains a programmatic list of items.  If you are not familiar with lists, please see Chapter 8 “Introduction to Lists” in my e-book “Guide to App Inventor 2:Tutorial – The fast and easy way to create Android apps”.

Using the WebViewer Control in App Inventor

Would it be neat if you could display pages from the web in your Android app? Well, you can!

And it is very easy to do in App Inventor.

In the Designer, drop a WebViewer control on to your app’s screen design area. The WebViewer control is located at the bottom of the user interface section of the Palette:

WebViewerControl

After dropping the control on to your user interface, select the control and set the Properties. In particular, set a default or initial web URL to display on the screen. Here, I have set the default URL to http://appinventor.pevest.com. Be sure to include the leading http://

WebViewerProperties

 

When your app runs, the WebViewer will automatically load the page at the HomeURL.

You can change this page, programmatically. Here, for example, in response to a button press, the URL is changed to my 3D blog.

WebChangeURL

Once this runs, the new page is read in to the viewer. Just like this!

Screenshot_2014-09-08-16-39-22

App Inventor is the quick and easy way to write Android Apps

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