A Single Thread of Execution in App Inventor Screens

The development documentation provides a very good example of why certain things in the User Interface (UI) of an app happen in the way they do. In App Inventor we use Screens to lay out components to create the UI of the app.

The main idea is that all actions that affect the UI happen sequentially. Some examples of these kind of actions could be changing the text of a label, or handling users interacting with the app via a button or a text box.

Sequential execution in App Inventor means that different blocks can never interrupt each other.

Different blocks and handlers will always be executed in sequence. Once a block starts executing, it will execute till the end, no matter if there are other blocks reacting to the one in progress, or users pressing buttons in the app. On more technical terms, we say that everything in the UI is executed in one Thread, the UI Thread, which is responsible for all updates and interaction that happen in each Screen of your app. For this to happen, we need some kind of mechanism to keep track of what needs to be done next, and when something should be done. A very simple model would be a Queue, just like the one you join at the supermarket to pay for your shopping. Tasks can be added to a Queue in the same way that people join in the queue from the end of it.

With all this in mind, let's see the example in the documentation. (From How to add a component )

“This gives a simple execution model for users: An App Inventor procedure will never be interrupted by an event handler, or vice versa; nor will one event handler be interrupted by another. This is illustrated in Figure 1:” (Ellen Spertus)

Figure 1: Sample program demonstrating serial semantics.

The example uses two Ball components, one CollidedWith handler that should fire as soon as Ball1 collides with anything, and an extra procedure called wait, which we don't really see what it does, but we are going to suppose that it takes 20 seconds to execute (imagine that it is loading content from the internet and it takes that long). All the code, except for the collision handler, is contained within a Click handler.

When Button1 is clicked, the two balls are positioned in exactly the same place, first Ball1, and then Ball2. Positioning the second ball in the same place as Ball1 should trigger the collision handler for Ball1. Some users would expect this to happen straight away, but if you were to execute this code, you would see that the collide handler will not execute until the click block finishes, and this would take at least 20 seconds because of the call to wait.

Following the same idea as above, you can think about the UI thread as if it was a Queue to get into the cinema; on arrival, ticket buyers will position themselves at the end of the queue.

When Button1 is clicked, we add the following things to the Queue:

• position Ball1
• position Ball2
• wait
• set Label1 text to : '...End of Button1.click...'

When Ball2 is positioned, the CollideWith handler is added to the Queue, but it cannot be executed straight away because the UI thread still has some tasks to do. So, after a couple of seconds we will see a queue such as:

• position Ball1 (DONE)
• position Ball2 (DONE)
• wait (IN PROGRESS)
• set Label1 to: '… End of Button1.click ...' (TO DO)
• CollidedWith (added to the queue when the collision happens)
  • set Label1 text to: '… Ball1.CollidedWith ...' (TO DO)

So Ball2 makes the handle trigger, but that does not mean that it will be executed straight away, instead it means that it will be added to the queue in the UI thread.

In the end, Label1 will always be assigned the value '...End of Button1.click...' first, and the value '...Ball1.CollidedWith...' later.
When both blocks finish, it will always read: “...End of Button1.Click......Ball1.CollidedWith...”.

Please note that having a procedure like wait blocking the UI is a really bad idea in an app, because as we saw in the explanation above, the app will be unresponsive for those 20 seconds, and this is not a pleasant experience for the users. If your app needs to do some expensive processing, it is quite possible that this interaction will block the app for as long as it takes.

From a design and development point of view, it is also important to note that certain blocks in App Inventor were designed with the UI thread in mind, and broken down into two different stages. For instance, the Web component Get call could be considered part 1 of a call to a web resource. This call will not block the UI because it is not performed in the UI Thread. When the response from the Get call is received, then part 2 will execute, which would be the event GotText (or GotFile, depending on how Get was configured). If you are planning on developing a new component for App Inventor, you need to take this into account.

Also related to this idea of sequential execution, a lot of users ask why their timers do not fire on time, for instance exactly every 2 seconds. Actually the timers fire at the time they are supposed to, but this only means that their handlers get added to the scheduling queue for UI updates. If at that particular moment something else is executing in the Screen, then the actions fired by the timer will have to wait for those other actions to finish, before they can be triggered. But once they get next to be executed in the Queue, they will fire one after the other, not waiting for any intervals. The waiting was already done before joining the Queue. 

This is actually something very usual in UI libraries, from Java (Swing) to Android, and even JavaScript. The browser, in which JavaScript executes, also has a single thread of execution, generally called the event loop, and it will behave in the same way as App Inventor does. Same happens if you run JavaScript on the server through something like node.js.

This is all a simplification of what really happens, and Android can create other Threads of execution that do not conflict with the UI thread to carry on with other operations. For a deeper dig in the matter, follow the link in the Android documentation to the Painless Threading article.