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App Inventor by Ellen Spertus, Liz Looney, Hal Abelson, David Wolber

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Chapter 14. Understanding an App’s Architecture

This chapter examines the structure of an app from a programmer’s perspective. It begins with the traditional analogy that an app is like a recipe and then proceeds to reconceptualize an app as a set of components that respond to events. The chapter also examines how apps can ask questions, repeat, remember, and talk to the Web, all of which will be described in more detail in later chapters.

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Many people can tell you what an app is from a user’s perspective, but understanding what it is from a programmer’s perspective is more complicated. Apps have an internal structure that we must fully understand in order to create them effectively.

One way to describe an app’s internals is to break it into two parts, its components and its behaviors. Roughly, these correspond to the two main windows you use in App Inventor: you use the Component Designer to specify the objects (components) of the app, and you use the Blocks Editor to program how the app responds to user and external events (the app’s behavior).

Figure 14-1 provides an overview of this app architecture. In this chapter, we’ll explore this architecture in detail.

The internal architecture of an App Inventor app
Figure 14-1. The internal architecture of an App Inventor app

Components

There are two main types of components in an app: visible ...

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