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Learning MIT App Inventor: A Hands-On Guide to Building Your Own Android Apps by Mark Sherman, Derek Walter

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Event Parameters: Special Variables

Think back to our first discussion on events. Everything in App Inventor is caused by an event. When a button is clicked, that is an event. When the phone is shaken, that is an event. When a text message arrives, that is an event. The events we’ve used so far have been pretty simple. All you needed to know was that the event was happening. The button click is an instance of this: When the button is clicked, it runs the Button.Click event block and all the blocks in it. But not all events are so simple.

Some events provide extra information to you, the programmer, when they happen. In the following exercise, we talk about touching a canvas. A canvas is much more than a button. A canvas has a coordinate system, ...

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