Hello everyone!
I'm Hardik and I will be working on the project "Implement a CheckBox Group in App Inventor" this summer.
Full Project Proposal: here (The Implementation plan in this is clearly outdated  ).
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Please do give it a read and provide your valuable feedback. You can comment on the doc or reply to this topic. Any feedback is most welcome  .
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Short Project Description:
The project aims to add a CheckBox Group (similar to RadioGroup in Android but for CheckBox) component to the existing App Inventor which would automate the task of unchecking the rest of CheckBoxes in a group once one of the CheckBox is checked.
Currently in MIT App Inventor if a user wants to have such functionality, they will have to use blockly like in the screenshot below:
However, this method is quite tedious and not scalable as for each CheckBox you will have to add a block which would set the other CheckBoxes to unchecked. This problem was also discussed here.
The following are the resources that Iām referring to plan the implementation of my project:
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Checkboxes - Introduction to CheckBox 
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RadioGroup - Documentation of RadioGroup 
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CheckBox - Documentation of CheckBox 
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RadioGroup.java - Source code for RadioGroup 
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xeoh/CheckBoxGroup - Android library for handling multiple checkbox 
 , however the developer will still have to populate it; when you design the  real GroupBox keep it simple.
 , however the developer will still have to populate it; when you design the  real GroupBox keep it simple.