Spring 2022
MAI203 – Tech
Bots w/ MIT AI2
Computer Storage and
Cloud Storage
https://appathon.appinventor.mit.edu/
Let us work together to take you to the winning stages.
Hackathons
Dr. R. Feng
https://gallery.appinventor.mit.edu/?galleryid=ac3e3f9c-d650-47bb-8f8c-3231e5bc83a0
4
Dr. R. Feng
Why do we need storage?
Cloud
Storage
CPU
Memory
Random Access
Memory
Dr. R. Feng
Why do we need storage?
Storage
Dr. R. Feng
Why do we need storage?
-
Information such as variables are temporary
When the app is restarted, the memory is cleared and you lose information.
File Storage:
-
Allows you to read/write text data from a file.
Applications: event logging, exporting data, etc
More permanent form of storage → will stay on your device until the file is
deleted
Dr. R. Feng
File Manipulation Functions
Adds the input text to the end of the designated file.
Ex. File1.txt contains “oldtext”
File1.AppendToFile(“newtext”, “File1.txt”) → File1.txt contains
“oldtextnewtext”
Deletes the designated file
* You cannot do this with protected files
Retrieves the text from the designated file
Saves input text to a file
If the file already exists, it will overwrite the contents of the file
Dr. R. Feng
File Events
Triggers after the file has successfully saved.
Means that the contents have already been written and confirms
that there were no errors in the writing process.
Triggers after text is successfully retrieved from the file.
Only way to confirm a file can be read and to access the text.
Dr. R. Feng
External Files
Files located elsewhere on your device.
Use slashes to denote the file’s directory, and use a leading slash to tell your app
to look outside of its private folder
Ex.
Will look in storage →
Download → filename.txt
Will look in storage → app
private folder → Download
→ filename.txt
Dr. R. Feng
App Data Files
Files located in sdcard (or phone internal storage) → AppInventor → data
Access files by using the file’s name
⇒ If you are on the companion, will read from AppInventor/data
⇒ If as a packaged app (.apk), will read from the app’s private directory
This means that other programs will not be able to access it!
Dr. R. Feng
Asset Files
Packaged with the application (.apk file)
Use “//” at the beginning of the fileName variable to tell the code to look in the
app’s asset files
Since asset files are packaged with the app, you cannot modify or delete them
Dr. R. Feng
Overview: Reading from files
1. “Filename.txt”
a. Reads from the app’s private directory
b. Not accessible by other apps
c. Use for data that you need to save but is not needed elsewhere
2. “/Filename.txt”
a. Reads from your device’s sdcard or internal storage
b. Can be accessed by other apps
c. Use for exporting data from your app
3. “//Filename.txt”
a. Reads from app’s asset files
b. Cannot be deleted or modified
c. Use for storing information the app needs on startup
Dr. R. Feng
CloudDB: Online Storage
Allows you to store data on an Internet connected server (Redis software)
Users of the app can share data with each other → All users will share the same
server!
CloudDB stores data based on a tag system, much like a dictionary
Tag, Data
Dr. R. Feng
CloudDB Methods
Appends the designated item to the end of the list
under the given tag.
* Multiple users can append at the same time
Clears the data under the given tag
Returns true/false based on whether the app is
connected to CloudDB
Retrieves a list of tags from CloudDB
Dr. R. Feng
CloudDB Methods Cont.
Retrieves the value at the given tag
Returns a specific value if the tag is not found
Removes the first item of the list located at the
specific tag
Stores the value at the specified tag.
Dr. R. Feng
CloudDB Events
Specifies actions if an error is encountered.
Specifies actions when data on CloudDB is changed
Triggered by the function RemoveFirstFromList
Triggered when a value is successfully retrieved
Triggers when the tag list is successfully retrieved
Dr. R. Feng
List View, Spinner, and ListPicker
Dr. R. Feng
Walkthrough Project # 1
Implement an app to create a file, append text to the file, and then delete the file.
Pay attention to the events the file component has. Understand event invocations
can be available in software objects or components, in additions to human beings.
Dr. R. Feng
Walkthrough Project # 2
Implment an app to store data in the cloud using CloudDB, retrieve data from the
CloudDB, and display the data with ListView, Spinner, or ListPicker.
Dr. R. Feng
Dr. R. Feng