Homework 4

Due: Saturday March 2.

Submission link: http://crowdlab.soe.ucsc.edu/crowdrank4/submission/submit/6

In this homework, we will extend the multi-button application created in Homework 2 to communicate with a server.

The application must continuously maintain and update a location estimate.

Scenario

You walk around, and switch on the Tagger app, which acquires your position, and downloads from the server a list of taggings, which it displays as a list sorted by distance from you. So you read, for instance,

• Pothole (400 ft, NE)
• Party (1500 ft, W)
• ...

(Yes, unfortunately, no tag filtering yet). As you walk around, every few seconds or so, those distance estimates are updated.

You come across a pothole, and you tag it by pressing the "Pothole" button on your phone (as in HW2, you have three buttons with labels on them, and one that leads you to a place where you can define a tag). This adds the pothole to your list, and also uploads it to the server.

Then, you come across a place selling roasted chestnuts, and you want to tag it. The tag "Chestnuts" is not labeling any of your buttons, so you press the "Other" button, you enter "Chestnut" as the label for a button, and once you go back to the original screen, the tag Chestnut and its coordinates have already been uploaded, and now "Chestnut" labels one of the three buttons.

Display nearby taggings

Upon startup, and periodically when the location changes, the application must query the server and download a number of taggings that are close to the location of the user. To do so, you can use this server call:

URL: http://crowdlab.soe.ucsc.edu/tagstore/default/get_tags.json

Parameters:

    lat_min

    lat_max

    lng_min

    lng_max

    n_taggings

    token = "CMPS121_yehaa"

    optional: username = ... (will cause only tags created by this username to be returned)

Return: 

A json list of dictionaries with fields:

• "lat"
• "lng"
• "tag"
• "nick"

Sample output:

{"tags": [{"lat": 100.0, "nick": "--anonymous--", "lng": 120.0, "tag": "pothole"}, {"lat": 100.0, "nick": "eduardo", "lng": 120.0, "tag": "pothole"}]}

The tags can be displayed in the usual list format, from the closest, to the farthest away. You should at least display Distance, Bearing, Tag, and Nickname of the tagger.

Add tagging

Users should be able to add tags. This is done by pressing one of the tag buttons, as in Homework 2. You should add the tag to the local list on the phone (to confirm to the user that the tag has been added), and you should upload the tag using this server call:

URL: http://crowdlab.soe.ucsc.edu/tagstore/default/add_tagging.json

Parameters:

    lat

    lng

    token = "CMPS121_yehaa"

    user = (your username)

    tag

Return: the id of the created tagging.

Sample output:

{"id": 7}

To check that the tag has been uploaded, you can look at the output of the call, or do a get_tags call using a lat/lng rectangle that includes the uploaded tag.

Managing the nickname

(this part is not required for this homework; I may put it back in one of the future ones)

Somewhere in a menu or setting button, users should be able to change their nickname:

URL: http://crowdlab.soe.ucsc.edu/tagstore/default/set_nickname.json

Parameters:

    user = (your username)

    nick = (your new nickname)

    token = "CMPS121_yehaa"

Sample output:

{"nick": "PeterPan"}

Optional: get common tag names

When a user defines a new tag, it may be useful to have a list of tag names, with their popularity, e.g. to suggest what tags to use. Obtain the list of tags in use with:

URL: http://crowdlab.soe.ucsc.edu/tagstore/default/get_popular_tags.json

Parameters:

    token = "CMPS121_yehaa"

Return: A JSON dictionary giving the popularity of the tags.

Sample output:

{"popular_tags": [{"cnt": 12, "name": "pothole"}, {"cnt": 2, "name": "firestation"}]}

Resources

• The web2py app running on the server, in case you want to run it on your own.
• The Tagger application, containing sample code for dealing with Json and for doing the HTTP calls to the server.
• The Tagger application as we modified and improved it during Tuesday's office hours.
• An image describing a possible implementation flow, as we drew on the whiteboard during office hours:

Some random advice

• Note that you do not need to store tag lists on the phone. You can just get them from the server dynamically.
• Yes, there is no way to remove tags. I should really implement a server-side process that expires old tags.
• You receive all tags. Much fun work would be needed to do this in a smarter way, e.g., receiving only tags from friends, or receiving only tags in some categories.
• Changing the ArrayAdapter to deal with the tag list with bearing, distance, etc, will be a bit of a hassle, unfortunately.
• If you try to upload a tag and you have no connectivity, it's ok to give up. You don't need to store on the phone a queue of tags that you are waiting to upload.
• Use git on your homework; it will help you if you take a wrong turn.
• Ask if you get stuck. And, do not wait till the last couple of days to get started. It is not an easy homework.

Things to watch out for

• Note that you need to URL encode your parameters; there is a Java class for doing this. Otherwise, things like tag names with spaces in them will break your code.
  • When you lose location services, make sure you don't crash -- you can either keep the last estimate, or say that the distance and bearing to other locations is unknown, at your choice.