AQI – Assignment

This is an iOS App which shows live AQI (Air quality index) data for different cities using Web-socket.


Simulator Screen Shot - iPhone 13 - 2021-12-29 at 19 11 18
Simulator Screen Shot - iPhone 13 - 2021-12-29 at 19 12 11
Simulator Screen Shot - iPhone 13 - 2021-12-29 at 19 13 45
Simulator Screen Shot - iPhone 13 - 2021-12-29 at 19 44 30

Basic details

  • Deployment target: iOS 13.0
  • Supported device: iPhone & iPad
  • Code architecture: MVVM


  • Live AQI data for 12 cities.
  • Detail screen which show live AQI chart for selected city.
  • History data upto 20 records.
  • Good chart data visulization & Zoom-in function.
  • Reconnection to Socket once user comes from App Background mode.
  • Dark mode support.
  • Error handling for Socket connection.


  • For the sake of assignment, history is kept upto 20 records.
  • Suppose AQI value is 50.23 which lies between Good and Satisfactory Categoty. In that case, it is consider to be in Good category. We can change this logic as required.
  • Swift Packgae Manager (SPM) is used to manage project dependencies.


  • MVVM code architecture pattern is used.

  • City list

    • ViewModel of City list connects to the web socket and listens to the callbacks.
    • It creates CityModel if it doesn’t already exist (using city name) and adds AQIModel in aqi history array. If city already exist, it just adds AQIModel.
    • Once datasource is prepared, ViewModel updates View using delegate to refresh the table.
    • If there is error in socket connection, it updates View and user is promoted with error message with Retry button.
  • AQI chart for city

    • Once user select any city, it will pass CityModel to the detail screen. As data is shared among two screen, Class is used so it automatically refresh the chart.
    • View will ask ViewModel to prepare datasource based on history data and after Chart data is prepared, it will updates View using delegate to refresh the chart.
    • If there is an AQI update for selected city, ViewModel of City list will update ViewModel of chart view, which will updated the chart datasource and will update to View to refresh the chart.


  • Create city list

    • Response array is looped through and existing datasource is checked whether it contains a city with cityName. If not, new instance is created for city and AQI record is added to history.
    • AQICategory Enum is created based on the AQI value and each category is assigned Title and Color.
  • Create chart data

    • AQI history is looped through and array of ChartDataEntry is created and then Data set is created.
    • X-Axis time values are assigned through dataPoints array.
    • For the sake of demo and for better chart UI, only 20 records are stored for AQI history.
  • Reconnect Socket

    • Observers for didEnterBackgroundNotification and willEnterForegroundNotification are used to open and close the connection to Web-socket. If not used reconnection logic, it will stop getting Socket updates while coming back from App background mode.


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