Handler in Android

While working on SUSI.AI app (a smart assistant app), I found the necessity of handling UI components along with background data queue. While initializing a text to speech(TTS) engine inside a fragment that already had a speech to text (stt) engine implemented, there was a necessity to run the TTS engine to run using handler and later make it interact with the main UI thread. Let’s see how I handled this situation in the SUSI.AI app.

Android usually handles all the UI operations and input events from one single thread which is known as the Main or UI thread. Android collects all events in this thread in a queue and processes this queue. If one needs to perform some tasks in parallel with the main thread, then the main thread needs to be synchronized. Each handler is associated with a thread. A handler is a way to solve the asynchronous problem in Android.

A handler is widely used for:

  • Managing and inserting messages in the queue
  • Performing a task at a scheduled time in a different thread
  • Implementing runnable 

How Handlers work?

A handler is used to send and process Message and Runnable objects associated with a thread. Each handler is associated with a different single thread. This helps to perform the task asynchronously. The task, messages or runnable associated with that handlers are executed when it comes out of the message queue.

In Android, Handler is mainly used to update the main thread from background thread or other than the main thread. There are two methods in the handler:

  • Post() − it posts a message from background thread to the main thread using looper.
  • sendmessage() − if we want to organize what we have sent to UI (message from background thread) or UI functions. we should use sendMessage().

Construction of Handler:

First of all, we need to create and reference to Handler. After Handler is being created, we create some runnable objects. These runnable objects get executed inside the Handler.

Construction of Runnables that can be used in the handler:

Here, in this example, I have used the Post method to update the main thread.

Clearly, the post method is used in the handler object to execute the task mentioned in the runnable, by using its reference. Also, we can see the use of postDelayed function. This function executes the runnable after a specific span of time(mentioned in milliseconds in the parameter along with the runnable reference).

Lastly, it is very important to note that we should clear all the references to the handlers during the destruction of the view or the activity else there might be memory leaks. 

Resources: 

Documentation: Handler

Reference: Handler in Android

SUSI.AI Android App: PlayStore GitHub

Tags:SUSI.AI Android App, Kotlin, SUSI.AI, FOSSASIA, GSoC, Android, Handler

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Implementing Scheduler Actions on Open Event Frontend

After the functionality to display scheduled sessions was added to Open Event Frontend, the read-only implementation of the scheduler had been completed. What was remaining now in the scheduler were the write actions, i.e., the sessions’ scheduling which event organizers do by deciding its timings, duration and venue.

First of all, these actions required the editable flag to be true for the fullcalendar plugin. This allowed the sessions displayed to be dragged and dropped. Once this was enabled, the next task was to embed data in each of the unscheduled sessions so that when they get dropped on the fullcalendar space, they get recognized by the calendar, which can place it at the appropriate location. For this functionality, they had to be jQuery UI draggables and contain an “event” data within them. This was accomplished by the following code:

this.$().draggable({
  zIndex         : 999,
  revert         : true,      // will cause the event to go back to its
  revertDuration : 0  //  original position after the drag
});

this.$().data('event', {
  title    : this.$().text().replace(/\s\s+/g, ' '), // use the element's text as the event title
  id       : this.$().attr('id'),
  serverId : this.get('session.id'),
  stick    : true, // maintain when user navigates (see docs on the renderEvent method)
  color    : this.get('session.track.color')
});

Here, “this” refers to each unscheduled session. Note that the session color is fetched via the corresponding session track. Once the unscheduled sessions contain enough relevant data and are of the right type (i.e, jQuery UI draggable type), they’re ready to be dropped on the fullcalendar space.

Now, when an unscheduled session is dropped on the fullcalendar space, fullcalendar’s eventReceive callback is triggered after its drop callback. In this callback, the code removes the session data from the unscheduled sessions’ list, so it disappears from there and gets stuck to the fullcalendar space. Then the code in the drop callback makes a PATCH request to Open Event Server with the relevant data, i.e, start and end times as well as microlocation. This updates the corresponding session on the server.

Similarly, another callback is generated when an event is resized, which means when its duration is changed. This again sends a corresponding session PATCH request to the server. Furthermore, the functionality to pop a scheduled event out of the calendar and add it back to the unscheduled sessions’ list is also implemented, just like in Eventyay version 1. For this, a cross button is implemented, which is embedded in each scheduled session. Clicking this pops the session out of the calendar and adds it back to the unscheduled sessions list. Again, a corresponding PATCH request is sent to the server.

After getting the response of such requests, a notification is displayed on the screen, which informs the users whether the action was successful or not. The main PATCH functionality is in a separate function which is called by different callbacks accordingly, so code reusability is increased:

updateSession(start, end, microlocationId, sessionId) {
    let payload = {
      data: {
        attributes: {
          'starts-at' : start ? start.toISOString() : null,
          'ends-at'   : end ? end.toISOString() : null
        },
        relationships: {
          microlocation: {
            data: {
              type : 'microlocation',
              id   : microlocationId
            }
          }
        },
        type : 'session',
        id   : sessionId
      }
    };

    let config = {
      skipDataTransform: true
    };
    return this.get('loader')
      .patch(`sessions/${sessionId}`, JSON.stringify(payload), config)
      .then(() => {
        this.get('notify').success('Changes have been made successfully');
      })
      .catch(reason => {
        this.set('error', reason);
        this.get('notify').error(`Error: ${reason}`);
      });
  },

This completes the scheduler implementation on Open Event Frontend. Here is how it looks in action:

scheduler actions.gif

Resources

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Implementing Scheduled Sessions in Open Event Scheduler

Until recently, the Open Event Frontend version 2 didn’t have the functionality to display the already scheduled sessions of an event on the sessions scheduler. Displaying the already scheduled sessions is important so that the event organizer can always use the sessions scheduler as a draft and not worry about losing progress or data about scheduled sessions’ timings. Therefore, just like a list of unscheduled sessions was implemented for the scheduler, the provision for displaying scheduled sessions also had to be implemented.

The first step towards implementing this was to fetch the scheduled sessions’ details from Open Event Server. To perform this fetch, an appropriate filter was required. This filter should ideally ask the server to send only those sessions that are “scheduled”. Thus, scheduled sessions need to be defined as sessions which have a non-null value of its starts-at and ends-at fields. Also, few more details are required to be fetched for a clean display of scheduled sessions. First, the sessions’ speaker details should be included so that the speakers’ names can be displayed alongside the sessions. Also, the microlocations’ details need to be included so that each session is displayed according to its microlocation. For example, if a session is to be delivered in a place named ‘Lecture Hall A’, it should appear under the ‘Lecture Hall A’ microlocation column. Therefore, the filter goes as follows:

let scheduledFilterOptions = [
      {
        and: [
          {
            name : 'starts-at',
            op   : 'ne',
            val  : null
          },
          {
            name : 'ends-at',
            op   : 'ne',
            val  : null
          }
        ]
      }
    ];

 

After fetching the scheduled sessions’ details, they need to be delivered to the fulllcalendar code for displaying on the session scheduler. For that, the sessions need to be converted in a format which can be parsed by the fullcalendar add-on of emberJS. For example, fullcalendar calls microlocations as ‘resources’. Here is the format which fullcalendar understands:

{
        title      : `${session.title} | ${speakerNames.join(', ')}`,
        start      : session.startsAt.format('YYYY-MM-DDTHH:mm:SS'),
        end        : session.endsAt.format('YYYY-MM-DDTHH:mm:SS'),
        resourceId : session.microlocation.get('id'),
        color      : session.track.get('color'),
        serverId   : session.get('id') // id of the session on BE
}

 

Once the sessions are in the appropriate format, their data is sent to the fullcalendar template, which renders them on the screen:

Screen Shot 2018-08-21 at 8.20.27 PM.png

This completes the implementation of displaying the scheduled sessions of an event on the Open Event Scheduler.

Resources

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Implementing Unscheduled Sessions List for Event Scheduler

Until recently, Open Event Server didn’t allow the storage of unscheduled sessions. However, having the provision of unscheduled sessions was necessary so that event organizers can easily schedule the unscheduled sessions and keep track of them. Also, it allows them to remove scheduled sessions from the scheduler and place them in the unscheduled sessions list, so that they can be scheduled later. Also, since the unscheduled sessions list was also present in Eventyay version 1, it was decided to have the same in version 2.

The first step was to enable the storage of unscheduled sessions on the server. For this, the starts-at and ends-at fields of the session model were modified to be non-required (earlier they were mandatory). Once this change was done, the next step was to fetch the list of unscheduled sessions on the frontend, from the server. Unscheduled sessions were the ones which had the starts-at and ends-at fields as null. Also, the speakers’ details needed to be fetched so that their names can be mentioned along with sessions’ titles, in accordance with Eventyay version 1. Thus, the following were the filter options for the unscheduled sessions’ fetching:

let unscheduledFilterOptions = [
      {
        and: [
          {
            name : 'starts-at',
            op   : 'eq',
            val  : null
          },
          {
            name : 'ends-at',
            op   : 'eq',
            val  : null
          }
        ]
      }
];
 
let unscheduledSessions = await eventDetails.query('sessions', {
      include : 'speakers,track',
      filter  : unscheduledFilterOptions
    });

 

This gave us the list of unscheduled sessions on the frontend appropriately. After this, the next step was to display this list to the event organizer. For this, the scheduler’s Handlebars template file was modified appropriately. The colors and sizes were chosen so that the list looks similar to the one in Eventyay version 1. Also, the Ember add-on named ember-drag-drop was used to make these unscheduled session components draggable, so that they can be ultimately scheduled on the scheduler. After installing this package and making the necessary changes to the project’s package.json file, the component file for unscheduled sessions was modified accordingly to adapt for the draggable components’ UI. This was the final step and completed the implementation of listing unscheduled sessions.

unscheduled_sessions.gif

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