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Write a widget using Dojo (behavior)

January 9th, 2014
7 minute read
Dojo JavaScript Web

In the previous tutorial we finally started writing our module. However, we didn’t do anything fancy yet except using inheritance and writing down our properties. If you remember the first tutorial, we created several attach points and events, which we’re going to use now to finish our stopwatch.

Event handling

If you remember the first tutorial, we connected to two events, the onChange event of the ToggleButton was connected to onStart and the onClick event of the normal Butto nwas connected to onReset. We’re going to implement these event handlers now. They’re nothing more than functions added to our module:

onStart: function(newValue) {
    if (newValue && !this.__started) {
    } else if (!newValue && this.__started) {
    } else {

onReset: function() {
    this.__pauseTime = 0;
    this.__started = false;

Our start button will have multiple purposes. When it’s clicked for the first time, it will start the stopwatch. When it’s clicked afterwards, it will pause the stopwatch and if it’s clicked another time, it will resume the stopwatch. That’s why we use this if structure to implement these situations. We also reference to three new functions called __start(), __stop() and __resume(). The reset event handler is a bit easier, it will actually disable the pause (because we’re actually resetting the timer to zero), then we set the __started property to false so when the stopwatch is restarted, it will actually act as if it’s started for the first time (from the beginning). It will also have to set the displayed time to zero as well, that’s what we will do using the __render function.

Starting the stopwatch

We’re now going to implement all these new functions we referred to already, the first one being __start(). This one is actually quite easy:

__start: function() {
    this.__started = true;
    this.__currentTime = this.__getTime();

It wil set __started to true (so next “start” is actually a “resume”), it will set the current time (milliseconds since epoch). This will be used to see what the difference is between the current time and the time the watch was started (which is actually the time we need to display). The code for the __getTime() function is the most easy one:

__getTime: function() {
    return new Date().getTime();

We will also start a timer using the __startTimer() function.

The __startTimer is easy as well, it actually uses setInterval() to repeat a block of code countless times. The implementation of this function is:

__startTimer: function() {
    this.__pauseTime = 0;
    this.startBtn.set("label", nls.stop);
    this.resetBtn.set("disabled", true);
    this.__timer = setInterval(lang.hitch(this, "__render"), this.__SECMS / this.updateRate);           

it will reset the __pauseTime (which I will talk about later), change the label of the start button to “Stop” and it will make sure the reset button is still disabled by setting the disabled property to true. If it’s called for the first time it isn’t really useful (since it’s already disabled), but we will reuse this function when we implement the resume functionality. If you remember our first tutorial, we gave both buttons a data-dojo-attach-point and because of this, we can now refer to these buttons using startBtn and resetBtn, all thanks to the dijit/_TemplatedMixin and dijit/_WidgetsInTemplateMixin modules.

The last thing we do is set an interval to call the __render() function. We use the dojo/_base/lang function for this, because the setInterval() will change the this otherwise (because it's not called from the widget anymore). To make sure that this still refers to the current widget, we use the hitch() function.

The repeat interval is calculated upon the updateRate property. This property is actually the intervals per second, so to know how long the interval should be, we divide 1000 (this.__SECMS) to the updateRate.

Stopping and resuming the stopwatch

If we started the stopwatch, the next thing we can do is stopping/pausing it using the __stop() function. This one is easy as well:

__stop: function() {
    this.startBtn.set("label", nls.resume);
    this.resetBtn.set("disabled", false);
    this.__pauseTime = this.__getTime();

The first thing we do is make sure that the interval we created before, is stopped (so the stopwatch stops as well). Then we change the label of the start button to Resume and enable the reset button. We now add the current time to __pauseTime. The reason for this is that if we resume the stopwatch, we need to make sure that it’s actually substracting the time that the stopwatch was paused.

The __resume() function is similar to __start(), but in stead of setting the current time (__currentTime) to the current time, we add the time it was paused to it. Now the difference between the current time and the value in __currentTime) is smaller, because the time it was paused is substracted from it. The code for this:

__resume: function() {
    if (this.__pauseTime !== 0) {
        this.__currentTime += (this.__getTime() - this.__pauseTime);

As you can see we also check if the __pauseTime is not zero. If you remember well, the onReset sets this property to zero, meaning that the pause should not be calculated.

Displaying the time

We now implemented starting, stopping, resuming and resetting the stopwatch, but what we didn’t provide yet is the __render() function we call from __setTimer() and onReset(). As you can see, this function accepts one optional parameter. If this parameter is added, it will use that as the current time (in stead of the difference of the current time and __currentTime. That means if we provide zero to it, it will actually reset the stopwatch and show only zeros.

The implementation looks hard, but it’s quite repetitive.

__render: function(milli) {
    if (milli === undefined) {
        milli = this.__getTime() - this.__currentTime;
    var hrs = milli / this.__HRSMS;
    milli = milli % this.__HRSMS;
    var mins = milli / this.__MINMS;
    milli = milli % this.__MINMS;
    var secs = milli / this.__SECMS;
    milli = milli % this.__SECMS;

    this.hoursNode.innerHTML = this.__format(hrs, "00");
    this.minutesNode.innerHTML = this.__format(mins, "00");
    this.secondsNode.innerHTML = this.__format(secs, "00");
    this.milliNode.innerHTML = this.__format(milli, "000");

I already explained the if statement already, the next step is that it actually calculates the “hour”, “minute”, “second” and “millisecond” part from milli. To calculate that, we actually need to divide that value by the number of milliseconds in an hour (__HRSMS). Then the part that is still left over which we calculate using the modulo operator (%) should be divided by the number of milliseconds in a minute (to get the minute part). We do the same for the seconds and the final leftover is what’s the real milliseconds value.

We can then display these values by setting the innerHTML of our different nodes. If you remember our first tutorial, we gave each part a data-dojo-attach-point, we can now use that attach point to set the displayed content all thanks to the dijit/_TemplatedMixin module.

You can see we actually use a __format function, which will use the dojo/number module to round the numbers and add leading zeros to it. The implementation of this function:

__format: function(value, pattern) {
    return NumberUtils.format(value, {
        pattern: pattern

Initializing the widget

There’s only one thing we’re going to do and that’s making sure that the stopwatch is reset when it’s loaded for the first time. Normally that’s already the case because we placed zeros in the template, but we can manually do that as well by resetting using the __render(0) function. Thanks to the dijit/_WidgetBase module, our widget has a postCreate function that is automatically called when the template is rendered and all widgets in the template are processed so that our widget is actually ready to use. We can use that function to reset the stopwatch at that time using:

postCreate: function() {

The first line, this.inherited(arguments); makes sure that the postCreate function of all inherited modules is also ran, in this case dijit/_WidgetBase, dijit/_TemplatedMixin and dijit/_WidgetsInTemplateMixin. This is always a good practice to do if you’re using functions that are inherited from one (or multiple) modules.

This makes our module complete and so it ends our tutorial. In the next tutorial I will finish the entire application and demonstrate what we actually have done until now.

Write a widget using Dojo series

  1. Application structure, templating and localization
  2. Modules, inheritance and object state
  3. Module behavior
  4. Finishing the application and demo