Save HTML5 Canvas Image to Gallery – Phonegap Android plugin

Update

Readers of this post have raised issues with the plugin. I have to personally check for the issues but while using the plugin if you find any issues, you might want to debug it in ADT. I will come up with a working plugin soon. Thanks.

Sometime back I worked on a Phonegap Android plugin that helps to save an HTML5 Canvas Image to the device’s gallery. Well, the title of this post may be misleading but note that when you save an image, you actually have to save it on the SD Card or the device’s memory. The Gallery is just an app that shows the collection of images from various locations on the SD Card. So, there’s nothing like saving an image directly to the Gallery.
I had this working with Phonegap 2.2.0 (the version that I used). Newer versions of Phonegap/Cordova are available and things might have changed a bit, specially in the way custom plugins are written. Hence, if you are using newer versions of Phonegap you should have a look at the official documentation before proceeding.
Alright, here is what I did. I tried to save an image using the FileWriter – Phonegap API, but as it turned out, it can save only text data on the device’s memory. So, only way to do it was write a Phonegap plugin, pass the Canvas details from JavaScript interface to the Java side and let the Java class save the Canvas image on the SD Card.
Note that I am not a Java developer, and there may be better ways of writing this plugin.

How to install the plugin?
You need to get hold of two files :
1) The Java class – SavePhotoPlugin.java
2) The JavaScript interface – SavePhotoPlugin.js

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JavaScript typeof and instanceof – the concept and differences

As a JavaScript developer you must have used the typeof and the instanceof operator sometime in your code to detect the the type of a variable in your program logic. And most often or not you might have been confused with the different results that show up while expecting a desired result. In this post I am gonna show you an easy technique to determine the type and instance of a variable. Let’s see some examples first.

The typeof operator

var str1 = "Hello, how are you?";
typeof str1;
Output: "string"

So, we have a string literal str1, and when we find the typeof str1, it gives “string”. Remember string is a primitive data type in JavaScript. Let’s see another example,

var str2 = new String("I am doing good. Thank you!");
typeof str2;
Output: "object"

Now, we have a String Object and we have created an instance of that Object using the new operator, which is str2. Now if we do a typeof, it gives object. Remember that in JavaScript, other than the primitive data types (and null, undefined, function), everything else is an Object. So as expected you get the type as object.

Now, let’s work with Array’s and try the same thing,

var arr1 = [2,3,4]; //defining an array literal
typeof arr1;
Output: "object"

var arr2 = new Array(4,5,6); //creating an Array Object instance
typeof arr2;
Output: "object"

Firstly, we have an Array literal – arr1 and if we do a typeof it gives “object”. Why? Since Array’s are Objects and they are not part of the primitive data set.
The next example is more obvious, we create an Array Object instance – arr2, and then do a typeof. As expected we get a “object”. So we have seen, if the typeof does not return a primitive data type (which are number, boolean, string), it will return an object. There are some special cases though, look at the bottom of this page to find out.

The instanceof operator

Now let’s take up some examples of the instanceof operator.

var str1 = "Hello, how are you?"; //defining a string literal, same as above
str1 instanceof String;
Output: false

If you see above, we have a string literal and when we do a instanceof to check if the variable is an instance of the String Object, it returns false. Since a string literal belongs to the string primitive data type. Now, consider the example below,

var str2 = new String("I am doing good. Thank you!"); //defining a String object instance
str2 instanceof String;
Output: true

Here, str2 is actually an instance, so the test returns true.
But What’s up with the Array’s though? Let’s see,

var arr1 = [2,3,4]; //defining an array literal
arr1 instanceof Array;
Output: true

var arr2 = new Array(4,5,6); //creating an Array Object instance
arr2 instanceof Array;
Output: true

Both returns true as both arr1 and arr2 are Array objects and they are not part of the primitve data set.

In fact the following test also returns true

arr1 instanceof Object;
Output: true

arr2 instanceof Object;
Output: true

This is because the Array object inherits from the more generic Object object. (Yes, you heard it right Object object!).

Still confused about finding types and instances? Here is the rule to do it easily.

Rule for the test
So, how do you guess if a variable is an instance of its Object or Type. Here is the rule,
1) First check the type of the variable using typeof.
2) Now do the instanceof test.
3) a) If the typeof test returns a primitive type – number, boolean, string. Then instanceof test will return false. Because these are just primitive type data. They are not instances of any Object.
b) But if typeof returns an object, then instanceof test will return true. So you know that you are dealing with objects and instances.
For eg.

var regexp = /d{3}/; //define a regular expression object
typeof regexp; -> "object", not a primitive type
regexp instanceof RegExp; -> true
regexp instanceof Object; -> true

Take another,

var str = "Hello, me again!";
typeof str; ->  "string" , a primitive type
str instanceof String; -> false

Now, you can think of some examples and put the rule to a test.

Special cases
1) typeof null = “object” , since null is a special value in JavaScript which represents emptyness and it does not have any type.
2) typeof undefined = “undefined”, as expected
3) typeof NaN = number, since NaN is actually a special number, although it says Not a Number. Confusing? Just keep in mind.
4) typeof Function = “function”
5) typeof xmlObj = “xml”

References
1) typeof operator : https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/typeof
2) instanceof operator: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/instanceof

HTML5 SVG Support check using JavaScript – the simple way

My last post talked about how you can detect whether your browser supports HTML5 Canvas or not using simple JavaScript technique. Based on the same principles, this post illustrates how you can detect if your browser supports SVG elements. You may note SVG is Scalable Vector Graphics and HTML5 has native support for SVG tags. The difference with Canvas is that in case of SVG elements, they become a part of the DOM and you can manipulate each SVG elements, whereas in case of Canvas, the entire <canvas></canvas> tag becomes one node in the DOM and anything inside the Canvas <canvas> tag (for eg. a Rectangle) cannot be manipulated individually using standard DOM methods. Browse the web and you will find a hell lot of topics on this.

Alright, now back to the detection code. Just copy/paste this inside your window load / body load or in case of JQuery the $(document).ready() method,

if(!document.createElement('svg').getAttributeNS){
  document.write('Your browser does not support SVG!');
  return;
}

So, if your browser does not support SVG, then it will not process further (in case your application cannot live without SVG). But if you have a back up plan and you can live without SVG in your browser, then you can add an else block along with the if so that you can write your backup logic.

Explanation
Well, the code is very simple. Inside the if condition I create a SVG element using the standard DOM createElement() method. Now this will create a <svg></svg> object. Now next thing I do is to check if the getAttributeNS() method is defined or not. If you browser does not support <svg> then there is no way it will understand getAttributeNS() and that’s the catch.

HTML5 Canvas Support check using JavaScript – the simple way

This is how you can check for HTML5 Canvas support in your browser by using JavaScript, there’s no need of an external library like Modernizer to do a simple check like this. In fact most of the feature checks can be done natively, that’s what I prefer.

Here is the code,

var canvasEl = document.createElement('canvas'); //create the canvas object
if(!canvasEl.getContext) //if the method is not supported, i.e canvas is not supported
{ 
  document.write("HTML5 Canvas Not Supported By Your Browser");
  return;
}

As you can see in the first line I create a HTML5 Canvas element by using createElement() DOM method. As you might know that to work with Canvas, you have to get it’s context object (2d for now), which can be retrieved by the getContext() method. So, if a browser does not support the <canvas> tag or the canvas element, then no way the getContext() method will be defined for it. That’s what I check in the second line – the if condition. Remember that a function/method name is a variable which holds a reference to the function block. So we can check whether it is defined or not, in a normal way like we do for object properties. IE8 doesn’t support HTML5 Canvas. So try this code in IE8 for a quick test.

How can I use it with my code?
Use this code block inside the window load event handler or when the DOM ready event is triggered (using document.onreadystatechange, more). You can modify it as per your need.
Leave behind a comment if you are upset with this attempt.

Augmenting the String object by extending its prototype – JavaScript

Every function in JavaScript has the prototype property which enables you to enhance the functionality of the current Object definition (the Class definition) either through adding properties or through methods. This stands true for both custom Objects as well as JavaScript defined Objects like Strings, Arrays.
Using prototypes to add functionality also has one more advantage and is performance optimized. Whatever methods or properties are defined using prototypes, they belong at the class level which means that all the instance of the Object (or rather the Class, in JavaScript there is no concept of Class, we say it Objects) will share the same copy of the method or property, rather than having their own individual copies.

In this post I am going to enhance the String Object which is already defined in JavaScript and has a lot of useful methods already available such as the split(), indexOf(), replace() etc and properties like length etc.

Now let’s add our new method to the String Object just to enhance its functionality. Let’s add a method that will remove the empty spaces between the words of a string value. For eg.

input value = The weather is very nice
output value = Theweatherisverynice

Defining our custom method
As you guessed it right we will be using the prototype property. Let’s try this thing out,

I have named our custom method as – removeEmptySpaces()

if(typeof(String.prototype.removeEmptySpaces) !== 'function'){ //you expect it to be a function if it is already there
    String.prototype.removeEmptySpaces = function(){                    
        return this.replace(/\s+/g,'');                   
    }
}

As you can see above, first we check if the removeEmptySpaces() method is already defined. If not (which is true in this case since we are adding out custom method. But the check is good to have if you do not want to overwrite a predefined method) then we define it inside the if block. So using the prototype property we add the custom method and from now on it is available to all the instances of the String Object. Inside the method definition we have a String.replace() method implemented. This will remove every occurrence of space between the words and will join them. I am using a regular expression inside the replace() method to match the spaces between the words. Once the match is found, it will be removed. You can also see the use of this keyword. This will represent the current instance that is calling our custom method – removeEmptySpaces(). 
You can learn more about the replace() method from w3schools.com

Putting it to test
Now let’s use our new method and see if it is actually doing the stuff that we wanted.

 var testStr = new String("The weather is very nice");   //create a new instance
 var newStr = testStr.removeEmptySpaces(); //call our custom method
 alert("New string after removing empty spaces is : " + newStr);

First we create a String Object instance – testStr and pass it a value. Then we call our custom method and store the returned value in newStr. Now if you alert the new value you can see that the empty spaces between the words have been removed. Here we have seen using our customized method with a String instance. What about String literals or constants? Well, we can use for them as well. Here is an example,

  var strConstant = "I am a String constant"; //defining our string literal
  var newStrConstant = strConstant.removeEmptySpaces(); 
  alert(newStrConstant); //alerts "IamaStringconstant"

So, we have seen how using prototypes we can add new functionality to our Objects. This was a simple example that I presented. You can customize any Objects as per your need.

Cooler modal popup example – how to open multiple popups, scrolling pop ups per page

Some time back I came up with a cooler modal pop up window using CSS3 and JavaScript specifically for mobile webkit browsers. The example that I presented had only a single button, a click on it would open a modal pop up window.

In this post I have a similar example but this time multiple pop ups can be opened from the same page. Opening multiple pop up’s from a single page was requested by some of my readers. And here it is. I will not go deep into explaining the bits and parts of how to create a pop up window. You can go through my previous post for that. But first let’s check a demo (meant only for web-kit based browsers):

Demo link (open in iOS or Android’s browser, you can also test this in Chrome or Safari in your computer) :  http://rialab.jbk404.site50.net/coolermultiplemodalpopup/

Open multiple pops from a single page

How to run this in Firefox, IE and other browsers?
For this you need to make changes in the CSS file. Add CSS3 prefixes for other browsers similarly as it is there for -webkit- . CSS3 junkies would already know what I am talking about.

CSS3 Transformations showing content outside overflow:hidden region in Firefox/Android

Some time back I was implementing a functionality for Firefox browsers on Android devices and I found a strange problem – For Firefox > 18 on Android devices if I move an element using CSS3 Transformations inside a parent container with overflow:hidden, then part of the element being moved is displayed outside the overflowed region. It seems like when you move the element it just discards the overflow:hidden property and everything is revealed rather. Normally the content outside overflow:hidden should be cut off from view.

I have a simple demo http://rialab.jbk404.site50.net/ffandroidissue/ – an image which is bigger than the container div is moved using CSS3 Transformations. The  div has a overflow:hidden set to it. There are two buttons. Click on them to see the various results. The first button causes the issue whereas the second button resolves it.

So this is what causes the issue – moving the image using CSS3 Transformations (for eg. transform:translateX(value);) inside a container with overflow:hidden

And this is what solves it – strange !!
Adding a background color and an opacity to the container element like this,

#container.noissue{
-moz-perspective: 1000px;
overflow: hidden;
background:#ccc;
width: 100px;
height: 50px;
display: block;
position:absolute;
top:50px;
right:50px;
  background:#ccc;
  opacity:0.99;
}

Check out the demo in Firefox on an Android device. I am not sure if anybody has encountered this situation. But this definitely seems to be a bug in Firefox for Android’s. This is not seen for other browsers.

Replicating the iPhone Swipe Gesture – Vertical swiping

For those who wanted a vertical swiping feature to the the swipe gesture gallery that I created earlier, this post has a new demo and minimal explanation about a vertical swipe gesture gallery. Now you can swipe the images up or down.
I will not go through the basics once again as I have explained them in details in my previous posts. You can refer them once again in these two tutorials – post 1, post 2. Check out the demo below. Open the link in a webkit browser in either your mobile device or your computer.

Demo link: http://rialab.jbk404.site50.net/swipegesture/vertical/

Below is a screenshot of the gallery in action. You can see that the images are being moved vertically.

Screenshot of vertical swiping through images
Screenshot of vertical swiping through images

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Adding Calendar Events – Phonegap Android plugin

Alright, I worked on this sometime back and I will share this with you. I was looking to programmatically  add events to the native Android calendar from a Phonegap android app. There is not an official plugin that is available as of now. So I wrote a custom workaround for this. Again, I am not a Java developer and there might be better ways of writing the plugin code. But just wanted to share the code, and any feedback would just be appreciated.

Alright, let’s get started. So, there are two files that I developed. One is the CalendarEventPlugin.java file which has the plugin’s JAVA code. The other one is the CalendarEventPlugin.js file which is the javascript interface of the plugin. At this point, if you are not sure of what goes into writing a custom Phonegap plugin for Android, then you might want to have a look at this. I am using Phonegap 2.2.0 for my purpose. There has been newer version of Phoengap that is available now, 2.4.0 being the latest at the time of writing. So you can use the latest version as well, but do check the documentation once, there might be some changes.

Check the two files out. I have inline comments that will help you to understand. Rest, is very simple to grasp. Now let’s check how to use and implement this plugin into your Phonegap Android project.

1) Firstly you have to add these two files to your Phonegap Android project. Add the CalendarEventPlugin.js file inside assets/www/ folder and provide a reference to it in your index.html file, like this

<script type="text/javascript" charset="utf-8" src="CalendarEventPlugin.js"></script>

2) Then create a directory inside your project’s src folder that matches the package name of CalendarEventPlugin.java class. For our case make a directory – /org/apache/cordova/plugin inside src and then paste CalendarEventPlugin.java file inside it. If you change the package name, make sure to change the directory structure as well. The package name can be found at the top of CalendarEventPlugin.java file.

3) Next thing to do is to register the plugin in the config file – open res/xml/config.xml and then add the plugin details given below to the <plugins></plugins> section of the XML file. The name attribute is the Java class name and the value is the path of the class. This should match the package name.

<plugin name="CalendarEventPlugin" value="org.apache.cordova.plugin.CalendarEventPlugin"/>

4) And then call the plugin inside your javascript (your script.js file or so) code like this. (You can call this inside a button click handler or so),
   window.addcalendareventplugin(function(val){
        alert(val);   //once success message come and you have tested it, you can remove this alert.
    });

Once everything runs fine, you will see that when you click a button or so in your demo app and it calls the callback function in step 4, the native Calendar Add event is popped up with values already set in it (which you can change in the java file of the plugin). From there on its just the normal way of adding events to Calendar.
Hope this helps!!

Note

  • I have tried this on an Android 4.0 device (Galaxy S3) and it works. I have not really tried this on lower Android versions.

Reset and clear the HTML form after submitting – when you hit browser’s back button

This is a small issue but sometimes important. Well this is not a major post by me nor a major find, but I did run into some trouble while trying this particular thing out. Hence thought of recording it somewhere, so that I do not have to re-invent the wheel again. Here is the use case – there is an HTML form which is to be filled up and submitted to a server side script (let’s say some PHP code). After successful submission the user hits the back button of the browser. All the form fields and values should be reset and set to its initial configuration.

I tried several different techniques and implementations but ran into some or the other problems mainly because of browsers – different browsers behaving differently. On some browsers something works while on the others it does not. Thanks to these differences in spite of a universal and a common W3C standard. But I finally found a rather easy solution and cached onto it. This post will talk on that particular method with an example.

So the clue is very simple – reset the form after it is submitted so that when the user hits the back button every field in the form is initialized and reset (no previous values are left behind). How?? Let’s see in this example,

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