Package com.codename1.javascript
NOTE: The com.codename1.javascript package is now deprecated. The preferred method of
Java/Javascript interop is to use BrowserComponent#execute(java.lang.String), com.codename1.util.SuccessCallback),
BrowserComponent#executeAndWait(java.lang.String), etc.. as these work asynchronously (except in the
XXXAndWait() variants, which use invokeAndBlock() to make the calls synchronously.
The Codename One JS Bridge package includes classes that facilitate the interaction between Java and Javascript in a Codename One application. It allows both Java to Javascript communication and the reverse via a mechanism similar to the one employed by Phone Gap/Apache Cordova.
Requirements
The bridge will only run on platforms that include a native browser component. This includes iOS, Android, Windows, Desktop & JavaScript at this time.
Usage
The com.codename1.javascript.JavascriptContext class lays the foundation by enabling you to call
Javascript code directly from Java. It provides automatic type conversion
between Java and Javascript types as follows:
Java to Javascript
Java Type Javascript Type
String String
Double/Integer/Float/Long Number
Boolean Boolean
JSObject Object
null null
Other Not Allowed
Javascript to Java
Javascript Type Java Type
Number Double
String String
Boolean Boolean
Object JSObject
Function JSObject
Array JSObject
null null
undefined null
Note that this conversion table is more verbose than necessary, since Javascript functions
and arrays are, in fact Objects themselves, so those rows are redundant. All Javascript
objects are converted to com.codename1.javascript.JSObjects.
Getting Started
In order to start interacting with a Javascript environment, you need to create a WebBrowser and load a page. Then inside the WebBrowser's onLoad() handler, you can create a JavascriptContext on the internal BrowserComponent object:
Getting Values
The com.codename1.javascript.JavascriptContext#get(String) method is used to get values from Javascript.
It takes an arbitrary Javascript expression, executes it, and returns the
result, after converting it to the corresponding Java type. E.g. if the result
is a String, it will return a String and if it is a Number it will return a
java Double object. If the result is an Object, it will return a com.codename1.javascript.JSObject.
The following is a simple example that retrieves the document content, which is a string:
If you run this example in the simulator, you'll see something like the following:
Note: You don't see the WebBrowser in the background here due to a bug in the simulator. If you run this example on a device, you will see the WebBrowser component in the background.
Returning Numeric Values
As mentioned in the conversion table above, numeric values are automatically converted to java.lang.Double objects. The following example, returns the width and height properties of the window for use in Java.
The result, when run in the simulator would be something like:
Returning Objects
The previous examples involved only primitive return values. The com.codename1.javascript.JavascriptContext
abstraction,
in these cases, doesn't offer a whole lot of added-value over just using the
BrowserComponent.executeJavascriptAndReturnString()
method. The real value is when we are dealing with objects.
The following example obtains a reference to the window object and wraps it in a
proxy com.codename1.javascript.JSObject class so that we can work directly with the window object:
This code produces the exact same result as the previous example. The difference
is the intermediary step of wrapping the window object in a com.codename1.javascript.JSObject, and
obtaining the outerHeight and outerWidth properties directly via that proxy object.
You can obtain a com.codename1.javascript.JSObject proxy for any Javascript object, even ones that you create
on the fly. The following example creates an anonymous object with some keys and values
and uses a com.codename1.javascript.JSObject proxy to interact with this object from Java.
The result is as follows:
See Working With Objects for more information
about working with the com.codename1.javascript.JSObject class.
Returning Functions and Arrays
In Javascript, functions and arrays are just objects, so these are also encapsulated as com.codename1.javascript.JSObject
proxies. See Working with Arrays and
Workin with Functions for more details on how to work
with these values via the com.codename1.javascript.JSObject interface.
Setting Values
Just as you can get values from Javascript using com.codename1.javascript.JavascriptContext's get() method,
you can
also set values via com.codename1.javascript.JavascriptContext#set(String,Object).
The following is a simple example that sets the location, and causes it to redirect to a different page:
If you run this example, you should see your browser display the Codename One website after a redirect.
Setting Object Values
The previous example showed us setting a primitive String value. You can do the same with other primitives like numbers and booleans, but you can also set Object values using the set() method.
The following example creates an anonymous Javascript object, wraps it in a com.codename1.javascript.JSObject
proxy,
sets some values on it, then sets the object as a property of the top-level window object.
As a result, you should see the following content set as the body of the HTML page in the WebBrowser. Note that we can refer to the "steve" object that we just set directly/globally because the "window" object's properties are always available directly through the global namespace in Javascript.
Working with Objects
Previously examples showed how to obtain a com.codename1.javascript.JSObject proxy to a Javascript object.
There are 4 ways to get a com.codename1.javascript.JSObject:
-
Create an anonymous object:
JSObject obj = (JSObject)ctx.get("{}"); -
Reference an existing object directly:
JSObject obj = (JSObject)ctx.get("window.location"); -
As the result of a Javascript expression or function call:
JSObject obj = (JSObject)ctx.get("document.getElementById('mydiv')")
- Retrieve an Object member variable from another JSObject:
JSObject obj = (JSObject)otherObj.get("aPropertyThatIsAnObject")
com.codename1.javascript.JSObjects are essentially just proxies around a Javascript object. Any calls to
retrieve
properties from a com.codename1.javascript.JSObject are just sent directly to the Javascript context, and
the result
returned. The com.codename1.javascript.JSObject object doesn't store copies the javascript object's
properties. It just
retrieves them as they are required via the com.codename1.javascript.JSObject#get(String) method.
Getting Simple Values
You can always retrieve the properties of an object using the com.codename1.javascript.JSObject#get(String)
method. It takes the name of the property
as a parameter, and returns its value, converted to the appropriate Java type. (e.g. if it is a String,
it returns a String, if it is a number it returns a Double, and if it is an Object, it returns an object.
E.g.
java String name = (String)obj.get("name"); Double age = (Double)obj.get("age");
Is equivalent to the following javascript:
`var name = obj.name; var age = obj.age;`
Assuming that the `obj` variable in Java is a proxy for the same obj variable
in the javascript example.
Getting Nested Object Values
Often, in Javascript, an object contains a heirarchy of nested child objects. E.g.
java var obj = { name : 'Steve', position : { x : 100, y : 105, z : -25 } }
In this case you may want to obtain the x coordinate of the nested position object. JSObject
allows
you to use the dot '.' notation for referencing sub-properties like this. E.g.
`````java Double x = (Double)obj.get("position.x") `````
This feature raises the issue of how, then, to access properties that contain a '.' in its name. E.g.
java var obj = { name : 'Steve', position : { x : 100, y : 105, z : -25 }, 'position.x' : 200 }
In this example there is a top-level property named 'position.x' as well as a property at the component address position.x. This is a contrived example that is meant to be somewhat confusing in order to demonstrate how to differentiate between requests for properties in the child object heirarchy and top-level properties that happen to include a '.' in the property name.
We can force the retrieval of a top-level property by wrapping the key in single quotes:
java Double x1 = (Double)obj.get("'position.x'")
This would return 200 for the above example, whereas:
java Double x2 = (Double)obj.get("position.x")
Would return 100
.
Setting Object Values
The com.codename1.javascript.JSObject#set(String,Object) method works the same as the com.codename1.javascript.JavascriptContext#set(String,Object) method except that it treats the local
object as the root node. It allows you to easily set properties on the object. Values set here should
be provided using Java values as they will automatically be converted to the appropriate associated Javascript
type. If you are setting an Object as a value, then you'll need to set it as a com.codename1.javascript.JSObject and not a string
representation of the object. This is because Strings will just be converted to Javascript strings.
Properties set via the com.codename1.javascript.JSObject#set(String,Object) method modify the underlying
Javascript object directly so that the change
is immediately effective inside the javascript environment.
Just as with the com.codename1.javascript.JSObject#get(String) method, you can set the values of direct
properties or nested properties using
the dot '.' notation. And just like com.codename1.javascript.JSObject#get(String), you can force setting a
direct property in cases where the property
name includes a '.', by wrapping the key inside single quotes.
E.g.:
java // Create a team object , and leave city null for now. JSObject blueJays = (JSObject)ctx.get("{name : 'Blue Jays', city : null}"); // Create a city object and leave country null for now. JSObject toronto = (JSObject)ctx.get("{name : 'Toronto', country : null}"); // Create a country object JSObject canada = (JSObject)ctx.get("{name : 'Canada'}"); // Set the team's city to toronto blueJays.set("city", toronto); // Set toronto's country to canada toronto.set("country", canada); // Retrieve the name of the country where the blue jays play String countryName = (String)blueJays.get("city.country.name"); // Should contain "Canada" // Change the name of Canada to "Canuck land" using nested // dot notation on the blueJays object. blueJays.set("city.country.name", "Canuck land"); String blueJaysCountry = (String)blueJays.get("city.country.name"); String torontoCountry = (String)tornoto.get("country.name"); String canadaName = (String)canada.get("name"); //Note that all of these should be equal and contain "Canuck land"
Calling Object Methods
The com.codename1.javascript.JSObject#call(String,Object) method allows you to call javascript methods that
are members of the underlying object. It arguments are passed as an
Object[] array. These will be automatically converted from the Java type to the corresponding
Javascript type. Java type conversion are the same as using the com.codename1.javascript.JavascriptContext#set(String,Object) method.
The following example shows an object with a simple add() method
that just adds two numbers together:
java JSObject obj = (JSObject)ctx.get("{ add : function(a,b){ return a+b;}}"); Double result = (Double)obj.call("add", new Object[]{new Integer(1), new Integer(3)} ); // Result should be 4.0
Working with Arrays
In javascript, arrays are just objects that include a special ability to be iterated. You can use the alternate
version of com.codename1.javascript.JSObject#get(int) which takes an int as a parameter to
retrieve the elements of an array.
For example, consider the following javascript object:
java var obj = { name : 'Blue Jays', players : [ { name : 'George Bell', age : 31}, { name : 'Tony Fernandez', age : 34}, { name : 'Lloyd Moseby', age : 29} ] }
Then assuming we have a com.codename1.javascript.JSObject proxy for this object, we could loop through the
players
array and output the name and age of each player as follows:
So far, our examples have been limited to Java calling into Javascript. However, it may be
useful to be able to also go the other way: call java methods from Javascript. Some applications
of this might include:
- Capturing video, or using the camera on the phone
- Getting the phone location
- Accessing the file system or storage API
The Codename One JS bridge supports javascript to java method calling by way of the `com.codename1.javascript.JSFunction` interface
and the `com.codename1.javascript.JSObject#set(String,Object)` methods on the `com.codename1.javascript.JSObject` class. You can implement a `com.codename1.javascript.JSFunction`
object and register it as a callback with a `com.codename1.javascript.JSObject`, then you will be able to
execute this object's apply method via a Javascript proxy.
As an example, let's implement a simple logging function:
`````java JSObject logger = (JSObject)ctx.get("{}"); logger.set("log", new JSFunction(){ public void apply(JSObject self, Object[] args) { String msg = (String)args[0]; Log.p("[Javascript Logger] "+msg); } }); ctx.set("window.logger", logger); c.executeAndReturnString("logger.log('This is a test message');"); `````
If you execute this code in the simulator, you'll see the following output in the console:
```java
[EDT] 0:0:0,0 - [Javascript Logger] This is a test message
```
Running it on a device will yield similar output in the device log file.
Let's step through this code to see what is happening. First we create a new, empty javascript object
and wrap it in a JSObject proxy. Then we use the `com.codename1.javascript.JSObject`'s `com.codename1.javascript.JSObject#set(String,Object)` method to add an anonymous `com.codename1.javascript.JSFunction`
object to it with the propery of "log". This step registers a method proxy on the Javascript side that acts just
like a normal javascript method, but which actually triggers the `com.codename1.javascript.JSFunction`'s
`com.codename1.javascript.JSFunction#apply(JSObject,Object[])` method.
We then set this logger object to the global javascript scope by making it a direct child
of the window object. Finally we issue a Javascript method call to `logger.log()`. This
is what effectively calls the apply() method on our `com.codename1.javascript.JSFunction` object.
Caveats
**JSFunction callbacks are executed asynchronously so as to prevent deadlocks.** This means that you
cannot
return a value from this method using a return statement (hence the reason why the interface definition for `com.codename1.javascript.JSFunction#apply(JSObject,Object[])` is void.
**If you want to return a value back to Javascript, then you'll need to do it by providing a
callback function as one of the parameters, and call this callback method from inside the `com.codename1.javascript.JSFunction#apply(JSObject,Object[])`
method upon completion.**
Example: Passing a Javascript Callback to Your Callback
Since `com.codename1.javascript.JSFunction` callbacks are executed asynchronously, if you want to be able to
return a result back to Javascript, you will
need to do this via a Javascript callback. This is quite a common pattern in Javascript since it is single threaded
and relies
upon non-blocking patterns.
As an example, let's create a `com.codename1.javascript.JSFunction` callback that adds two numbers together and
returns the result to Javascript via a callback:
First we will create the `com.codename1.javascript.JSFunction` object to perform the addition, as follows:
`````java WebBrowser b = new WebBrowser(){ protected void onLoad(String url){ JSObject window = (JSObject)ctx.get("window"); window.set("addAsync", new JSFunction(){ public void apply(JSObject self, final Object[] args) { Double a = (Double)args[0]; Double b = (Double)args[1]; JSObject callback = (JSObject)args[2]; double result = a.doubleValue() + b.doubleValue(); callback.call(new Object[]{new Double(result)}); } }); } }; b.setURL("jar:///ca/weblite/codename1/tests/AddAsync.html"); `````
In this snippet, we start by obtaining a reference to the "window" object. We then add a method to this object named
"addAsync". This method is a `com.codename1.javascript.JSFunction`
object that we implement inline. The apply() method of the `com.codename1.javascript.JSFunction` object is the
Java method that will be executed when the addAsync method is called
from Javascript. In this case the addAsync method expects three parameters:
- The two numbers that are being added together.
- A Javascript callback method that will be executed when completed and passed the result of the addition
as a parameter.
Notice that all numerical arguments are converted to Java Double objects, and the callback function is converted to a
`com.codename1.javascript.JSObject` object. Also notice the use
of callback.call(), which just calls the callback as a function itself. With this variant of
the call() method,
the window object is used as this. Notice also that we pass the result inside an
Object[] array. This array will be expanded to
the direct javascript function parameters. (i.e. it will not pass an array as the parameter to the javascript
method, the array elements are extracted
and passed individually.
Now, let's look at the HTML page contents for our example:
`````java Addition Example Addition Example
+ =
Calculate
`````
Our HTML simply includes two text fields (to input the values to be added together), a button to initiate the
calculation,
and a <span> tag where the result will be placed when the calculation is complete.
Finally it includes the AddAsync.js Javascript file (which is placed in the same directory as the AddAsync.html file.
Its
contents are as follows:
`````java document .getElementById('calculate') .addEventListener('click', function(){ var aField = document.getElementById('input1'); var bField = document.getElementById('input2'); var a = parseFloat(aField.value); var b = parseFloat(bField.value); window.addAsync(a, b, function(result){ document.getElementById('result').innerHTML = result; }); }, true); `````
This script attaches an event handler to the calculate
button that gets the values from the two input fields and
passes it to the window.addAsync() method for calculation. The addAsync()
method is actually our java JSFunction that we implemented earlier.
One small word about the placement of these files: This example is taken from a class ca.weblite.codename1.tests.CodenameOneTests.
The AddAsync.html and AddAsync.js files are included in the same directory as the CodenameOneTests.java file (
i.e. /ca/weblite/codename1/tests). We used the WebBrowser's setURL() method to load the AddAsync.html
file from an
absolute path using jar: protocol. Currently this is the best way of loading local HTML files into
a WebBrowser object (i.e. use the jar: protocol and provide an absolute path).
The result of running this app is as follows:
Example: Exposing the Camera to Javascript
The following creates a Javascript function for taking photos on a mobile device. It involves a simple webpage with a
"Capture" button. When the user
clicks this button, it will dispatch a function call to CodenameOne to access the device's camera. After the user
takes a picture, CodenameOne will
execute a Javascript callback to add the picture to the webpage.
The HTML page source is as follows:
This loads the CameraExample.js script:
`````java document.getElementById('capture') .addEventListener('click', function(){ camera.capture(function(url){ if ( url == null ){ // No image was provided return; } var results = document.getElementById('results'); results.appendChild(document.createTextNode("testing")); var img = document.createElement('img'); img.setAttribute('src',url); img.setAttribute('width', '100'); img.setAttribute('height', '100'); results.appendChild(img); }) }, true); `````
The CameraExample.js script attaches a listener to the 'click' event of the "Capture" button which simply calls the
`camera.capture()` method,
which is actually a JSFunction that has been registered with the Javascript runtime. This actually calls into Java.
We pass a callback function into `camera.capture()` which will be executed upon successfully completion of
the camera. This is a common
programming pattern in Javascript. If a non-null URL is passed to this callback function, it is expected to be the
URL of the image (it will be
a local file URL.
The Java code that powers this example is as follows:
This example puts together most of the features of the CodenameOne-JS library.
- It creates a new JSObject from within Java code to serve as the camera object.
- It registers a JSFunction callback as a Javascript function which is added as a method to the camera object.
- It shows the use of the `call()` method of `JSObject` to call the callback function that
was
provided by Javascript from inside
the JSFunction's `apply()` method.
-
ClassDescriptionRepresents a Javascript context of a single BrowserComponent.The JSFunction interface allows you to implement functions in Java that can be called from Javascript.A Java Wrapper around a Javascript object.