com.codename1.javascript.package.html Maven / Gradle / Ivy
NOTE: The {@link com.codename1.javascript } package is now deprecated. The preferred method of
Java/Javascript interop is to use {@link BrowserComponent#execute(java.lang.String) }, {@link BrowserComponent#execute(java.lang.String, com.codename1.util.SuccessCallback) },
{@link 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 {@link 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 {@link com.codename1.javascript.JSObject}s.
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 {@link 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 {@link 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 {@link 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 {@link 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 {@link com.codename1.javascript.JSObject}, and
obtaining the outerHeight and outerWidth properties directly via that proxy object.
You can obtain a {@link 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 {@link 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 {@link com.codename1.javascript.JSObject} class.
Returning Functions and Arrays
In Javascript, functions and arrays are just objects, so these are also encapsulated as {@link 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 {@link com.codename1.javascript.JSObject} interface.
Setting Values
Just as you can get values from Javascript using {@link com.codename1.javascript.JavascriptContext}'s get() method, you can
also set values via {@link 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 {@link 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 {@link com.codename1.javascript.JSObject} proxy to a Javascript object. There are 4 ways to get a {@link 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")
{@link com.codename1.javascript.JSObject}s are essentially just proxies around a Javascript object. Any calls to retrieve
properties from a {@link com.codename1.javascript.JSObject} are just sent directly to the Javascript context, and the result
returned. The {@link com.codename1.javascript.JSObject} object doesn't store copies the javascript object's properties. It just
retrieves them as they are required via the {@link com.codename1.javascript.JSObject#get(String)} method.
Getting Simple Values
You can always retrieve the properties of an object using the {@link 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.
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.
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. {@link JSObject} allows
you to use the dot '.' notation for referencing sub-properties like this. E.g.
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.
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:
Double x1 = (Double)obj.get("'position.x'")
This would return 200 for the above example, whereas:
Double x2 = (Double)obj.get("position.x")
Would return 100
.
Setting Object Values
The {@link com.codename1.javascript.JSObject#set(String,Object)} method works the same as the {@link 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 {@link 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 {@link 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 {@link 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 {@link 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.:
// 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 {@link 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 {@link 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:
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 {@link 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:
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 {@link 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:
JObject players = (JObject)obj.get("players");
int len = ((Double)players.get("length")).intValue();
for ( int i=0; i<len; i++){
JSObject player = (JSObject)players.get(i);
Log.p("Name : "+player.get("name")+" age : "+player.get("age"));
}
Calling Java Methods from Javascript
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 {@link com.codename1.javascript.JSFunction} interface
and the {@link com.codename1.javascript.JSObject#set(String,Object)} methods on the {@link com.codename1.javascript.JSObject} class. You can implement a {@link com.codename1.javascript.JSFunction}
object and register it as a callback with a {@link 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:
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:
[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 {@link com.codename1.javascript.JSObject}'s {@link com.codename1.javascript.JSObject#set(String,Object)} method to add an anonymous {@link 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 {@link com.codename1.javascript.JSFunction}'s {@link 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 {@link 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 {@link 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 {@link com.codename1.javascript.JSFunction#apply(JSObject,Object[])}
method upon completion.
Example: Passing a Javascript Callback to Your Callback
Since {@link 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 {@link com.codename1.javascript.JSFunction} callback that adds two numbers together and returns the result to Javascript via a callback:
First we will create the {@link com.codename1.javascript.JSFunction} object to perform the addition, as follows:
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 {@link com.codename1.javascript.JSFunction}
object that we implement inline. The apply() method of the {@link 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 {@link 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:
<!DOCTYPE html>
<html>
<head>
<title>Addition Example</title>
</head>
<body>
<div>Addition Example</div>
<p><input type="text" size="4" id="input1"/> +
<input type="text" size="4" id="input2"/> =
<span id="result"></span>
</p>
<p><button id="calculate">Calculate</button></p>
<script src="AddAsync.js"></script>
</body>
</html>
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:
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:
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.