JSContext Class

Definition

Namespace:

JavaScriptCore

Assembly:

Xamarin.iOS.dll

Encapsulates a JavaScript engine.

[Foundation.Register("JSContext", true)]
[ObjCRuntime.Introduced(ObjCRuntime.PlatformName.MacOSX, 10, 9, ObjCRuntime.PlatformArchitecture.Arch64, null)]
[ObjCRuntime.Introduced(ObjCRuntime.PlatformName.iOS, 7, 0, ObjCRuntime.PlatformArchitecture.All, null)]
public class JSContext : Foundation.NSObject

type JSContext = class
    inherit NSObject

Inheritance

Object

NSObject

JSContext

Attributes

RegisterAttribute IntroducedAttribute

Remarks

The JSContext is the central object of the JavaScriptCore namespace. The JSContext maintains a JavaScript environment (manipulated by the Item[NSObject] property) and evaluates scripts with the EvaluateScript(String, NSUrl) method.

Application developers will often want to assign a delegate to the ExceptionHandler property to gain access, in their Xamarin.iOS code, of exceptions raised in the JavaScript realm.

The following example shows the basic use of JSContext. The context is instantiated and a simple exception handler is assigned. One of the M:JavaScriptCore.JSValue.From* method overloads is used to assign values to the JavaScript variables arg1 and arg2. The EvaluateScript(String, NSUrl) method evaluates the JavaScript and returns the result, which is converted back into a .NET object with the ToInt32() method.

jsContext = new JSContext();

jsContext.ExceptionHandler = (context, exception) => {
	Console.WriteLine(exception);
};

jsContext[new NSString("arg1")] = JSValue.From(2, jsContext);
jsContext[new NSString("arg2")] = JSValue.From(2, jsContext);

var jsResult = jsContext.EvaluateScript("arg1 + arg2;");

var four = jsResult.ToInt32();

The JSContext contains the global JavaScript context, including variables set by JavaScript calculations, as shown in the following example:

jsContext.EvaluateScript("sum = 2 + 2;");
var four = jsContext[(NSString)"sum"].ToInt32();

Calling C# code from JavaScript

Developers can extend the T:JavascriptCore.IJSExport interface to define methods that can be called from JavaScript. Developers must mark that interface with the ProtocolAttribute attribute and must mark JavaScript-callable methods with the ExportAttribute attribute. They must also add --registrar:static to the arguments used by mtouch during the build. For example:

[Protocol ()]
interface IMyJSVisibleProtocol : IJSExport {
    [Export ("myFunc")]
    int MyFunc ();

    [Export ("Arity2:With:")]
    NSObject Arity2With(NSObject arg1, NSObject arg2);
}

class MyJSExporter : NSObject, IMyJSVisibleProtocol
{
    public int MyFunc ()
    {
        Console.WriteLine ("Called!");
        return 42;
    }

    public NSObject Arity2With(NSObject arg1, NSObject arg2)
    {
        Console.WriteLine ("Arity 2 function called with " + arg1 + " " + arg2);
        return (NSNumber) 42;
    }
}

The above example:

• Defines IMyJSVisibleProtocol as extending IJSExport;:
• Decorates IMyJSVisibleProtocol and it's method MyFunc with the ProtocolAttribute and ExportAttribute attributes;:
• Implements the interface:

To expose the IMyJSVisibleProtocol to JavaScript, the developer could use code like the following in the ViewDidLoad() method of their UIViewController:

webView = new UIWebView(UIScreen.MainScreen.Bounds);
var context = (JSContext) webView.ValueForKeyPath ((NSString) "documentView.webView.mainFrame.javaScriptContext");
context.ExceptionHandler = (JSContext context2, JSValue exception) => 
    {
        Console.WriteLine ("JS exception: {0}", exception);
    };
var myExporter = new MyJSExporter ();
context [(NSString) "myCSharpObject"] = JSValue.From (myExporter, context);
webView.LoadRequest(NSUrlRequest.FromUrl(new NSUrl("MyHtmlFile.html", false)));

The above C# code:

• Creates a UIWebView which will be displayed to the end user;:
• Gets the JSContext of the UIWebView object's main frame;:
• Adds an exception handler so that JavaScript trouble will be visible to the Xamarin project;:
• Instantiates a new MyJSExporter object that, as described above, implements the IMyJSVisibleProtocol;:
• Adds that object to the JSContext with the name myCSharpObject;:
• Loads an HTML file (see below):

Finally, the HTML file that is loaded into the T:UIKIt.UIWebView and into whose JSContext the MyJSExporter object has been placed can access the object from within JavaScript:

<html>
<head>
<title></title>
<script type="text/javascript">
            function callXamObject() {
                // `myCSharpObject` injected into JS context by C# code `context [(NSString) "myCSharpObject"] = JSValue.From (...etc...`
                var resultCalculatedInCSharp = myCSharpObject.myFunc();
                document.getElementById("Output").innerHTML = resultCalculatedInCSharp;
            }

            function callArity2Method() {
                //Note how this is mapped by [Export ("Arity2:With:")]
                var result = myCSharpObject.Arity2With("foo", "bar");
            }
</script>
</head>
<body>
<div onclick="callXamObject()" class="button">
            Click Me
</div>
<div id="Output">Value</div>
</body>
</html>

In order to export a C# object so that it is visible to JavaScript, the developer must add the --registrar:static argument to the arguments used by mtouch. In Xamarin Studio, this is done in the Project Options dialog, in the Build Options / iOS Build pane:

Another technique for calling C# code from Xamarin.iOS is to use REST, as shown in the following:

In the JavaScript code, use XMLHttpRequest and standard JSON techniques to post and parse a query to a REST service running on the local device:

<html>
<head>
<title></title>
<script type="text/javascript">
           function callCSharp(msg) {
                var request = new XMLHttpRequest();
                request.open('GET','http://127.0.0.1:1711/', false);
                request.send();

                if(request.status == 200){
                    alert(JSON.parse(request.responseText));
                }else{
                    alert("Error");
                }
            }
</script>
</head>
<body>
<div onclick="callCSharp('this is a test')" class="button">
            Click Me
</div>
</body>
</html>

In the application, use T:System.Net.HttpListener to listen and respond to that request:

//Wire up listener
listener = new HttpListener();
listener.Prefixes.Add("http://*:1711/");
listener.Start();
listener.BeginGetContext(new AsyncCallback(Callback), listener);

//....etc...

void Callback(IAsyncResult result)
{
	//Get the listener context
	var context = listener.EndGetContext(result);

	//Start listening for the next request
	listener.BeginGetContext(new AsyncCallback(Callback), listener);

	var response = CalculateResponse();
	var responseBytes = System.Text.Encoding.UTF8.GetBytes(response);

	context.Response.ContentType = "text/json";
	context.Response.StatusCode = HttpStatusCode.OK;
	context.Response.ContentLength64 = responseBytes.Length;
	context.Response.OutputStream.Write(responseBytes, 0, responseBytes.Length);
	context.Response.OutputStream.Close();
}

Finally, a third technique is to poll the JSContext for a flag set by a JavaScript calculation.

Constructors

JSContext()	Default constructor, initializes a new instance of this class.
JSContext(IntPtr)	A constructor used when creating managed representations of unmanaged objects; Called by the runtime.
JSContext(JSVirtualMachine)
JSContext(NSObjectFlag)	Constructor to call on derived classes to skip initialization and merely allocate the object.

Properties

Class	(Inherited from NSObject)
ClassHandle	The handle for this class.
CurrentArguments
CurrentCallee
CurrentContext
CurrentThis
DebugDescription	A developer-meaningful description of this object. (Inherited from NSObject)
Description	Description of the object, the Objective-C version of ToString. (Inherited from NSObject)
Exception
ExceptionHandler
GlobalObject
Handle	Handle (pointer) to the unmanaged object representation. (Inherited from NSObject)
IsDirectBinding	(Inherited from NSObject)
IsProxy	(Inherited from NSObject)
Item[NSObject]
JSGlobalContextRefPtr
Name
RetainCount	Returns the current Objective-C retain count for the object. (Inherited from NSObject)
Self	(Inherited from NSObject)
Superclass	(Inherited from NSObject)
SuperHandle	Handle used to represent the methods in the base class for this NSObject. (Inherited from NSObject)
VirtualMachine
Zone	(Inherited from NSObject)

Methods

AddObserver(NSObject, NSString, NSKeyValueObservingOptions, IntPtr)	Registers an object for being observed externally (using NSString keyPath).   Observed changes are dispatched to the observer’s object ObserveValue(NSString, NSObject, NSDictionary, IntPtr) method. (Inherited from NSObject)
AddObserver(NSObject, String, NSKeyValueObservingOptions, IntPtr)	Registers an object for being observed externally (using string keyPath).   Observed changes are dispatched to the observer’s object ObserveValue(NSString, NSObject, NSDictionary, IntPtr) method. (Inherited from NSObject)
AddObserver(NSString, NSKeyValueObservingOptions, Action<NSObservedChange>)	Registers an object for being observed externally using an arbitrary method. (Inherited from NSObject)
AddObserver(String, NSKeyValueObservingOptions, Action<NSObservedChange>)	Registers an object for being observed externally using an arbitrary method. (Inherited from NSObject)
AwakeFromNib()	Called after the object has been loaded from the nib file. Overriders must call base.AwakeFromNib(). (Inherited from NSObject)
BeginInvokeOnMainThread(Action)	(Inherited from NSObject)
BeginInvokeOnMainThread(Selector, NSObject)	Invokes asynchrously the specified code on the main UI thread. (Inherited from NSObject)
Bind(NSString, NSObject, String, NSDictionary)	(Inherited from NSObject)
Bind(String, NSObject, String, NSDictionary)	(Inherited from NSObject)
BindingInfo(String)	(Inherited from NSObject)
BindingOptionDescriptions(String)	(Inherited from NSObject)
BindingValueClass(String)	(Inherited from NSObject)
CommitEditing()	(Inherited from NSObject)
CommitEditing(NSObject, Selector, IntPtr)	(Inherited from NSObject)
ConformsToProtocol(IntPtr)	Invoked to determine if this object implements the specified protocol. (Inherited from NSObject)
Copy()	Performs a copy of the underlying Objective-C object. (Inherited from NSObject)
DangerousAutorelease()	(Inherited from NSObject)
DangerousRelease()	(Inherited from NSObject)
DangerousRetain()	(Inherited from NSObject)
DidChange(NSKeyValueChange, NSIndexSet, NSString)	Indicates a change occurred to the indexes for a to-many relationship. (Inherited from NSObject)
DidChange(NSString, NSKeyValueSetMutationKind, NSSet)	(Inherited from NSObject)
DidChangeValue(String)	Indicates that a change occurred on the specified key. (Inherited from NSObject)
Dispose()	Releases the resources used by the NSObject object. (Inherited from NSObject)
Dispose(Boolean)	Releases the resources used by the NSObject object. (Inherited from NSObject)
DoesNotRecognizeSelector(Selector)	Indicates that this object does not recognize the specified selector. (Inherited from NSObject)
Equals(NSObject)	(Inherited from NSObject)
Equals(Object)	(Inherited from NSObject)
EvaluateScript(String, NSUrl)
EvaluateScript(String)
ExposedBindings()	(Inherited from NSObject)
FromJSGlobalContextRef(IntPtr)
GetBindingInfo(NSString)	(Inherited from NSObject)
GetBindingOptionDescriptions(NSString)	(Inherited from NSObject)
GetBindingValueClass(NSString)	(Inherited from NSObject)
GetDictionaryOfValuesFromKeys(NSString[])	Retrieves the values of the specified keys. (Inherited from NSObject)
GetHashCode()	Generates a hash code for the current instance. (Inherited from NSObject)
GetMethodForSelector(Selector)	(Inherited from NSObject)
GetNativeField(String)	Obsolete. (Inherited from NSObject)
GetNativeHash()	(Inherited from NSObject)
Init()	(Inherited from NSObject)
InitializeHandle(IntPtr, String)	(Inherited from NSObject)
InitializeHandle(IntPtr)	(Inherited from NSObject)
Invoke(Action, Double)	(Inherited from NSObject)
Invoke(Action, TimeSpan)	(Inherited from NSObject)
InvokeOnMainThread(Action)	(Inherited from NSObject)
InvokeOnMainThread(Selector, NSObject)	Invokes synchrously the specified code on the main UI thread. (Inherited from NSObject)
IsEqual(NSObject)	(Inherited from NSObject)
IsKindOfClass(Class)	(Inherited from NSObject)
IsMemberOfClass(Class)	(Inherited from NSObject)
MarkDirty()	Promotes a regular peer object (IsDirectBinding is true) into a toggleref object. (Inherited from NSObject)
MutableCopy()	Creates a mutable copy of the specified NSObject. (Inherited from NSObject)
ObjectDidEndEditing(NSObject)	(Inherited from NSObject)
ObserveValue(NSString, NSObject, NSDictionary, IntPtr)	Indicates that the value at the specified keyPath relative to this object has changed. (Inherited from NSObject)
PerformSelector(Selector, NSObject, Double, NSString[])	(Inherited from NSObject)
PerformSelector(Selector, NSObject, Double)	Invokes the selector on the current instance and if the obj is not null, it passes this as its single parameter. (Inherited from NSObject)
PerformSelector(Selector, NSObject, NSObject)	(Inherited from NSObject)
PerformSelector(Selector, NSObject)	(Inherited from NSObject)
PerformSelector(Selector, NSThread, NSObject, Boolean, NSString[])	(Inherited from NSObject)
PerformSelector(Selector, NSThread, NSObject, Boolean)	(Inherited from NSObject)
PerformSelector(Selector)	(Inherited from NSObject)
PrepareForInterfaceBuilder()	(Inherited from NSObject)
RemoveObserver(NSObject, NSString, IntPtr)	Stops the specified observer from receiving further notifications of changed values for the specified keyPath and context. (Inherited from NSObject)
RemoveObserver(NSObject, NSString)	Stops the specified observer from receiving further notifications of changed values for the specified keyPath. (Inherited from NSObject)
RemoveObserver(NSObject, String, IntPtr)	Stops the specified observer from receiving further notifications of changed values for the specified keyPath and context. (Inherited from NSObject)
RemoveObserver(NSObject, String)	Stops the specified observer from receiving further notifications of changed values for the specified keyPath. (Inherited from NSObject)
RespondsToSelector(Selector)	Whether this object recognizes the specified selector. (Inherited from NSObject)
SetNativeField(String, NSObject)	Obsolete. (Inherited from NSObject)
SetNilValueForKey(NSString)	Sets the value of the specified key to null. (Inherited from NSObject)
SetValueForKey(NSObject, NSString)	Sets the value of the property specified by the key to the specified value. (Inherited from NSObject)
SetValueForKeyPath(IntPtr, NSString)	A constructor used when creating managed representations of unmanaged objects; Called by the runtime. (Inherited from NSObject)
SetValueForKeyPath(NSObject, NSString)	Sets the value of a property that can be reached using a keypath. (Inherited from NSObject)
SetValueForUndefinedKey(NSObject, NSString)	Indicates an attempt to write a value to an undefined key. If not overridden, raises an NSUndefinedKeyException. (Inherited from NSObject)
SetValuesForKeysWithDictionary(NSDictionary)	Sets the values of this NSObject to those in the specified dictionary. (Inherited from NSObject)
ToString()	Returns a string representation of the value of the current instance. (Inherited from NSObject)
Unbind(NSString)	(Inherited from NSObject)
Unbind(String)	(Inherited from NSObject)
ValueForKey(NSString)	Returns the value of the property associated with the specified key. (Inherited from NSObject)
ValueForKeyPath(NSString)	Returns the value of a property that can be reached using a keypath. (Inherited from NSObject)
ValueForUndefinedKey(NSString)	Indicates an attempt to read a value of an undefined key. If not overridden, raises an NSUndefinedKeyException. (Inherited from NSObject)
WillChange(NSKeyValueChange, NSIndexSet, NSString)	Indicates that the values of the specified indices in the specified key are about to change. (Inherited from NSObject)
WillChange(NSString, NSKeyValueSetMutationKind, NSSet)	(Inherited from NSObject)
WillChangeValue(String)	Indicates that the value of the specified key is about to change. (Inherited from NSObject)

Extension Methods

GetDebugDescription(INSObjectProtocol)
GetAccessibilityCustomRotors(NSObject)	Gets the array of UIAccessibilityCustomRotor objects appropriate for this object.
SetAccessibilityCustomRotors(NSObject, UIAccessibilityCustomRotor[])	Sets the array of UIAccessibilityCustomRotor objects appropriate for this object.

See also

• Apple documentation for JSContext