GWT JSNI javascript to Java not working

Question

I am trying to invoke a Java Method from my Javascript code. This is for a Windows Phone 7 app using Phonegap.

I have the following in my javascript code.

document.addEventListener("backbutton", onBackKeyDown, false);
 function onBackKeyDown(){

    }

And in my Java code I have the following.

  public static native void exportStaticMethod() /*-{
    $wnd.onBackKeyDown = 
        $entry([email protected]::hideSettingsWidgets());

    }-*/;

Then in the on onModuleLoad() I am calling it like so:

MyApp.exportStaticMethod();

It does not work I have an alert in the hideSettingsWidgets() but it never gets shown.

*EDIT* Here is some more code. The EventListener is not added in the Javascript. It is specifically added withing the java code. I could not get the listeners to register originally so here is what I added.

public static native void removeBackListener() /*-{
   $wnd.removeTheListener();
}-*/;

And in my JavaScript

function removeTheListener(){
        document.removeEventListener("backbutton", onBackKeyDown, false);

        }

Here is my call to hideSettingsWidgets()

public void hideSettingsWidgets(){
        for(int i=0;i<settingsScreenWidgets.length;i++){
            settingsScreenWidgets[i].setVisible(false);
        }
        alertString("Working");
        removeBackListener();



    }

And I am calling the method you gave me inside showSettingsWidgets()

p

rivate void showSettingsWidgets(){
        for(int i=0;i<settingsScreenWidgets.length;i++){
            settingsScreenWidgets[i].setVisible(true);
        }
        setCurrentImage();
        setOnOffImage();
        setupJavaHandler();

    }

It does seem to be adding the EventListener that is inside your

public native void setupJavaHandler() /*-{
      var app = this;
      var onBackKeyDown = $entry(function() {
        [email protected]::hideSettingsWidgets();
      });
      $doc.addEventListener("backbutton", onBackKeyDown, false);
    }-*/;

So I am not sure where I am going wrong. I did not add the ArrayList<> you mentioned because was not sure to and the Event Listener was not running when page was Loaded.

Seems like showSettingsWidgets() never gets run

Community · Accepted Answer · 2017-05-23 12:33:14Z

2

The addEventListener code is probably running when the page loads, right? This will map your empty function onBackKeyDown to the backbutton event. Then, when your module loads, you attempt to redefine the onBackKeyDown function to be a new one - but the old one was already attached to the event you are trying to listen to.

This is roughly the equivalent of this (with strings instead of listener functions):

// first, make the thing to hold the 'listener', and define the first one
List<String> strings = new ArrayList<String>();
String onBackKeyDown = "abcd";
strings.add(onBackKeyDown);

// then, redefine the string, but don't change the list!
onBackKeyDown = "zyxw";

assert strings.contains(onBackKeyDown) : "Whoops, reassigned, but not added!";

To fix this, you need a cross between what you are doing in your other question, Adding Eventlisteners to document with GWT JSNI, and what you are doing here. Wrapping the Java function in an $entry call, and passing that to $doc.addEventListener makes the most logical sense (though I don't know a lot about WP7).

public static native void setupJavaHandler() /*-{
  var onBackKeyDown = $entry([email protected]::hideSettingsWidgets());
  $doc.addEventListener("backbutton", onBackKeyDown, false);
}-*/;

One more thing - remembering that we are writing JavaScript in that native code, what is going to be this when that hideSettingsWidgets() method is called? JavaScript doesn't know that all Java instance methods need a this to run on (and JavaScript has no problem running methods for object A on B - A.method.call(B) is totally legal, and often helpful). We need to be sure that this means what we think it does:

public static native void setupJavaHandler() /*-{
  var app = this;
  var onBackKeyDown = $entry(function() {
    [email protected]::hideSettingsWidgets();
  });
  $doc.addEventListener("backbutton", onBackKeyDown, false);
}-*/;

Edit: Oops, turns out your method was static anyway, so this doesn't actually mean anything! Either change exportStaticMethod/setupJavaHandler to be non static and call it directly (probably in your onModuleLoad as you have it now), or pass in an instance to call hideSettingsWidgets() on, like we are doing with app in the previous sample.

public native void setupJavaHandler() /*-{
  var app = this;
  var onBackKeyDown = $entry(function() {
    [email protected]::hideSettingsWidgets();
  });
  $doc.addEventListener("backbutton", onBackKeyDown, false);
}-*/;
// in onModuleLoad:
setupJavaHandler();

or

public static native void setupJavaHandler(MpApp app) /*-{
  //var app = this;
  var onBackKeyDown = $entry(function() {
    [email protected]::hideSettingsWidgets();
  });
  $doc.addEventListener("backbutton", onBackKeyDown, false);
}-*/;

// in onModuleLoad:
MyApp.setupJavaHandler(this);

edited May 23, 2017 at 12:33

CommunityBot

11 silver badge

answered Mar 31, 2012 at 1:05

Colin Alworth

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5 Comments

james Over a year ago

Hi. The addEventListener code is not added or running when the page loads it is specifically added in the java code. I also remove it by calling a javascript function. I have added my relevant code above. Thanks for the help.

james Over a year ago

Also just to be sure do I still need the empty onBackKeyDown function in my hosted html page?

james Over a year ago

It must be working right. I downloaded GWT Phonegap and it is doing the same thing. I would not be surprised if IE on mobile does not support JSNI fully.

james Over a year ago

Hi I have confirmed it was an issue with GWT JSNI. I made a plain HTML5 app and the 'backbutton' code is working right. Thanks for the help.

funkybro Over a year ago

I'm not quite sure how your GWT app integrates with Phonegap, but if it is running locally on the browser (rather than loaded from a server) you need to make sure GWT is using the ie6 user agent target.

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