Qt Android: How to call Toast.makeText from Java?

[Ibrahim]

Hi; I want to call Toast.makeText method from Java in Qt / C++. Codes:
I added QT += androidextras in .pro file.

Toast.java:

package com.classes.java;

public class Toast {
  public static void makeText(String text, int duration) {
    Toast.makeText(getApplicationContext(), text, duration).show();
  }
}

toast.h:

#ifndef TOAST_H
#define TOAST_H

#include <QObject>
#include <QAndroidJniObject>

class Toast : public QObject
{
  Q_OBJECT
public:
  explicit Toast(QObject* parent = 0);
  Q_INVOKABLE void make_text(QString& text, int duration);

signals:

public slots:
};

#endif // TOAST_H

toast.cpp:

#include "toast.h"

Toast::Toast(QObject* parent) : QObject(parent)
{
}

void Toast::make_text(QString& text, int duration)
{
  QAndroidJniObject txt = QAndroidJniObject::fromString(text);
  QAndroidJniObject::callStaticObjectMethod(
      "com/classes/java/Toast",
      "makeText",
      "(Ljava/lang/String;I)V",
      txt.object<jstring>(), duration);
}

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQml>

#include "toast.h"

int main(int argc, char* argv[])
{
  QGuiApplication app(argc, argv);

  QQmlApplicationEngine engine;
  qmlRegisterType<Toast>("java.class", 1, 0, "Toast");
  engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

  return app.exec();
}

main.qml:

import QtQuick 2.6
import QtQuick.Window 2.2
import Qt.labs.controls 1.0
import java.class 1.0;

Window {
  visible: true

  Toast {
    id: toast
  }

  Button {
    id: button
    text: "Click Me!"
    onClicked: toast.make_text("Hello World!", 2000)
  }
}

I get this error message:

UserPath\build-QtMobileTest-Android_for_armeabi_v7a_GCC_4_9_Qt_5_6_0-Release\android-build\src\com\classes\java\Toast.java:9: error: cannot find symbol
    Toast.makeText(getApplicationContext(), text, duration).show();
                   ^
  symbol:   method getApplicationContext()
  location: class Toast
Note: UserPath\build-QtMobileTest-Android_for_armeabi_v7a_GCC_4_9_Qt_5_6_0-Release\android-build\__qt5__android__files__\src\org\qtproject\qt5\android\bindings\QtActivity.java uses or overrides a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
1 error
:compileDebugJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileDebugJava'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

I think I need android context for getApplicationContext() but I do not know how can I get context? How can I call Toast.makeText method using Qt / C++ JNI? Thanks.

[Carmoneer]

You need to create a static instance of your main java class object and call the method that way. So in your 'JavaMain':

public class MyJavaMain extends QtActivity {

    // Singleton Instance
    public  static MyJavaMain mInstance;

    ...

    public MyJavaMain () {
        Log.d(QtApplication.QtTAG, "Java constructor called.");
        mInstance = this;
    }

    ...

    public static MyMethodCallToast(final String message) {
         mInstance.runOnUiThread(new Runnable() {
            public void run() {
               Toast.makeText(mInstance.getApplicationContext(),
                              message,
                              Toast.LENGTH_SHORT).show();
           }
        });
    }  
}

Hopefully that helps. Cheers!

[Ibrahim]

@Carmoneer Thanks, I get this error:

W/dalvikvm( 7847): Exception thrown (Ljava/lang/NullPointerException;) while throwing internal exception (Ljava/lang/NullPointerException;)
W/InputMethodManagerService(  589): Starting input on non-focused client com.android.internal.view.IInputMethodClient$Stub$Proxy@429dfe58 (uid=10170 pid=7847)

NullPointerException error?

[Carmoneer]

I think it may be an issue with the signature call on the .cpp side, specifically "(Ljava/lang/String;I)V". Try:

QString toastMessage = "My message";
QAndroidJniObject javaMessage = QAndroidJniObject::fromString(toastMessage);
QAndroidJniObject::callStaticMethod<void>("com/app/myApp/MyJavaMain",
                                          "MyMethodCallToast",
                                          "(Ljava/lang/String;)V",
                                          javaMessage.object<jstring>());

[Ibrahim]

@Carmoneer Thanks, but I get same error again. This NullPointerException is text.

[Ibrahim]

I tried:

// ToastMessage.java file
...
public static void makeText(final String text) {
    mInstance.runOnUiThread(new Runnable() {
      public void run() {
        Toast.makeText(mInstance.getApplicationContext(), "text", Toast.LENGTH_LONG).show();
      }
    });
  }
...

This code give error. I put the string ("text") but I get still NullPointerException error.

[Carmoneer]

The only other thing I can think of is maybe because you have a separate 'Toast' class, it's calling that nullpointerexception. I don't have a separate class for my Toast calls.

Maybe

Toast.makeText(Classname.class, message, Toast.LENGTH_SHORT).show();

is what you need to do?

I simply import android.widget.Toast and make my Toast calls.

[Ibrahim]

@Carmoneer My ToastMessage.java file:

package com.classes.java;

import org.qtproject.qt5.android.bindings.QtActivity;
import android.widget.Toast;

public class ToastMessage extends QtActivity {
  public static ToastMessage mInstance;

  public ToastMessage() {
    mInstance = this;
  }

  public static String msg() {
    return "<HELLO WORLD!>";
  }

  public static void makeText(final String text) {
    mInstance.runOnUiThread(new Runnable() {
      public void run() {
        Toast.makeText(mInstance.getApplicationContext(), "text", Toast.LENGTH_LONG).show();
      }
    });
  }
}

I want to learn how to call java codes from C++. Have you an example project for the android toast message?

[Carmoneer]

This is how I extend the framework for QtActivity:

package com.app.myapp;

import org.qtproject.qt5.android.bindings.QtApplication;
import org.qtproject.qt5.android.bindings.QtActivity;


import android.widget.Toast;


/** --------------------------------------------------------------------------
** @MyJavaNatives
** Static methods called from .cpp side. Sends data to .cpp
** -------------------------------------------------------------------------*/
class MyJavaNatives
{
    public static native void showToast(final String message);
}

/** --------------------------------------------------------------------------
** @JavaMain (Only one class can extend QtActivity)
** -------------------------------------------------------------------------*/
public class JavaMain extends QtActivity {
    // Singleton Instance
    public  static JavaMain mInstance;

    public JavaMain() {
        Log.d(QtApplication.QtTAG, "Java constructor called.");
        mInstance = this;
    }

    // Called at the start of the full lifetime.
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(QtApplication.QtTAG, "onCreate Called");
    }


    // Called at the start of the visible lifetime.
    @Override
    protected void onStart() {
        super.onStart();
    }

    // Called at the start of the active lifetime.
    @Override
    public synchronized void onResume() {
        super.onResume();
    }

    // Called at the end of the active lifetime.
    @Override
    protected synchronized void onPause() {
        super.onPause();
    }

    // Called at the end of the full lifetime.
    @Override
    protected void onDestroy() {
        // Clean up any resources including ending threads,
        // closing database connections etc.
        super.onDestroy();
    }

    // Called at the end of the visible lifetime.
    @Override
    protected void onStop() {
        // Suspend remaining UI updates, threads, or processing
        // that aren’t required when the Activity isn’t visible.
        // Persist all edits or state changes
        // as after this call the process is likely to be killed.
        super.onStop();
 
    }

    // Called before subsequent visible lifetimes
    // for an activity process.
    @Override
    protected void onRestart() {
        // Load changes knowing that the activity has already
        // been visible within this process.
        super.onRestart();
    }


    /** ------------------------------------------------------------------------
    ** @Toast
    ** Show toast messge.
    ** -----------------------------------------------------------------------*/
    public static void showToast(final String message) {
        mInstance.runOnUiThread(new Runnable() {
           public void run() {
               Toast.makeText(mInstance.getApplicationContext(),
                              message,
                              Toast.LENGTH_SHORT).show();
           }
        });
    }
}

and then on the .cpp to call the message:

            QAndroidJniObject::callStaticMethod<void>("com/app/myapp/JavaMain",
                                                      "showToast",
                                                      "(Ljava/lang/String;)V",
                                                      QAndroidJniObject::fromString("This is my toast message!").object<jstring>());

That is the basics to setting up a 'Main' java class that extends QtActivity. Hopefully that should at least steer you in the right direction. Good luck :)

[Ibrahim]

@Carmoneer Ohh thanks. I tried this project, but it give same error:

W/dalvikvm(18212): Exception thrown (Ljava/lang/NullPointerException;) while throwing internal exception (Ljava/lang/NullPointerException;)
I/InputDispatcher(  589): Delivering touch to (18212): action: 0x0, toolType: 1
I/InputDispatcher(  589): Delivering touch to (18212): action: 0x1, toolType: 1

I am sharing this project source codes: Project Source Codes.

[douyw]

There is an android-extra example called 'notification' in Qt SDK examples. Just change the notify function in NotificationClient.java to display the toast. Maybe it will work.

[Ibrahim]

@douyw thanks, I tried this method and it is work. Interesting! I do not understand. What is the problem? How can I learn it?