Use XML onClick field to call function in DrawingView

Question

I'm developing an application in android studio. I currently have two parts, a MainActivity and a DrawingView. In the DrawingView is both a Canvas and a Button.

<com.jerem.simpledraw.DrawingView
    android:id="@+id/DrawingView"
    android:layout_width="wrap_content"
    android:layout_height="554dp"
    android:background="#ffffff"
    android:paddingBottom="40dp"
    android:paddingLeft="20dp"
    app:exampleColor="#33b5e5"
    app:exampleDimension="24sp"
    app:exampleDrawable="@android:drawable/ic_menu_add"
    app:exampleString="Hello, DrawingView" />

<Button
    android:text="Refresh"
    android:layout_height="wrap_content"
    android:id="@+id/buttonRefresh"
    android:layout_width="match_parent"
    android:layout_alignParentBottom="true"
    android:layout_alignParentStart="true"
    android:onClick="refreshCallback"/>

Above is the layout file for the drawing view. I want the button, when pressed, to call the method "refreshCallback", which is in the class DrawingView.

However, this warns me that it cannot find the method in the MainActivity.

I want to call a method in the DrawingView, not in the Main Activity. How can I fix this?

EDIT: I am updating to include the entire DrawingView class. I want to refresh the page.

package com.jerem.simpledraw;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Button;
import android.view.View.OnClickListener;

public class DrawingView extends View
{
//Constant
private int Colour = 0xFF000000;
private int screen_width;
private int screen_height;

//Drawing Var
private Canvas drawCanvas;
private Bitmap canvasBitmap;
private Path drawPath;
private Paint drawPaint;
private Paint canvasPaint;

public DrawingView(Context context, AttributeSet attributeSet){
    super(context, attributeSet);
    setupDrawing();
    setupUI();
}

private void setupUI() {

}

private void setupDrawing() {
    drawPath = new Path();
    drawPaint = new Paint();

    drawPaint.setColor(Colour);

    drawPaint.setAntiAlias(true);
    drawPaint.setStrokeWidth(20);
    drawPaint.setStyle(Paint.Style.STROKE);
    drawPaint.setStrokeJoin(Paint.Join.ROUND);
    drawPaint.setStrokeCap(Paint.Cap.ROUND);

    canvasPaint = new Paint(Paint.DITHER_FLAG);
}

@Override
protected void onDraw(Canvas canvas){
    canvas.drawBitmap(canvasBitmap, 0, 0, canvasPaint);
    canvas.drawPath(drawPath, drawPaint);
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh){
    super.onSizeChanged(w, h, oldw, oldh);
    canvasBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
    drawCanvas = new Canvas(canvasBitmap);
}

private void handelAction(int action, int x, int y){
    switch(action) {
        case 0:
            drawPath.moveTo(x, y);
            break;
        case 2:
            drawPath.lineTo(x, y);
            break;
        case 1:
            drawCanvas.drawPath(drawPath, drawPaint);
            break;
    }
    {
        invalidate();
    }
}

@Override
public boolean onTouchEvent(MotionEvent event){
    float x = event.getX();
    float y = event.getY();

    System.out.println(event.getAction());
    handelAction(event.getAction(), (int) x, (int) y);
    return true;
}

public void refreshCallback(View v){
    System.out.println("Refresh Command Sent");
    this.setupDrawing();
}

}

Answer 1

You can only call the activity's methods through the xml onClick. Therefore, you need a method in your MainActivity to pass the function call to the sibling view. If you're wondering why this is, consider this: How should the runtime decide which of the current views' methods should be called when the user clicks the button?

EDIT: Today I learned that it is in fact possible to refer to a static method in your activity class through onClick. If you intent to use non-static methods of your activity, you shouldn't declare the method as static, (and vice versa, I suppose).

Answer 2

The fact is onclick only tries to find the method that the current class is in which is using it. You can do this if you want, follow the steps :-

MainActivity:

public void callRefreshCallback(View v){
   (new DesignView()).refreshCallback(v);
}

DesignView:

public static void refreshCallback(View v){
//dosomething.
}

MainActivity.xml:

android:onClick="callRefreshCallback" 

Answer 3

Try this code in Activity class.

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.sample_drawing_view);
        DrawingView drawingView = (DrawingView) findViewById(R.id.DrawingView);
        Button buttonRefresh = (Button) findViewById(R.id.buttonRefresh);
        buttonRefresh.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                drawingView.refreshCallback(view);
            }
        });
}