Question

At first my programm looks like this:

But when i am moving the picture around the screen programm turns to:

Red graph is random-generated in onDraw method. So i want to draging picture around and have static background that not inavalidates while im moving picture.

here is my code:
Custom View:
package com.graph.base;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.View;
import java.util.Random;

public class CustomView extends View
{
Random random=new Random();
public CustomView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
}

public CustomView(Context context, AttributeSet attrs) {
    super(context, attrs);
}

public CustomView(Context context) {
    super(context);
}
public void onDraw (Canvas canvas)
{
    super.onDraw(canvas);
    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    paint.setStyle(Paint.Style.FILL);
    paint.setColor(Color.WHITE);
    canvas.drawPaint(paint);
    Path path=new Path();
    path.moveTo(10, 10);
    for (float i=0.5f; i<=140; i+=10)
    {
        path.quadTo(10+i*5, 10+random.nextInt(500), 10+(i+10)*5,     10+random.nextInt(500));
    }
    paint.setDither(true);
    paint.setColor(Color.RED);
    paint.setStyle(Paint.Style.STROKE);    
    paint.setStrokeJoin(Paint.Join.ROUND);
    paint.setStrokeCap(Paint.Cap.ROUND);
    paint.setStrokeWidth(2);
    canvas.drawPath(path, paint);
}

}

MainActivity

package com.graph.base;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.MotionEvent;
import android.view.View;
import android.widget.FrameLayout;
import android.widget.ImageView;

public class MainActivity extends Activity {
FrameLayout frame;
ImageView image;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    frame=(FrameLayout)findViewById(R.id.frameLayout1);
    image=(ImageView)findViewById(R.id.imageView1);
    image.setOnTouchListener(new View.OnTouchListener() {

        public boolean onTouch(View v, MotionEvent event) {
            Log.e("TAG", "Touch happend");
            switch (event.getAction())
            {
            case MotionEvent.ACTION_MOVE:
            {
                image.setPadding((int)event.getX(),     (int)event.getY(), 0, 0);
                break;
            }

            }

            return true;
        }
    });
}

public void invalidateButton_Click (View v)
{
    frame.invalidate();
}
}

Answer 1

You’re drawing random lines onto your canvas. This code is called each time the screen is invalidated. If you want to keep the drawing from changing, then set a bitmap into a canvas and draw into the bitmap. In your onDraw() method you’ll then draw the bitmap onto the canvas.

// Outside of onDraw()
Bitmap bmDest = Bitmap.createBitmap(width, height, Bitmap.Config.RGB_565);
Canvas mycanvas = new Canvas(bmDest);
// Do your drawing on the canvas outside of the onDraw() method

In onDraw(), you’ll simply do this:

canvas.drawBitmap(bmDest, x, y, null);