[android-developers] sliding image usging "onDraw()" and "invalidate()"

[Walton]

Hi all,

I write a simple program that can slide image by finger touch.

It seems to work fine but the edge of the image looks flickered  when
sliding.

if anyone can help me to figure out that problem?


thanks in advance,

Walton

here is the demo on Youtube:
http://www.youtube.com/watch?v=bNaJ3YGN9Co


my source code: (please put three pictures(320x480) in /res/drawable,
named a.jpg, b.jpg and c.jpg)
----------------------------

package com.example.android.picSlide;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;


public class picSlide extends Activity {
        final int width = 320, height = 480;
        final int numPic = 3;
    float [] x = new float[numPic];
    float y, xLast, yLast;
    int curIndex = 0;
    boolean drag = false;
    View myView;
    Bitmap [] img = new Bitmap[numPic];

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.
FLAG_FULLSCREEN ,WindowManager.LayoutParams. FLAG_FULLSCREEN);
        getImg();

        myView = new MyView(this);
        setContentView(myView);
    }

    public boolean onTouchEvent(MotionEvent event) {
                float x1 = event.getX(),
                          y1 = event.getY();

          switch(event.getAction()){
              case MotionEvent.ACTION_DOWN:
                  xLast = x1;
                  yLast = y1;
                  drag = false;
                  break;
              case MotionEvent.ACTION_UP:
                  changeImg();
                  drag = true;

                  myView.invalidate(0, 0, width, height);
                  break;
              case MotionEvent.ACTION_MOVE:
                  x[curIndex] += (x1-xLast);

                  if (x[curIndex] < 0 && curIndex+1 <= numPic-1) //drag to left
                          x[curIndex+1] = width + x[curIndex];
                  else if (x[curIndex] > 0 && curIndex-1 >= 0) //drag to right
                          x[curIndex-1] = (-width) + x[curIndex];

                  xLast = x1;

                  myView.invalidate(0, 0, width, height);
                  break;
          }

          return super.onTouchEvent(event);
    }

    class MyView extends View{

                @Override
            protected void onDraw(Canvas canvas) {
                    Paint mPaint = new Paint();
                    //mPaint.setAntiAlias(true);
                    slide();

                    canvas.drawBitmap(img[curIndex], x[curIndex], y, mPaint);
                    if (x[curIndex] < 0 && curIndex+1 <= numPic-1) //drag to
left
                        canvas.drawBitmap(img[curIndex+1], x[curIndex+1], y,
mPaint);
                    else if (x[curIndex] > 0 && curIndex-1 >= 0) //drag to
right
                        canvas.drawBitmap(img[curIndex-1], x[curIndex-1], y,
mPaint);

                    super.onDraw(canvas);
            }

            public MyView(Context context) {
                    super(context);
            }
    }

    private void changeImg()
    {
        final int factor = 6;

        if(x[curIndex] > width/factor && curIndex-1 >= 0){ //drag to
right
                curIndex--;
                }
                else if(x[curIndex] < -width/factor && curIndex+1 <= numPic-1){
                        curIndex++;
                }
    }

    private void getImg()
    {
        int [] pic = {R.drawable.a, R.drawable.b, R.drawable.c};

        for (int i=0; i < numPic; i++)
                img[i] = BitmapFactory.decodeResource(getResources(), pic[i]);
    }

    private void slide()
    {
        final float factor = 1.1f; //greater indicates move faster

        if (drag){
                if ((int)x[curIndex] > 0){ //drag to right
                        x[curIndex] /= factor;
                        if (curIndex-1 >= 0)
                                x[curIndex-1] = (-width) + x[curIndex];
                }
                else if ((int)x[curIndex] < 0){ //drag to left
                        x[curIndex] /= factor;
                        if (curIndex+1 <= numPic-1)
                                x[curIndex+1] = width + x[curIndex];
                }
                else{
                        x[curIndex] = 0;
                        drag = false;
                }
                myView.invalidate(0, 0, width, height);
        }
    }

}

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