java - How does TouchImageView works?

Question

I have good working TouchImageView and i want to know how does it work in a code : What i just want to do is be able to pinch to zoom, or use double tap to zoom on any imageview i choose, and when i zoom back i return to the original size of the image. TouchImageView.java :

public class TouchImageView extends ImageView {

Matrix matrix = new Matrix();

static final int NONE = 0;
static final int DRAG = 1;
static final int ZOOM = 2;
int mode = NONE;

PointF last = new PointF();
PointF start = new PointF();
float minScale = 1f;
float maxScale = 3f;
float[] m;

float redundantXSpace, redundantYSpace;

float width, height;
static final int CLICK = 3;
float saveScale = 1f;
float right, bottom, origWidth, origHeight, bmWidth, bmHeight;

ScaleGestureDetector mScaleDetector;

Context context;


public TouchImageView(Context context) {
super(context);
super.setClickable(true);
this.context = context;
mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
matrix.setTranslate(1f, 1f);
m = new float[9];
setImageMatrix(matrix);
setScaleType(ScaleType.MATRIX);

setOnTouchListener(new OnTouchListener() {

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        mScaleDetector.onTouchEvent(event);

        matrix.getValues(m);
        float x = m[Matrix.MTRANS_X];
        float y = m[Matrix.MTRANS_Y];
        PointF curr = new PointF(event.getX(), event.getY());

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                last.set(event.getX(), event.getY());
                start.set(last);
                mode = DRAG;
                break;
            case MotionEvent.ACTION_MOVE:
                if (mode == DRAG) {
                    float deltaX = curr.x - last.x;
                    float deltaY = curr.y - last.y;
                    float scaleWidth = Math.round(origWidth * saveScale);
                    float scaleHeight = Math.round(origHeight * saveScale);
                    if (scaleWidth < width) {
                        deltaX = 0;
                        if (y + deltaY > 0)
                            deltaY = -y;
                        else if (y + deltaY < -bottom)
                            deltaY = -(y + bottom); 
                    } else if (scaleHeight < height) {
                        deltaY = 0;
                        if (x + deltaX > 0)
                            deltaX = -x;
                        else if (x + deltaX < -right)
                            deltaX = -(x + right);
                    } else {
                        if (x + deltaX > 0)
                            deltaX = -x;
                        else if (x + deltaX < -right)
                            deltaX = -(x + right);

                        if (y + deltaY > 0)
                            deltaY = -y;
                        else if (y + deltaY < -bottom)
                            deltaY = -(y + bottom);
                    }
                    matrix.postTranslate(deltaX, deltaY);
                    last.set(curr.x, curr.y);
                }
                break;

            case MotionEvent.ACTION_UP:
                mode = NONE;
                int xDiff = (int) Math.abs(curr.x - start.x);
                int yDiff = (int) Math.abs(curr.y - start.y);
                if (xDiff < CLICK && yDiff < CLICK)
                    performClick();
                break;

            case MotionEvent.ACTION_POINTER_UP:
                mode = NONE;
                break;
        }
        setImageMatrix(matrix);
        invalidate();
        return true; // indicate event was handled
    }

});
}

@Override
public void setImageBitmap(Bitmap bm) { 
super.setImageBitmap(bm);
bmWidth = bm.getWidth();
bmHeight = bm.getHeight();
}

public void setMaxZoom(float x)
{
maxScale = x;
}

private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
@Override
public boolean onScaleBegin(ScaleGestureDetector detector) {
    mode = ZOOM;
    return true;
}

@Override
public boolean onScale(ScaleGestureDetector detector) {
    float mScaleFactor = (float)Math.min(Math.max(.95f, detector.getScaleFactor()), 1.05);
    float origScale = saveScale;
    saveScale *= mScaleFactor;
    if (saveScale > maxScale) {
        saveScale = maxScale;
        mScaleFactor = maxScale / origScale;
    } else if (saveScale < minScale) {
        saveScale = minScale;
        mScaleFactor = minScale / origScale;
    }
    right = width * saveScale - width - (2 * redundantXSpace * saveScale);
    bottom = height * saveScale - height - (2 * redundantYSpace * saveScale);
    if (origWidth * saveScale <= width || origHeight * saveScale <= height) {
        matrix.postScale(mScaleFactor, mScaleFactor, width / 2, height / 2);
        if (mScaleFactor < 1) {
            matrix.getValues(m);
            float x = m[Matrix.MTRANS_X];
            float y = m[Matrix.MTRANS_Y];
            if (mScaleFactor < 1) {
                if (Math.round(origWidth * saveScale) < width) {
                    if (y < -bottom)
                        matrix.postTranslate(0, -(y + bottom));
                    else if (y > 0)
                        matrix.postTranslate(0, -y);
                } else {
                    if (x < -right) 
                        matrix.postTranslate(-(x + right), 0);
                    else if (x > 0) 
                        matrix.postTranslate(-x, 0);
                }
            }
        }
    } else {
        matrix.postScale(mScaleFactor, mScaleFactor, detector.getFocusX(), detector.getFocusY());
        matrix.getValues(m);
        float x = m[Matrix.MTRANS_X];
        float y = m[Matrix.MTRANS_Y];
        if (mScaleFactor < 1) {
            if (x < -right) 
                matrix.postTranslate(-(x + right), 0);
            else if (x > 0) 
                matrix.postTranslate(-x, 0);
            if (y < -bottom)
                matrix.postTranslate(0, -(y + bottom));
            else if (y > 0)
                matrix.postTranslate(0, -y);
        }
    }
    return true;

}
}

@Override
protected void onMeasure (int widthMeasureSpec, int heightMeasureSpec)
{
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
width = MeasureSpec.getSize(widthMeasureSpec);
height = MeasureSpec.getSize(heightMeasureSpec);
//Fit to screen.
float scale;
float scaleX =  (float)width / (float)bmWidth;
float scaleY = (float)height / (float)bmHeight;
scale = Math.min(scaleX, scaleY);
matrix.setScale(scale, scale);
setImageMatrix(matrix);
saveScale = 1f;

// Center the image
redundantYSpace = (float)height - (scale * (float)bmHeight) ;
redundantXSpace = (float)width - (scale * (float)bmWidth);
redundantYSpace /= (float)2;
redundantXSpace /= (float)2;

matrix.postTranslate(redundantXSpace, redundantYSpace);

origWidth = width - 2 * redundantXSpace;
origHeight = height - 2 * redundantYSpace;
right = width * saveScale - width - (2 * redundantXSpace * saveScale);
bottom = height * saveScale - height - (2 * redundantYSpace * saveScale);
setImageMatrix(matrix);
}

}

To use is i created a private class :

private class CreateImage extends AsyncTask<String, Void, Drawable> {
    protected void onPreExecute() {
    }

    protected Drawable doInBackground(String... urls) {
        InputStream is;
        Drawable d = null ;
        try {
            is = (InputStream)new URL(urls[0]).getContent();
            d = Drawable.createFromStream(is, "Image");
            return d;
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return d;
    }
    protected void onPostExecute(Drawable d) {
        touch.setMaxZoom(4f);
        touch.setImageDrawable(d);
        setContentView(touch);
    }
}
public void createUrlImage(String url){
    new CreateImage().execute(url);
}

and in the onCreate() i put createUrlImage(url). I modified TouchImageView by adding :

public void setImageDrawable(Drawable dr) { 
super.setImageDrawable(dr);
bmWidth = dr.getIntrinsicWidth();
bmHeight = dr.getIntrinsicHeight();
}

Answer 1

EDIT: Double Tap Zoom, Fling and other features have been added to TouchImageView since I originally answered this question. You can check it out on github here.

---

I added some usage details to the original post here. The code already has pinch zoom and panning, along with boundaries. Also, zooming out will return you to the original sized image.

Adding double tap zoom will take more work. You'll have to use a GestureDetector and override onDoubleTap and onSingleTapConfirmed. You'll then want to make sure you pass your touch events to gestureDetector, without interfering with the rest of the code (see how events are passed to mScaleDetector at the beginning of onTouch). You'll want to get rid of the call to performClick() in ACTION_UP and instead place it in onSingleTapConfirmed. You can check this answer for some skeletal code to get you started in implementing GestureDetector.

Let me know if you manage to get double tap zoom solid and I'll add your changes to the original post and the Github repo.