ZoomableImageView

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Matrix;
import android.graphics.PointF;
import android.graphics.drawable.BitmapDrawable;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.view.View;
import android.widget.ImageView;


public class ZoomableImageView extends ImageView {

    static final int NONE = 0;

    static final int DRAG = 1;

    static final int ZOOM = 2;

    static final int CLICK = 3;

    Matrix matrix = new Matrix();

    int mode = NONE;

    PointF last = new PointF();

    PointF start = new PointF();

    float minScale = 1f;

    float maxScale = 4f;

    float[] m;

    float redundantXSpace, redundantYSpace;

    float width, height;

    float saveScale = 1f;

    float right, bottom, origWidth, origHeight, bmWidth, bmHeight;

    ScaleGestureDetector mScaleDetector;

    Context context;

    public ZoomableImageView(Context context, AttributeSet attr) {
        super(context, attr);
        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()) {
                    //when one finger is touching
                    //set the mode to DRAG
                    case MotionEvent.ACTION_DOWN:
                        last.set(event.getX(), event.getY());
                        start.set(last);
                        mode = DRAG;
                        break;
                    //when two fingers are touching
                    //set the mode to ZOOM
                    case MotionEvent.ACTION_POINTER_DOWN:
                        last.set(event.getX(), event.getY());
                        start.set(last);
                        mode = ZOOM;
                        break;
                    //when a finger moves
                    //If mode is applicable move image
                    case MotionEvent.ACTION_MOVE:
                        //if the mode is ZOOM or
                        //if the mode is DRAG and already zoomed
                        if (mode == ZOOM || (mode == DRAG && saveScale > minScale)) {
                            float deltaX = curr.x - last.x;// x difference
                            float deltaY = curr.y - last.y;// y difference
                            float scaleWidth = Math.round(origWidth * saveScale);// width after applying current scale
                            float scaleHeight = Math.round(origHeight * saveScale);// height after applying current scale
                            //if scaleWidth is smaller than the views width
                            //in other words if the image width fits in the view
                            //limit left and right movement
                            if (scaleWidth < width) {
                                deltaX = 0;
                                if (y + deltaY > 0) {
                                    deltaY = -y;
                                } else if (y + deltaY < -bottom) {
                                    deltaY = -(y + bottom);
                                }
                            }
                            //if scaleHeight is smaller than the views height
                            //in other words if the image height fits in the view
                            //limit up and down movement
                            else if (scaleHeight < height) {
                                deltaY = 0;
                                if (x + deltaX > 0) {
                                    deltaX = -x;
                                } else if (x + deltaX < -right) {
                                    deltaX = -(x + right);
                                }
                            }
                            //if the image doesnt fit in the width or height
                            //limit both up and down and left and 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);
                                }
                            }
                            //move the image with the matrix
                            matrix.postTranslate(deltaX, deltaY);
                            //set the last touch location to the current
                            last.set(curr.x, curr.y);
                        }
                        break;
                    //first finger is lifted
                    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;
                    // second finger is lifted
                    case MotionEvent.ACTION_POINTER_UP:
                        mode = NONE;
                        break;
                }
                setImageMatrix(matrix);
                invalidate();
                return true;
            }

        });
        resetView();
    }

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

    public void resetView() {
        BitmapDrawable bitmapDrawable = (BitmapDrawable) getDrawable();
        setImageBitmap(bitmapDrawable.getBitmap());
    }

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

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

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

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {

        @Override
        public boolean onScaleBegin(ScaleGestureDetector detector) {
            mode = ZOOM;
            return true;
        }

        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            float mScaleFactor = detector.getScaleFactor();
            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;
        }
    }
}

Example of usage :

  <com.monpackage.ZoomableImageView
        android:id="@+id/store_detail_map"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scaleType="center"
        android:layout_weight="1"
        android:src="@drawable/mon_image" />