Double tap to zoom on ImageView

Question

I am trying to implement zoom functionality on an ImageView, similar to the built-in Photos app on iOS (I don't have a physical android device to make a more suitable analogy). Pinch-to-zoom would be nice to have, but zoom on double-tap is essential.

I've tried some sample code (ex. http://android-journey.blogspot.com/2010/01/android-gestures.html), but this does not work for me, for some reason; none of the event handlers fire (I may be omitting something simple?).

Can someone point me in the right direction?

For troubleshooting purposes, I used sample code unmodified from the above link, on an Android 2.3 emulator in an application with android:minSdkVersion=7 and the following layout:

<?xml version="1.0" encoding="utf-8"?>
<ImageView  
xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:src="@drawable/android"
    android:id="@+id/image"
    />

Answer 1

Here I explained and wrote a code for zoom, but not on double tap, its a zooming by spanning image, try this, It worked for me:

package com.example.myapp

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

public class TouchImageView extends ImageView {

Matrix matrix;

// We can be in one of these 3 states
static final int NONE = 0;
static final int DRAG = 1;
static final int ZOOM = 2;
int mode = NONE;

// Remember some things for zooming
PointF last = new PointF();
PointF start = new PointF();
float minScale = 1f;
float maxScale = 3f;
float[] m;

int viewWidth, viewHeight;
static final int CLICK = 3;
float saveScale = 1f;
protected float origWidth, origHeight;
int oldMeasuredWidth, oldMeasuredHeight;

ScaleGestureDetector mScaleDetector;

Context context;

public TouchImageView(Context context) {
    super(context);
    sharedConstructing(context);
}

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

private void sharedConstructing(Context context) {
    super.setClickable(true);
    this.context = context;
    mScaleDetector = new ScaleGestureDetector(context, new ScaleListener());
    matrix = new Matrix();
    m = new float[9];
    setImageMatrix(matrix);
    setScaleType(ScaleType.MATRIX);

    setOnTouchListener(new OnTouchListener() {

        @Override
        public boolean onTouch(View v, MotionEvent event) {
            mScaleDetector.onTouchEvent(event);
            PointF curr = new PointF(event.getX(), event.getY());

            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                   last.set(curr);
                    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 fixTransX = getFixDragTrans(deltaX, viewWidth, origWidth * saveScale);
                        float fixTransY = getFixDragTrans(deltaY, viewHeight, origHeight * saveScale);
                        matrix.postTranslate(fixTransX, fixTransY);
                        fixTrans();
                        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
        }

    });
}

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 = detector.getScaleFactor();
        float origScale = saveScale;
        saveScale *= mScaleFactor;
        if (saveScale > maxScale) {
            saveScale = maxScale;
            mScaleFactor = maxScale / origScale;
        } else if (saveScale < minScale) {
            saveScale = minScale;
            mScaleFactor = minScale / origScale;
        }

        if (origWidth * saveScale <= viewWidth || origHeight * saveScale <= viewHeight)
            matrix.postScale(mScaleFactor, mScaleFactor, viewWidth / 2, viewHeight / 2);
        else
            matrix.postScale(mScaleFactor, mScaleFactor, detector.getFocusX(), detector.getFocusY());

        fixTrans();
        return true;
    }
}

void fixTrans() {
    matrix.getValues(m);
    float transX = m[Matrix.MTRANS_X];
    float transY = m[Matrix.MTRANS_Y];

    float fixTransX = getFixTrans(transX, viewWidth, origWidth * saveScale);
    float fixTransY = getFixTrans(transY, viewHeight, origHeight * saveScale);

    if (fixTransX != 0 || fixTransY != 0)
        matrix.postTranslate(fixTransX, fixTransY);
}

float getFixTrans(float trans, float viewSize, float contentSize) {
    float minTrans, maxTrans;

    if (contentSize <= viewSize) {
        minTrans = 0;
        maxTrans = viewSize - contentSize;
    } else {
        minTrans = viewSize - contentSize;
        maxTrans = 0;
    }

    if (trans < minTrans)
        return -trans + minTrans;
    if (trans > maxTrans)
        return -trans + maxTrans;
    return 0;
}

float getFixDragTrans(float delta, float viewSize, float contentSize) {
    if (contentSize <= viewSize) {
        return 0;
    }
    return delta;
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    viewWidth = MeasureSpec.getSize(widthMeasureSpec);
    viewHeight = MeasureSpec.getSize(heightMeasureSpec);

    //
    // Rescales image on rotation
    //
    if (oldMeasuredHeight == viewWidth && oldMeasuredHeight == viewHeight
            || viewWidth == 0 || viewHeight == 0)
        return;
    oldMeasuredHeight = viewHeight;
    oldMeasuredWidth = viewWidth;

    if (saveScale == 1) {
        //Fit to screen.
        float scale;

        Drawable drawable = getDrawable();
        if (drawable == null || drawable.getIntrinsicWidth() == 0 || drawable.getIntrinsicHeight() == 0)
            return;
        int bmWidth = drawable.getIntrinsicWidth();
        int bmHeight = drawable.getIntrinsicHeight();

        Log.d("bmSize", "bmWidth: " + bmWidth + " bmHeight : " + bmHeight);

        float scaleX = (float) viewWidth / (float) bmWidth;
        float scaleY = (float) viewHeight / (float) bmHeight;
        scale = Math.min(scaleX, scaleY);
        matrix.setScale(scale, scale);

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

        matrix.postTranslate(redundantXSpace, redundantYSpace);

        origWidth = viewWidth - 2 * redundantXSpace;
        origHeight = viewHeight - 2 * redundantYSpace;
        setImageMatrix(matrix);
    }
    fixTrans();
}

}

This is class for zooming: Now in activity_main.xml file, replace your ImageView with this class name with your package name like:

<ImageView  
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:src="@drawable/android"
    android:id="@+id/image"/>

with

<com.example.myapp.TouchImageView   
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:src="@drawable/android"
    android:id="@+id/image"/>

In your MainActivity:

TouchImageView fullImageView = (TouchImageView)findViewById(R.id.image);
fullImageView.setMaxZoom(4f);

That's it. :)

Answer 2

Depending on the application's requirements, you might want to consider having the Image be loaded in a webview. This has all of the functionality of a browser (pinch to zoom if available, double tap to zoom, overlayed buttons to zoom/zoom out. You just need to load the html with the tag referring to the image on the phone.