android - Select a portion of image in ImageView and retrieve the end points of selected rectangle

Question

I need to drag and select a portion of an image set in an ImageView and retrieve the end points of the selected rectangle without causing any modifications (such as crop).

So far all I've been able to do is figure out the co-ordinates of the point where the user taps the screen.

ImageView imageView = (ImageView) findViewById(R.id.imageView);
imageView.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        //Simply displays a toast
        Utilities.displayToast(getApplicationContext(), "Touch coordinates : " +
                String.valueOf(event.getX()) + "x" + String.valueOf(event.getY())); 
        return true;
    }
});

But this is far from where I want to be. And I really haven't been able to find anything related on StackOverFlow/Google.

How can I implement it?

Answer 1

Here's one way, that You can use (however, there's lot of possibilities to implement the same). It's based on creation of custom View for drawing and tracking of selection rectangle. Also, You can just apply the logic from onTouch() of custom view in yours OnTouchListener().

Main layout:

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:id="@+id/root"
    android:background="@android:color/background_dark">

    <ImageView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/image"
        android:src="@drawable/up_image"
        android:scaleType="fitXY" />

    <com.example.TestApp.DragRectView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/dragRect" />

</RelativeLayout>

Custom view:

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.text.TextPaint;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class DragRectView extends View {

    private Paint mRectPaint;

    private int mStartX = 0;
    private int mStartY = 0;
    private int mEndX = 0;
    private int mEndY = 0;
    private boolean mDrawRect = false;
    private TextPaint mTextPaint = null;

    private OnUpCallback mCallback = null;

    public interface OnUpCallback {
        void onRectFinished(Rect rect);
    }

    public DragRectView(final Context context) {
        super(context);
        init();
    }

    public DragRectView(final Context context, final AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public DragRectView(final Context context, final AttributeSet attrs, final int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    /**
     * Sets callback for up
     *
     * @param callback {@link OnUpCallback}
     */
    public void setOnUpCallback(OnUpCallback callback) {
        mCallback = callback;
    }

    /**
     * Inits internal data
     */
    private void init() {
        mRectPaint = new Paint();
        mRectPaint.setColor(getContext().getResources().getColor(android.R.color.holo_green_light));
        mRectPaint.setStyle(Paint.Style.STROKE);
        mRectPaint.setStrokeWidth(5); // TODO: should take from resources

        mTextPaint = new TextPaint();
        mTextPaint.setColor(getContext().getResources().getColor(android.R.color.holo_green_light));
        mTextPaint.setTextSize(20);
    }

    @Override
    public boolean onTouchEvent(final MotionEvent event) {

        // TODO: be aware of multi-touches
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mDrawRect = false;
                mStartX = (int) event.getX();
                mStartY = (int) event.getY();
                invalidate();
                break;

            case MotionEvent.ACTION_MOVE:
                final int x = (int) event.getX();
                final int y = (int) event.getY();

                if (!mDrawRect || Math.abs(x - mEndX) > 5 || Math.abs(y - mEndY) > 5) {
                    mEndX = x;
                    mEndY = y;
                    invalidate();
                }

                mDrawRect = true;
                break;

            case MotionEvent.ACTION_UP:
                if (mCallback != null) {
                    mCallback.onRectFinished(new Rect(Math.min(mStartX, mEndX), Math.min(mStartY, mEndY),
                            Math.max(mEndX, mStartX), Math.max(mStartY, mEndY)));
                }
                invalidate();
                break;

            default:
                break;
        }

        return true;
    }

    @Override
    protected void onDraw(final Canvas canvas) {
        super.onDraw(canvas);

        if (mDrawRect) {
            canvas.drawRect(Math.min(mStartX, mEndX), Math.min(mStartY, mEndY),
                    Math.max(mEndX, mStartX), Math.max(mEndY, mStartY), mRectPaint);
            canvas.drawText("  (" + Math.abs(mStartX - mEndX) + ", " + Math.abs(mStartY - mEndY) + ")",
                    Math.max(mEndX, mStartX), Math.max(mEndY, mStartY), mTextPaint);
        }
    }
}

Activity is simple:

public class MyActivity extends Activity {

    private static final String TAG = "MyActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        final DragRectView view = (DragRectView) findViewById(R.id.dragRect);

        if (null != view) {
            view.setOnUpCallback(new DragRectView.OnUpCallback() {
                @Override
                public void onRectFinished(final Rect rect) {
                    Toast.makeText(getApplicationContext(), "Rect is (" + rect.left + ", " + rect.top + ", " + rect.right + ", " + rect.bottom + ")",
                            Toast.LENGTH_LONG).show();
                }
            });
        }
    }
}

The output is like the following: