AlphaImageView.java

import android.content.Context;
import android.os.Build;
import android.support.annotation.RequiresApi;
import android.support.v7.widget.AppCompatImageView;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

import static android.view.MotionEvent.ACTION_DOWN;
import static android.view.MotionEvent.ACTION_MOVE;
import static android.view.MotionEvent.ACTION_UP;

@RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN)
public class AlphaImageView extends AppCompatImageView {

    private float rawX;
    private int alpha;
    private AlphaChangeListener listener;

    public AlphaImageView(Context context) {
        super(context);
        this.rawX = -1.0f;
        this.alpha = -1;
    }

    public AlphaImageView(Context context, AttributeSet attributeSet) {
        super(context, attributeSet);
        this.rawX = -1.0f;
        this.alpha = -1;
    }

    public AlphaImageView(Context context, AttributeSet attributeSet, int i) {
        super(context, attributeSet, i);
        this.rawX = -1.0f;
        this.alpha = -1;
    }

    public void setAlphaListener(AlphaChangeListener listener) {
        this.listener = listener;
    }


    public void reset() {
        setImageAlpha(255);
    }

    @Override
    public boolean onTouchEvent(MotionEvent motionEvent) {
        switch (motionEvent.getAction()) {
            case ACTION_DOWN:

                rawX = motionEvent.getRawX();
                alpha = getImageAlpha();

                if (listener != null) {
                    float alpha = (float) (((double) getImageAlpha()) / 255.0d);
                    listener.onStart(alpha);
                    break;
                }
                break;
            case ACTION_UP:
                rawX = -1.0f;
                alpha = -1;

                if (listener != null) {
                    listener.onStop();
                    break;
                }
                break;
            case ACTION_MOVE:
                if (getVisibility() == View.VISIBLE && getDrawable() != null &&
                        motionEvent.getActionIndex() == ACTION_DOWN && rawX >= 0.0f) {
                    int min = Math.min(alpha -
                            ((int) ((((double) (rawX - motionEvent.getRawX())) / (((double) getMeasuredWidth()) * 0.6d)) * 255.0d)), 255);
                    int max = Math.max(0, min);
                    setImageAlpha(max);
                    if (listener != null) {
                        float alpha = (float) (((double) max) / 255.0d);
                        listener.onChange(alpha);
                        break;
                    }
                }
                break;
        }
        return true;
    }

    public interface AlphaChangeListener {
        void onStop();
        void onStart(float alpha);
        void onChange(float alpha);
    }
}