Handle custom Gesture

package com.java2s.myapplication3.app;
/*from w  ww. j a  va 2 s  . c  om*/
import android.content.Context;
import android.util.AttributeSet;
import android.view.GestureDetector;
import android.view.GestureDetector.SimpleOnGestureListener;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewConfiguration;
import android.widget.FrameLayout;
import android.widget.Scroller;
import android.app.Activity;
import android.os.Bundle;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.LinearLayout;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        PanGestureScrollView scrollView = new PanGestureScrollView(this);
        LinearLayout layout = new LinearLayout(this);
        layout.setOrientation(LinearLayout.VERTICAL);
        for(int i=0; i < 5; i++) {
            ImageView iv = new ImageButton(this);
            iv.setImageResource(R.drawable.ic_launcher);
            layout.addView(iv, new LinearLayout.LayoutParams(1000, 500));
        }

        scrollView.addView(layout);
        setContentView(scrollView);
    }
}


class PanGestureScrollView extends FrameLayout {

    private GestureDetector mDetector;
    private Scroller mScroller;

    /* Positions of the last motion event */
    private float mInitialX, mInitialY;
    /* Drag threshold */
    private int mTouchSlop;

    public PanGestureScrollView(Context context) {
        super(context);
        init(context);
    }

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

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

    private void init(Context context) {
        mDetector = new GestureDetector(context, mListener);
        mScroller = new Scroller(context);
        // Get system constants for touch thresholds
        mTouchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
    }

    /*
     * Override the measureChild... implementations to guarantee that the child
     * view gets measured to be as large as it wants to be. The default
     * implementation will force some children to be only as large as this view.
     */
    @Override
    protected void measureChild(View child, int parentWidthMeasureSpec,
                                int parentHeightMeasureSpec) {
        int childWidthMeasureSpec;
        int childHeightMeasureSpec;

        childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(0,
                MeasureSpec.UNSPECIFIED);
        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0,
                MeasureSpec.UNSPECIFIED);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

    @Override
    protected void measureChildWithMargins(View child,
                                           int parentWidthMeasureSpec, int widthUsed,
                                           int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child
                .getLayoutParams();

        final int childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                lp.leftMargin + lp.rightMargin, MeasureSpec.UNSPECIFIED);
        final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                lp.topMargin + lp.bottomMargin, MeasureSpec.UNSPECIFIED);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

    // Listener to handle all the touch events
    private SimpleOnGestureListener mListener = new SimpleOnGestureListener() {
        public boolean onDown(MotionEvent e) {
            // Cancel any current fling
            if (!mScroller.isFinished()) {
                mScroller.abortAnimation();
            }
            return true;
        }

        public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,
                               float velocityY) {
            // Call a helper method to start the scroller animation
            fling((int) -velocityX / 3, (int) -velocityY / 3);
            return true;
        }

        public boolean onScroll(MotionEvent e1, MotionEvent e2,
                                float distanceX, float distanceY) {
            // Any view can be scrolled by simply calling its scrollBy() method
            scrollBy((int) distanceX, (int) distanceY);
            return true;
        }
    };

    @Override
    public void computeScroll() {
        if (mScroller.computeScrollOffset()) {
            // This is called at drawing time by ViewGroup. We use
            // this method to keep the fling animation going through
            // to completion.
            int oldX = getScrollX();
            int oldY = getScrollY();
            int x = mScroller.getCurrX();
            int y = mScroller.getCurrY();

            if (getChildCount() > 0) {
                View child = getChildAt(0);
                x = clamp(x, getWidth() - getPaddingRight() - getPaddingLeft(),
                        child.getWidth());
                y = clamp(y,
                        getHeight() - getPaddingBottom() - getPaddingTop(),
                        child.getHeight());
                if (x != oldX || y != oldY) {
                    scrollTo(x, y);
                }
            }

            // Keep on drawing until the animation has finished.
            postInvalidate();
        }
    }

    // Override scrollTo to do bounds checks on any scrolling request
    @Override
    public void scrollTo(int x, int y) {
        // we rely on the fact the View.scrollBy calls scrollTo.
        if (getChildCount() > 0) {
            View child = getChildAt(0);
            x = clamp(x, getWidth() - getPaddingRight() - getPaddingLeft(),
                    child.getWidth());
            y = clamp(y, getHeight() - getPaddingBottom() - getPaddingTop(),
                    child.getHeight());
            if (x != getScrollX() || y != getScrollY()) {
                super.scrollTo(x, y);
            }
        }
    }

    /*
     * Monitor touch events passed down to the children and intercept as soon as
     * it is determined we are dragging
     */
    @Override
    public boolean onInterceptTouchEvent(MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mInitialX = event.getX();
                mInitialY = event.getY();
                // Feed the down event to the detector so it has
                // context when/if dragging begins
                mDetector.onTouchEvent(event);
                break;
            case MotionEvent.ACTION_MOVE:
                final float x = event.getX();
                final float y = event.getY();
                final int yDiff = (int) Math.abs(y - mInitialY);
                final int xDiff = (int) Math.abs(x - mInitialX);
                // Verify that either difference is enough to be a drag
                if (yDiff > mTouchSlop || xDiff > mTouchSlop) {
                    // Start capturing events
                    return true;
                }
                break;
        }

        return super.onInterceptTouchEvent(event);
    }

    /*
     * Feed all touch events we receive to the detector for processing.
     */
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        return mDetector.onTouchEvent(event);
    }

    /*
     * Utility method to initialize the Scroller and start redrawing
     */
    public void fling(int velocityX, int velocityY) {
        if (getChildCount() > 0) {
            int height = getHeight() - getPaddingBottom() - getPaddingTop();
            int width = getWidth() - getPaddingLeft() - getPaddingRight();
            int bottom = getChildAt(0).getHeight();
            int right = getChildAt(0).getWidth();

            mScroller.fling(getScrollX(), getScrollY(), velocityX, velocityY,
                    0, Math.max(0, right - width), 0,
                    Math.max(0, bottom - height));

            invalidate();
        }
    }

    /*
     * Utility method to assist in doing bounds checking
     */
    private int clamp(int n, int my, int child) {
        if (my >= child || n < 0) {
            /*
             * my >= child is this case: |--------------- me ---------------|
             * |------ child ------| or |--------------- me ---------------|
             * |------ child ------| or |--------------- me ---------------|
             * |------ child ------|
             *
             * n < 0 is this case: |------ me ------| |-------- child --------|
             * |-- mScrollX --|
             */
            return 0;
        }
        if ((my + n) > child) {
            /*
             * this case: |------ me ------| |------ child ------| |-- mScrollX
             * --|
             */
            return child - my;
        }
        return n;
    }
}