自定義View,包括事件分發，滑動衝突，測量以及佈局

/**
 * Created by Venn on 2016/4/11.
 * 水平滑動的ScrollView,內部可嵌套豎直滑動的佈局(ListView,ScrollView等)
 */
public class HorizontalScrollView extends ViewGroup {

    private Context mContext;

    private Scroller mScroller;
    private VelocityTracker mTracker;
    private int mLastX;
    private int mLastY;
    private int mInterceptedX;
    private int mInterceptedY;

    private int currentChildIndex;

    public HorizontalScrollView(Context context) {
        this(context, null);
    }

    public HorizontalScrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.mContext = context;
        init();
    }

    public void init() {
        if (mScroller == null) {
            mScroller = new Scroller(mContext);
        }
        mTracker = VelocityTracker.obtain();
    }

    //1 測量過程
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        measureChildren(widthMeasureSpec, heightMeasureSpec);

        final int childCount = getChildCount();
        //child 的getLayoutParams()得到的爲parent裏面的
        ViewGroup.LayoutParams params = getLayoutParams();
        int defaultWidth = params.width;
        int childTotalWidth = 0;
        int defaultHeight = params.height;

        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            if (child != null) {
                LayoutParams marginParams = (LayoutParams) child.getLayoutParams();
                int childHeight = child.getMeasuredHeight() + child.getPaddingTop() + child
                        .getPaddingBottom() + marginParams.leftMargin + marginParams.rightMargin
                        + getPaddingTop() + getPaddingBottom();
                defaultHeight = Math.max(defaultHeight, childHeight);

                int childWidth = child.getMeasuredWidth() + child.getPaddingLeft() + marginParams
                        .leftMargin + marginParams.rightMargin + child
                        .getPaddingRight() + getPaddingLeft() + getPaddingRight();
                childTotalWidth += childWidth;
            }
        }
        defaultWidth = Math.max(defaultWidth, childTotalWidth);
        if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(defaultWidth, defaultHeight);
        } else if (widthMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(defaultWidth, heightSize);
        } else if (heightMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSize, defaultHeight);
        }
    }

    //2 佈局過程
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int childCount = getChildCount();
        int childLeft = getPaddingLeft();
        int childTop = getPaddingTop();
        for (int i = 0; i < childCount; i++) {
            View child = getChildAt(i);
            if (child != null && child.getVisibility() != View.GONE) {
                int childMeasuredWidth = child.getMeasuredWidth();
                LayoutParams marginParams = (LayoutParams) child.getLayoutParams();
                childLeft += marginParams.leftMargin;
                child.layout(childLeft, childTop + marginParams.topMargin, childLeft +
                                childMeasuredWidth,
                        childTop + marginParams.topMargin + child.getMeasuredHeight());
                childLeft += childMeasuredWidth + marginParams
                        .rightMargin;
            }
        }
    }

    //3 繪製過程
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
    }

    //4 事件分發過程
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        return super.dispatchTouchEvent(ev);
    }

    //5 事件攔截過程
    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        boolean intercepted = false;
        int x = (int) ev.getX();
        int y = (int) ev.getY();
        switch (ev.getAction()) {
            case MotionEvent.ACTION_DOWN:
                intercepted = false;
                if (!mScroller.isFinished()) {
                    mScroller.abortAnimation();
                    intercepted = true;
                }
                break;
            case MotionEvent.ACTION_MOVE:
                if (Math.abs(x - mInterceptedX) > Math.abs(y - mInterceptedY)) {
                    intercepted = true;
                } else {
                    intercepted = false;
                }
                break;
            case MotionEvent.ACTION_UP:
                intercepted = false;
                break;
            default:
                break;
        }

        mLastX = x;
        mLastY = y;
        mInterceptedX = x;
        mInterceptedY = y;

        return intercepted;
    }

    //6 事件處理過程
    @Override
    public boolean onTouchEvent(MotionEvent event) {

        int x = (int) event.getX();
        int y = (int) event.getY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                if (!mScroller.isFinished()) {
                    mScroller.abortAnimation();
                }
                break;
            case MotionEvent.ACTION_MOVE:
                int dX = x - mLastX;
                int dY = y - mLastY;
                scrollBy(-dX, 0);
                currentChildIndex = dX / 30;
                break;
            case MotionEvent.ACTION_UP:
                //使用彈性滑動,使得手指擡起後自動滑動到當前child
                int scrollX = getScrollX();
                mTracker.computeCurrentVelocity(1000);
                int xVelocity = (int) mTracker.getXVelocity();
                if (Math.abs(xVelocity) > 50) {
                    currentChildIndex = xVelocity > 0 ? currentChildIndex-- : currentChildIndex++;
                }

                smoothScrollBy(-(30 * currentChildIndex - scrollX), 0, 3000);
                mTracker.clear();
                break;
        }

        mLastX = x;
        mLastY = y;
        mTracker.clear();

        return true;
    }

    //7 對View移除的處理
    @Override
    protected void onDetachedFromWindow() {
        mTracker.recycle();
        super.onDetachedFromWindow();
    }

    @Override
    public void computeScroll() {
        if (mScroller.computeScrollOffset()) {
            scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
            postInvalidate();
        }
    }

    //水平彈性滑動
    private void smoothScrollBy(int dX, int dY, int duration) {
        mScroller.startScroll(mScroller.getStartX(), 0, dX, 0, duration);
        invalidate();
    }

    //重載使得margin有效
    @Override
    public ViewGroup.LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new HorizontalScrollView.LayoutParams(mContext, attrs);
    }

    //需要重寫layoutParams類使得支持當前ViewGroup的LayoutParams
    public static class LayoutParams extends MarginLayoutParams {
        public LayoutParams(Context c, AttributeSet attrs) {
            super(c, attrs);
        }
    }
}