参考资料

郭霖 Scroller完全解析
鸿洋 ViewDragHelper完全解析
鸿洋 ViewDragHelper实战 自己打造Drawerlayout

-目录

• 1）layout
• 2）offsetLeftAndRight() offsetTopAndBottom()
• 3）LayoutParams()
• 4）scrollTo() scrollBy()
• 5）Scroller
• 6）属性动画
• 7）ViewDragHelper

-实现滑动的7种方法

public class DragView extends View {
    private static final String TAG = "DragView";
    private int lastX, lastY;
    private Scroller scroller;

    public DragView(Context context, AttributeSet attrs) {
        super(context, attrs);
        scroller = new Scroller(context);
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int x = (int) event.getX();
        int y = (int) event.getY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                lastX = x;
                lastY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                int offsetX = x - lastX;
                int offsetY = y - lastY;

                //方法一
//                layout(getLeft() + offsetX, getTop() + offsetY, getRight() + offsetX, getBottom() + offsetY);
                //方法二
//                offsetLeftAndRight(offsetX);
//                offsetTopAndBottom(offsetY);
                //方法三
//                LinearLayout.LayoutParams layoutParams = (LinearLayout.LayoutParams) getLayoutParams();
//                layoutParams.leftMargin = getLeft()+offsetX;
//                layoutParams.topMargin = getTop()+offsetY;
//                setLayoutParams(layoutParams);
                //方法四
                ((View)getParent()).scrollBy(-offsetX,-offsetY);

                break;
            case MotionEvent.ACTION_UP:
                View view =  (View)getParent();
                Log.i(TAG, "getScrollX: "+view.getScrollX());
                Log.i(TAG, "getScrollY: "+view.getScrollY());
                scroller.startScroll(view.getScrollX(),view.getScrollY(),-view.getScrollX(),-view.getScrollY());
                invalidate();
                break;
        }
        return true;
    }

    @Override
    public void computeScroll() {
        super.computeScroll();
        if (scroller.computeScrollOffset()){
            Log.i(TAG, "getCurrX: "+scroller.getCurrX());
            Log.i(TAG, "getCurrY: "+scroller.getCurrY());
            ((ViewGroup)getParent()).scrollTo(scroller.getCurrX(),scroller.getCurrY());
            invalidate();
        }
    }
}

1) layout

2) offsetLeftAndRight() offsetTopAndBottom()

3) LayoutParams()

//使用MarginLayoutParams更加方便还不用考虑父布局是LinearLayout还是RelativeLayout
ViewGroup.MarginLayoutParams layoutParams = (ViewGroup.MarginLayoutParams) getLayoutParams();
layoutParams.leftMargin = getLeft()+offsetX;
layoutParams.rightMargin = getRight()+offsetY;
setLayoutParams(layoutParams);

4) scrollTo() scrollBy()

任何一个控件都是可以滚动的，因为View类中有scrollTo()和scrollBy()两个方法，scrollBy()是让View相对于当前位置滚动某段距离，scrollTo()是让View相对于初始位置滚动某段距离。

scrollTo,scrollBy方法移动的是View的内容，如果ViewGroup中使用scrollTo，scrollBy，那么移动的将是所有子View。

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        layout = (LinearLayout) findViewById(R.id.layout);
        scrollToBtn = (Button) findViewById(R.id.scroll_to_btn);
        scrollByBtn = (Button) findViewById(R.id.scroll_by_btn);
        scrollToBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                layout.scrollTo(-60, -100); //注意此处是layout的scrollTo()
            }
        });
        scrollByBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                layout.scrollBy(-60, -100);//注意此处是layout的scrollBy()
            }
        });
    }

下图中为什么scrollBy(-60, -100)，按钮确是向手机坐标系的x和y轴正向移动呢？
答：可以想象屏幕是一个放大镜，而下面是一个巨大的画布，使用scrollBy方法，将layout向X轴负方向(左)平移60，向Y轴负方向(上)平移100，则layout内的子view相当于向X轴和Y轴的正方向上移动了。

20160110164232041.gif

5) Scroller

使用Scroller模仿ViewPager的例子

startScroll(int startX,int startY,int dx, int dy,int duration)
startScroll(int startX,int startY,int dx, int dy)

20160114230048304.gif

/**
 * Created by 涂高峰 on 2017/6/21.
 */
public class ScrollerLayout extends ViewGroup {
    private static final String TAG = "ScrollerLayout";
    private Scroller mScroller;
    private int mDownX,mMoveX;
    private int leftBorder,rightBorder;
    private int mTouchSlop;
    public ScrollerLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
        mScroller = new Scroller(context);
        //大于这个距离,系统认为是移动
        mTouchSlop = ViewConfiguration.get(context).getScaledPagingTouchSlop();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int count = getChildCount();
        for (int i=0; i<count; i++){
            View child = getChildAt(i);
            measureChild(child,widthMeasureSpec,heightMeasureSpec);
        }
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        int count = getChildCount();
        for (int i=0; i<count; i++){
            View child = getChildAt(i);
            child.layout(i*child.getMeasuredWidth(), 0, (i+1)*child.getMeasuredWidth(), child.getMeasuredHeight());
        }
        leftBorder = getChildAt(0).getLeft();
        rightBorder = getChildAt(getChildCount()-1).getRight();
        Log.i(TAG, "leftBorder: "+leftBorder);
        Log.i(TAG, "rightBorder: "+rightBorder);
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        int x = (int) ev.getRawX();
        switch (ev.getAction()){
            case  MotionEvent.ACTION_DOWN:
                mDownX = x;
                mMoveX = x;
                break;
            case MotionEvent.ACTION_MOVE:
                //按下的坐标与当前移动坐标绝对值 大于 系统默认的移动距离
                //拦截此移动事件,不向子view传递,进入自身的onTouchEvent
                if (Math.abs(mDownX - x)>mTouchSlop){
                    return true;
                }
                break;
        }
        return super.onInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int x = (int) event.getRawX();
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                //如果子控件为Button之类的clickable控件,则会由button消费掉down事件,当viewgroup滑动时,会拦截move事件并处理
                //但是若子控件为TextView之类的非clickable控件,则viewgroup和textview都不会消费掉down事件.
                //由于没有任何view消费down事件,后续事件将由上层消费,而不会往下传递给viewgroup.所以此处需要将down事件消费掉,从而能继续接收后续事件
                return true;
            case MotionEvent.ACTION_MOVE:
                //偏移量
                int offsetX = mMoveX-x;
                //左边界处理
                if (getScrollX()+offsetX < leftBorder){
                    scrollTo(leftBorder,0);
                    return true;
                }
                //右边界处理
                if (getScrollX()+offsetX + getWidth()> rightBorder){
                    scrollTo(rightBorder-getWidth(),0);
                    return true;
                }
                //滑动处理
                scrollBy(offsetX,0);
                mMoveX = x;
                break;
            case MotionEvent.ACTION_UP:
                //手指抬起,判断是哪个子控件的index
                //小于第一个子控件的一半宽度则认为是第一个子控件
                //大于第一个子控件的一半宽度则认为是下一个子控件
                int index = (getScrollX()+getWidth()/2)/getWidth();
                Log.i(TAG, "index: "+index); //结果为  0  1  2
                //根据子空间index计算偏移量
                int dy = index * getWidth() - getScrollX();
                Log.i(TAG, "dy: "+dy);
                mScroller.startScroll(getScrollX(),0,dy,0);
                invalidate();
                break;
        }
        return super.onTouchEvent(event);
    }

    //重绘会调用此方法,此方法中的invalidate又会触发重绘,从而循环实现弹性滑动
    @Override
    public void computeScroll() {
        super.computeScroll();
        if (mScroller.computeScrollOffset()){
            scrollTo(mScroller.getCurrX(),mScroller.getCurrY());
            invalidate();
        }
    }
}

6) 属性动画（动画中讲解）

7) ViewDragHelper

在自定义ViewGroup中，很多效果都包含用户手指去拖动其内部的某个View(eg:侧滑菜单等)，针对具体的需要去写好onInterceptTouchEvent和onTouchEvent这两个方法是一件很不容易的事，需要自己去处理：多手指的处理、加速度检测等等。
好在官方在v4的支持包中提供了ViewDragHelper这样一个类来帮助我们方便的编写自定义ViewGroup

1)ViewDragHelper类相关的API:

2）ViewDragHelper.Callback相关API：

//方法的大致的回调顺序：

1)shouldInterceptTouchEvent：

DOWN:
    getOrderedChildIndex(findTopChildUnder)
    ->onEdgeTouched

MOVE:
    getOrderedChildIndex(findTopChildUnder)
    ->getViewHorizontalDragRange & 
      getViewVerticalDragRange(checkTouchSlop)(MOVE中可能不止一次)
    ->clampViewPositionHorizontal&
      clampViewPositionVertical
    ->onEdgeDragStarted
    ->tryCaptureView
    ->onViewCaptured
    ->onViewDragStateChanged

2)processTouchEvent:

DOWN:
    getOrderedChildIndex(findTopChildUnder)
    ->tryCaptureView
    ->onViewCaptured
    ->onViewDragStateChanged
    ->onEdgeTouched
MOVE:
    ->STATE==DRAGGING:dragTo
    ->STATE!=DRAGGING:
        onEdgeDragStarted
        ->getOrderedChildIndex(findTopChildUnder)
        ->getViewHorizontalDragRange&
          getViewVerticalDragRange(checkTouchSlop)
        ->tryCaptureView
        ->onViewCaptured
        ->onViewDragStateChanged

例子
1）任意移动
2）移动完毕后回到原位
3）边界移动时对View进行捕获（未成功。。）

20150713095339390.gif

public class VDHDemo extends LinearLayout {
    private static final String TAG = "VDHDemo";
    private ViewDragHelper mDragger;

    private View mDragView;
    private View mAutoBackView;
    private Point mAutoBackOriPos = new Point();

    public VDHDemo(Context context, AttributeSet attrs) {
        super(context, attrs);
        //第二个参数为敏感度(sensitivity),敏感度越大mTouchSlop就越小
        //mTouchSlop为系统认为是移动的最小距离,即ViewConfiguration.get(context).getScaledPagingTouchSlop()
        mDragger = ViewDragHelper.create(this, 1.0f, new ViewDragHelper.Callback() {
            @Override
            public boolean tryCaptureView(View child, int pointerId) {
                //返回true表示可以捕获该view,可根据第一个参数决定捕获哪个view
                //如: return xxView == child;
                return mDragView==child || mAutoBackView==child;
//                return true;
            }

            //边界控制
            @Override
            public int clampViewPositionHorizontal(View child, int left, int dx) {
                final int leftBound = getPaddingLeft(); //左边界为viewgroup的paddingleft
                final int rightBound = getWidth() - leftBound - getPaddingRight() - 200; //200为子view的宽度

                final int newLeft = Math.min(Math.max(left, leftBound), rightBound);
                return newLeft;
            }

            //边界控制
            @Override
            public int clampViewPositionVertical(View child, int top, int dy) {
                return top;
            }

            //手指释放时回调
            @Override
            public void onViewReleased(View releasedChild, float xvel, float yvel) {
//                super.onViewReleased(releasedChild, xvel, yvel);
                //若为mAutoBackView,则回到初始位置,调用settleCapturedViewAt()
                //其内部为mScroller.startScroll(),别忘了invalidate和computeScroll
                //注意你拖动的越快，返回的越快
                if (releasedChild == mAutoBackView){
                    mDragger.settleCapturedViewAt(mAutoBackOriPos.x,mAutoBackOriPos.y);
                    invalidate();
                }
            }
            //如果子View不消耗事件，那么整个手势（DOWN-MOVE*-UP）都是直接进入onTouchEvent，
            // 在onTouchEvent的DOWN的时候就确定了captureView

            //如果消耗事件，那么就会先走onInterceptTouchEvent方法，判断是否可以捕获，
            // 而在判断的过程中会去判断另外两个回调的方法：getViewHorizontalDragRange和getViewVerticalDragRange，
            // 只有这两个方法返回大于0的值才能正常的捕获。
            @Override
            public int getViewHorizontalDragRange(View child)
            {
                return getMeasuredWidth()-child.getMeasuredWidth();
            }

            @Override
            public int getViewVerticalDragRange(View child)
            {
                return getMeasuredHeight()-child.getMeasuredHeight();
            }
        });
    }

    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        return mDragger.shouldInterceptTouchEvent(ev);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mDragger.processTouchEvent(event);
        return true;
    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        mDragView = getChildAt(0);
        mAutoBackView = getChildAt(1);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        //onLayout结束后将mAutoBackView的返回原点设置为其初始的点
        mAutoBackOriPos.x = mAutoBackView.getLeft();
        mAutoBackOriPos.y = mAutoBackView.getTop();
    }

    @Override
    public void computeScroll() {
        if (mDragger.continueSettling(true)){
            invalidate();
        }
    }
}