Android scrollTo() scrollBy() Scroller讲解及应用

protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        ...
        ...
        ...

    if (!concatMatrix && canvas.quickReject(cl, ct, cr, cb, Canvas.EdgeType.BW) &&
                (child.mPrivateFlags & DRAW_ANIMATION) == 0) {
            return more;
        }

        child.computeScroll();

        final int sx = child.mScrollX;
        final int sy = child.mScrollY;

        boolean scalingRequired = false;
        Bitmap cache = null;

        ...
        ...
        ...

}

public class Scroller  {
    private int mMode;

    private int mStartX;
    private int mStartY;
    private int mFinalX;
    private int mFinalY;

    private int mMinX;
    private int mMaxX;
    private int mMinY;
    private int mMaxY;

    private int mCurrX;
    private int mCurrY;
    private long mStartTime;
    private int mDuration;
    private float mDurationReciprocal;
    private float mDeltaX;
    private float mDeltaY;
    private boolean mFinished;
    private Interpolator mInterpolator;

    private float mVelocity;
    private float mCurrVelocity;
    private int mDistance;

    private float mFlingFriction = ViewConfiguration.getScrollFriction();

    private static final int DEFAULT_DURATION = 250;
    private static final int SCROLL_MODE = 0;
    private static final int FLING_MODE = 1;

    /**
     * Create a Scroller with the default duration and interpolator.
     */
    public Scroller(Context context) {
        this(context, null);
    }

    /**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. "Flywheel" behavior will
     * be in effect for apps targeting Honeycomb or newer.
     */
    public Scroller(Context context, Interpolator interpolator) {
        this(context, interpolator,
                context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
    }

    /**
     * Create a Scroller with the specified interpolator. If the interpolator is
     * null, the default (viscous) interpolator will be used. Specify whether or
     * not to support progressive "flywheel" behavior in flinging.
     */
    public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
        mFinished = true;
        mInterpolator = interpolator;
        mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
        mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
        mFlywheel = flywheel;

        mPhysicalCoeff = computeDeceleration(0.84f); // look and feel tuning
    }

    /**
     * Call this when you want to know the new location.  If it returns true,
     * the animation is not yet finished.
     */
    public boolean computeScrollOffset() {
        if (mFinished) {
            return false;
        }

        int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
   
        if (timePassed < mDuration) {
            switch (mMode) {
            case SCROLL_MODE:
                float x = timePassed * mDurationReciprocal;
   
                if (mInterpolator == null)
                    x = viscousFluid(x);
                else
                    x = mInterpolator.getInterpolation(x);
   
                mCurrX = mStartX + Math.round(x * mDeltaX);
                mCurrY = mStartY + Math.round(x * mDeltaY);
                break;
            case FLING_MODE:
                final float t = (float) timePassed / mDuration;
                final int index = (int) (NB_SAMPLES * t);
                float distanceCoef = 1.f;
                float velocityCoef = 0.f;
                if (index < NB_SAMPLES) {
                    final float t_inf = (float) index / NB_SAMPLES;
                    final float t_sup = (float) (index + 1) / NB_SAMPLES;
                    final float d_inf = SPLINE_POSITION[index];
                    final float d_sup = SPLINE_POSITION[index + 1];
                    velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
                    distanceCoef = d_inf + (t - t_inf) * velocityCoef;
                }

                mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;
               
                mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
                // Pin to mMinX <= mCurrX <= mMaxX
                mCurrX = Math.min(mCurrX, mMaxX);
                mCurrX = Math.max(mCurrX, mMinX);
               
                mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
                // Pin to mMinY <= mCurrY <= mMaxY
                mCurrY = Math.min(mCurrY, mMaxY);
                mCurrY = Math.max(mCurrY, mMinY);

                if (mCurrX == mFinalX && mCurrY == mFinalY) {
                    mFinished = true;
                }

                break;
            }
        }
        else {
            mCurrX = mFinalX;
            mCurrY = mFinalY;
            mFinished = true;
        }
        return true;
    }
   
    /**
     * Start scrolling by providing a starting point and the distance to travel.
     * The scroll will use the default value of 250 milliseconds for the
     * duration.
     *
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     */
    public void startScroll(int startX, int startY, int dx, int dy) {
        startScroll(startX, startY, dx, dy, DEFAULT_DURATION);
    }

    /**
     * Start scrolling by providing a starting point, the distance to travel,
     * and the duration of the scroll.
     *
     * @param startX Starting horizontal scroll offset in pixels. Positive
     *        numbers will scroll the content to the left.
     * @param startY Starting vertical scroll offset in pixels. Positive numbers
     *        will scroll the content up.
     * @param dx Horizontal distance to travel. Positive numbers will scroll the
     *        content to the left.
     * @param dy Vertical distance to travel. Positive numbers will scroll the
     *        content up.
     * @param duration Duration of the scroll in milliseconds.
     */
    public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

    /**
     * Start scrolling based on a fling gesture. The distance travelled will
     * depend on the initial velocity of the fling.
     *
     * @param startX Starting point of the scroll (X)
     * @param startY Starting point of the scroll (Y)
     * @param velocityX Initial velocity of the fling (X) measured in pixels per
     *        second.
     * @param velocityY Initial velocity of the fling (Y) measured in pixels per
     *        second
     * @param minX Minimum X value. The scroller will not scroll past this
     *        point.
     * @param maxX Maximum X value. The scroller will not scroll past this
     *        point.
     * @param minY Minimum Y value. The scroller will not scroll past this
     *        point.
     * @param maxY Maximum Y value. The scroller will not scroll past this
     *        point.
     */
    public void fling(int startX, int startY, int velocityX, int velocityY,
            int minX, int maxX, int minY, int maxY) {
        // Continue a scroll or fling in progress
        if (mFlywheel && !mFinished) {
            float oldVel = getCurrVelocity();

            float dx = (float) (mFinalX - mStartX);
            float dy = (float) (mFinalY - mStartY);
            float hyp = FloatMath.sqrt(dx * dx + dy * dy);

            float ndx = dx / hyp;
            float ndy = dy / hyp;

            float oldVelocityX = ndx * oldVel;
            float oldVelocityY = ndy * oldVel;
            if (Math.signum(velocityX) == Math.signum(oldVelocityX) &&
                    Math.signum(velocityY) == Math.signum(oldVelocityY)) {
                velocityX += oldVelocityX;
                velocityY += oldVelocityY;
            }
        }

        mMode = FLING_MODE;
        mFinished = false;

        float velocity = FloatMath.sqrt(velocityX * velocityX + velocityY * velocityY);
    
        mVelocity = velocity;
        mDuration = getSplineFlingDuration(velocity);
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;

        float coeffX = velocity == 0 ? 1.0f : velocityX / velocity;
        float coeffY = velocity == 0 ? 1.0f : velocityY / velocity;

        double totalDistance = getSplineFlingDistance(velocity);
        mDistance = (int) (totalDistance * Math.signum(velocity));
       
        mMinX = minX;
        mMaxX = maxX;
        mMinY = minY;
        mMaxY = maxY;

        mFinalX = startX + (int) Math.round(totalDistance * coeffX);
        // Pin to mMinX <= mFinalX <= mMaxX
        mFinalX = Math.min(mFinalX, mMaxX);
        mFinalX = Math.max(mFinalX, mMinX);
       
        mFinalY = startY + (int) Math.round(totalDistance * coeffY);
        // Pin to mMinY <= mFinalY <= mMaxY
        mFinalY = Math.min(mFinalY, mMaxY);
        mFinalY = Math.max(mFinalY, mMinY);
    }
   
}

  public void startScroll(int startX, int startY, int dx, int dy, int duration) {
        mMode = SCROLL_MODE;
        mFinished = false;
        mDuration = duration;
        mStartTime = AnimationUtils.currentAnimationTimeMillis();
        mStartX = startX;
        mStartY = startY;
        mFinalX = startX + dx;
        mFinalY = startY + dy;
        mDeltaX = dx;
        mDeltaY = dy;
        mDurationReciprocal = 1.0f / (float) mDuration;
    }

  这个方法更像是一个构造方法用来初始化赋值的，比如设置滚动模式、开始时间，持续时间、起始坐标、结束坐标等等，并没有任何对View的滚动操作，当然还有一个重要的变量：mDurationReciprocal。因为这个变量要在接下来介绍的computeScrollOffset()方法使用，computeScrollOffset()方法主要是根据当前已经消逝的时间来计算当前的坐标点，并且保存在mCurrX和mCurrY值中，那这个消逝的时间就是如何计算出来的呢？之前在startScroll()方法的时候获取了当前的动画毫秒并赋值给了mStartTime，在computeScrollOffset()中再一次调用AnimationUtils.currentAnimationTimeMillis()来获取动画毫秒减去mStartTime就是消逝时间了。然后进去if判断，如果动画持续时间小于设置的滚动持续时间mDuration，则是SCROLL_MODE，再根据Interpolator来计算出在该时间段里面移动的距离，移动的距离是根据这个消逝时间乘以mDurationReciprocal，就得到一个相对偏移量，再进行Math.round(x * mDeltaX)计算，就得到最后的偏移量，然后赋值给mCurrX， mCurrY，所以mCurrX、 mCurrY 的值也是一直变化的。总结一下该方法的作用就是，计算在0到mDuration时间段内滚动的偏移量，并且判断滚动是否结束，true代表还没结束，false则表示滚动结束了。

  之前说到是Scroller配合computeScroll()方法来实现移动的，那是如何配合的呢？

  1、首先调用Scroller的startScroll()方法来进行一些滚动的初始化设置，

  scroller.startScroll(getScrollX(), 0, distance, 0);

  

  2、然后调用View的invalidate()或postInvalidate()进行重绘。

  

 invalidate(); // 刷新视图 

  3、绘制View的时候会触发computeScroll()方法，接着重写computeScroll()，在computeScroll()里面先调用Scroller的computeScrollOffset()方法来判断滚动是否结束，如果滚动没有结束就调用scrollTo()方法来进行滚动。

       @Override
     public void computeScroll() {
          if (scroller.computeScrollOffset()) {
               scrollTo(scroller.getCurrX(), 0);
          }
     }

  4、scrollTo()方法虽然会重新绘制View,但是还是要手动调用下invalidate()或者postInvalidate()来触发界面重绘，重新绘制View又触发computeScroll()，所以就进入一个递归循环阶段，这样就实现在某个时间段里面滚动某段距离的一个平滑的滚动效果。

       @Override
     public void computeScroll() {
          if (scroller.computeScrollOffset()) {
               scrollTo(scroller.getCurrX(), 0);
               invalidate();
          }
     }

   具体流程图如下：

  了解完Scroller之后，我们就对之前的例子进行一下改进，不直接使用scrollTo()、ScrollBy()方法了，而是使用Scroller来实现一个平滑的移动效果。只需把代码稍微改一下就可以了，如下：

/**
*
*/
package com.kince.scrolldemo;

import android.content.Context;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Scroller;

/**
* @author kince
*
*
*/
public class CusScrollView extends ViewGroup {

     private int lastX = 0;
     private int currX = 0;
     private int offX = 0;
     private Scroller mScroller;

     /**
     * @param context
     */
     public CusScrollView(Context context) {
          this(context, null);
          // TODO Auto-generated constructor stub
     }

     /**
     * @param context
     * @param attrs
     */
     public CusScrollView(Context context, AttributeSet attrs) {
          this(context, attrs, 0);
          // TODO Auto-generated constructor stub
     }

     /**
     * @param context
     * @param attrs
     * @param defStyle
     */
     public CusScrollView(Context context, AttributeSet attrs, int defStyle) {
          super(context, attrs, defStyle);
          // TODO Auto-generated constructor stub
          mScroller = new Scroller(context);
     }

     /*
     * (non-Javadoc)
     *
     * @see android.view.ViewGroup#onLayout(boolean, int, int, int, int)
     */
     @Override
     protected void onLayout(boolean changed, int l, int t, int r, int b) {
          // TODO Auto-generated method stub

          for (int i = 0; i < getChildCount(); i++) {
               View v = getChildAt(i);
               v.layout(0 + i * getWidth(), 0, getWidth() + i * getWidth(),
                         getHeight());
          }
     }

     @Override
     public boolean onTouchEvent(MotionEvent event) {
          // TODO Auto-generated method stub
          switch (event.getAction()) {
          case MotionEvent.ACTION_DOWN:
               // 只考虑水平方向
               lastX = (int) event.getX();
               return true;

          case MotionEvent.ACTION_MOVE:
               currX = (int) event.getX();
               offX = currX - lastX;
               // scrollBy(-offX, 0);
               mScroller.startScroll(getScrollX(), 0, -offX, 0);

               break;

          case MotionEvent.ACTION_UP:
//               scrollTo(0, 0);
               mScroller.startScroll(getScrollX(), 0, -100, 0);
               break;
          }
          invalidate();
          return super.onTouchEvent(event);
     }

     @Override
     public void computeScroll() {
          // TODO Auto-generated method stub
          if (mScroller.computeScrollOffset()) {
               scrollTo(mScroller.getCurrX(), 0);
               invalidate();
          }
     }

}

  这样就实现了一个平滑的移动效果。关于scrollTo() 、scrollBy()、 Scroller讲解就进行到这里。之后会更新两篇关于这方面的UI效果开发，一篇是模仿Zaker的开门效果；另一篇是首页推荐图片轮播效果。