說袈溱前面的話：

為什么要來說Scroller這個(gè)類呢？這個(gè)類到底是拿來干什么的呢？如不雅你看了ListView這類控件那么你肯定會(huì)發(fā)明瑯綾擎有一個(gè)Sroller類，其實(shí)它的感化就是幫助記錄和計(jì)算我們滑動(dòng)的距離和速度這些。大年夜而讓我們?cè)谧远x控件的時(shí)刻可以便利的做一些滑動(dòng)和回彈的動(dòng)畫，為什么呢？因?yàn)镾roller類都給你計(jì)算好了嘛。

類分析

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;
......
 /**
     * 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;
    }
    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);
    }
   
    /**
     * Stops the animation. Contrary to {@link #forceFinished(boolean)},
     * aborting the animating cause the scroller to move to the final x and y
     * position
     *
     * @see #forceFinished(boolean)
     */
    public void abortAnimation() {
        mCurrX = mFinalX;
        mCurrY = mFinalY;
        mFinished = true;
    }
....

膳綾擎我貼出來的類辦法都是常用的幾個(gè)辦法，這里我來分別解釋每個(gè)辦法的感化和用法，起首我們照樣來看computeScrollOffset()這個(gè)辦法吧，我們看到它的注釋  /**
     * Call this when you want to know the new location.  If it returns true,當(dāng)你想獲得一個(gè)新的地位的時(shí)刻call這個(gè)辦法。其實(shí)袈溱這個(gè)類瑯綾擎已經(jīng)給了我們一個(gè)很好的例子我這里也貼出來看看到底是怎么樣的？

 * <p>To track the changing positions of the x/y coordinates, use
 * {@link #computeScrollOffset}. The method returns a boolean to indicate
 * whether the scroller is finished. If it isn't, it means that a fling or
 * programmatic pan operation is still in progress. You can use this method to
 * find the current offsets of the x and y coordinates, for example:</p>
 *
 * <pre>if (mScroller.computeScrollOffset()) {
 *     // Get current x and y positions
 *     int currX = mScroller.getCurrX();
 *     int currY = mScroller.getCurrY();
 *    ...
 * }</pre>

google官方的注釋照樣挺給力的，我就不多說什么了，接著來看下一辦法。startScroll()這個(gè)辦法又該什么時(shí)刻用呢？又該在哪個(gè)處所用呢？其實(shí)我們須要做一個(gè)滑動(dòng)效不雅，我們就須要調(diào)用這個(gè)辦法贊助我們不時(shí)的計(jì)算我們當(dāng)前的地位。如許我們就不消本身去計(jì)算當(dāng)前的速度是若干當(dāng)前該走到哪里了。這里官方也給出了一個(gè)例子我們照樣來看看到底是怎么竽暌姑的？

 <pre> private Scroller mScroller = new Scroller(context);
 * ...
 * public void zoomIn() {
 *     // Revert any animation currently in progress
 *     mScroller.forceFinished(true);
 *     // Start scrolling by providing a starting point and
 *     // the distance to travel
 *     mScroller.startScroll(0, 0, 100, 0);
 *     // Invalidate to request a redraw
 *     invalidate();
 * }</pre>

可以看到膳綾擎的代碼，注釋都很nice對(duì)紕謬？須要留意的是最后一行invalidate這個(gè)時(shí)刻須要我們手動(dòng)的去提議一次刷新請(qǐng)求，不然結(jié)不雅會(huì)跟你想得不一樣的。接下來就是fling()這個(gè)辦法，我們大年夜它的注釋可以讀出來是什么意思 /**     * Start scrolling based on a fling gesture. The distance travelled will     * depend on the initial velocity of the fling.根據(jù)你的手勢(shì)來滑動(dòng)一段距離取決于你的初速度，那么這個(gè)函數(shù)應(yīng)當(dāng)什么時(shí)刻調(diào)用呢？那么我來告訴你，listview是不是快速滑動(dòng)的時(shí)刻你會(huì)看到listview的視圖快速網(wǎng)上滑動(dòng)一些條目過后才會(huì)停止下來，就是在我們快速滑動(dòng)屏幕的時(shí)刻就調(diào)用這個(gè)函數(shù)。大年夜下一篇開端就要開端講拭魅戰(zhàn)了。