在上一篇中介紹了View類的scrollTo和scrollBy兩個方法，對這兩個方法不太瞭解的朋友可以先看《自定義View及ViewGroup》

scrollTo和scrollBy雖然實現了檢視的偏移，但是卻沒有更好的控制移動過程，移動是瞬間進行的。Scroller類就是為解決這個問題而設計的。

開啟Scroller的原始碼，可以看到startScroll方法：

    /**
     * Start scrolling by providing a starting point and the distance to travel.
     * 
     * @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;
    }
 

   /**
     * Called by a parent to request that a child update its values for mScrollX
     * and mScrollY if necessary. This will typically be done if the child is
     * animating a scroll using a {@link android.widget.Scroller Scroller}
     * object.
     */
    public void computeScroll() {
    }
 

為了易於控制滑屏控制，Android框架提供了 computeScroll()方法去控制這個流程。在繪製View時，會在draw()過程呼叫該方法。為了實現偏移控制，一般自定義View/ViewGroup都需要過載該方法 。

@Override  
protected void dispatchDraw(Canvas canvas){  
    ...  
      
    for (int i = 0; i < count; i++) {  
        final View child = children[getChildDrawingOrder(count, i)];  
        if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {  
            more |= drawChild(canvas, child, drawingTime);  
        }  
    }  
}  
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {  
    ...  
    child.computeScroll();  
    ...  
} 
 

package com.example.testscrollto;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Scroller;

public class MainActivity extends Activity {
	private static final String TAG = "TestScrollerActivity";
	LinearLayout lay1, lay2, lay0;
	private Scroller mScroller;

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		
		mScroller = new Scroller(this);
		lay1 = new MyLinearLayout(this);
		lay2 = new MyLinearLayout(this);

		lay1.setBackgroundColor(this.getResources().getColor(
				android.R.color.darker_gray));
		lay2.setBackgroundColor(this.getResources().getColor(
				android.R.color.white));
		lay0 = new ContentLinearLayout(this);
		lay0.setOrientation(LinearLayout.VERTICAL);
		LinearLayout.LayoutParams p0 = new LinearLayout.LayoutParams(
				LinearLayout.LayoutParams.FILL_PARENT,
				LinearLayout.LayoutParams.FILL_PARENT);
		this.setContentView(lay0, p0);

		LinearLayout.LayoutParams p1 = new LinearLayout.LayoutParams(
				LinearLayout.LayoutParams.FILL_PARENT,
				LinearLayout.LayoutParams.FILL_PARENT);
		p1.weight = 1;
		lay0.addView(lay1, p1);
		
		LinearLayout.LayoutParams p2 = new LinearLayout.LayoutParams(
				LinearLayout.LayoutParams.FILL_PARENT,
				LinearLayout.LayoutParams.FILL_PARENT);
		p2.weight = 1;
		
		lay0.addView(lay2, p2);
		MyButton btn1 = new MyButton(this);
		MyButton btn2 = new MyButton(this);
		btn1.setText("btn in layout1");
		btn2.setText("btn in layout2");
		btn1.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				mScroller.startScroll(0, 0, -30, -30, 50);
			}
		});
		btn2.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				mScroller.startScroll(20, 20, -50, -50, 50);
			}
		});
		lay1.addView(btn1);
		lay2.addView(btn2);
	}

	class MyButton extends Button {
		public MyButton(Context ctx) {
			super(ctx);
		}

		@Override
		protected void onDraw(Canvas canvas) {
			super.onDraw(canvas);
			Log.d(TAG, this.toString() + " onDraw------");
		}
	}

	class MyLinearLayout extends LinearLayout {
		public MyLinearLayout(Context ctx) {
			super(ctx);
		}

		@Override
		/**
		 * Called by a parent to request that a child update its values for mScrollX
		 * and mScrollY if necessary. This will typically be done if the child is
		 * animating a scroll using a {@link android.widget.Scroller Scroller}
		 * object.
		 */
		public void computeScroll() {
			Log.d(TAG, this.toString() + " computeScroll-----------");
			if (mScroller.computeScrollOffset())// 如果mScroller沒有呼叫startScroll，這裡將會返回false。
			{
				// 因為呼叫computeScroll函式的是MyLinearLayout例項，
				// 所以呼叫scrollTo移動的將是該例項的孩子，也就是MyButton例項
				scrollTo(mScroller.getCurrX(), 0);
				Log.d(TAG, "getCurrX = " + mScroller.getCurrX());

				// 繼續讓系統重繪
				getChildAt(0).invalidate();
			}
		}
	}

	class ContentLinearLayout extends LinearLayout {
		public ContentLinearLayout(Context ctx) {
			super(ctx);
		}

		@Override
		protected void dispatchDraw(Canvas canvas) {
			Log.d(TAG, "contentview dispatchDraw");
			super.dispatchDraw(canvas);
		}
	}
}

當點選Button的時候背景發生了變化，就需要重繪，這時button呼叫invalidate方法請求重繪，這就是scroll動態效果的觸發源，Scroller物件的例項是一個封裝位置和速度的變數，startScroll()方法是對一些成員變數進行設定，設定的唯一效果是導致mScroller.computeScrollOffset()方法返回true.在button重繪的同時mScroller.startScroll()方法被呼叫，此時mScroller變數設定了有效的值。接下來的過程就是View自上而下的繪製了。在繪製lay1的時候就會呼叫drawChild方法，這時候就會執行computeScrll()方法：

		public void computeScroll() {
			Log.d(TAG, this.toString() + " computeScroll-----------");
			if (mScroller.computeScrollOffset())// 如果mScroller沒有呼叫startScroll，這裡將會返回false。
			{
				// 因為呼叫computeScroll函式的是MyLinearLayout例項，
				// 所以呼叫scrollTo移動的將是該例項的孩子，也就是MyButton例項
				scrollTo(mScroller.getCurrX(), 0);
				Log.d(TAG, "getCurrX = " + mScroller.getCurrX());

				// 繼續讓系統重繪
				getChildAt(0).invalidate();
			}
		}