Android Scroller 使用详解

1.Scroller到底是什么？

答：Scroller只是个计算器，提供插值计算，让滚动过程具有动画属性，但它并不是View，只是单纯地为滑动提供计算。

2.Scroller只是提供计算，那谁来让ViewGroup滑动

答：真正让ViewGroup滑动的是scrollTo,scrollBy 方法。Scroller 的 computeScrollOffset()方法的作用是计算ViewGroup 在当前时间点应该滑动到哪个位置。

在这里我想说一下，Scroller 的使用流程，首先，你创建一个scroller,然后你在computerScroll()方法里面调用scroller 去计算当前应该滚动到的坐标位置。然后scrollTo 去使view滚动。

Scroller 使用

1.创建Scroller

        mScroller = new Scroller(context);

2.在onTouchEvent 方法里面调用mScroller fling 方法，开始快速滚动

case MotionEvent.ACTION_UP:mVelocityTracker.computeCurrentVelocity(1000,mScaledMaximumFlingVelocity);int yVelocity = (int) mVelocityTracker.getYVelocity();if (Math.abs(yVelocity) > mScaledMinimumFlingVelocity) {if (!mScroller.isFinished()) {mScroller.abortAnimation();}mScroller.fling(0,getScrollY(),0, -yVelocity,0,0,0,getLayout().getHeight() - getHeight());//之前老是掉帧的感觉   原来是因为fling 之后 并没有去执行invalidate 有时候可以是因为可能刚好到了60帧刷新的节点postInvalidateOnAnimation();}

3.在View 的 computeScroll 方法里面实现View 的Scroll

    @Overridepublic void computeScroll() {super.computeScroll();if (mScroller.computeScrollOffset()) {scrollBy(mScroller.getCurrX() - getScrollX() ,mScroller.getCurrY() - getScrollY());Log.d("ScrollTextView", "computescorll");}

注意：computerScroll 方法会在 View 的 onDraw() 方法里面调用

下面是一个内容过长可以滚动的TextView:

package com.example.widget;import android.content.Context;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.ViewConfiguration;
import android.widget.OverScroller;
import android.widget.Scroller;import androidx.annotation.Nullable;
import androidx.core.view.ViewCompat;
import androidx.core.view.ViewConfigurationCompat;/*** =======================================================================================* 作    者：caoxinyu* 创建日期：2020/5/10.* 类的作用:可以上下滑动的TextView* 修订历史：*** 问题1.textView 高度有问题* 终于知道MeasureSpec.UNSPECIFIED 有什么用了* 因为有些是无限大的 比如 listView* 所以 他的尺寸应该是UNSPECIFIED 的* =======================================================================================*/
public class ScrollTextView extends androidx.appcompat.widget.AppCompatTextView {private Scroller mScroller;private OverScroller mOverScroller;private int mLastY;private VelocityTracker mVelocityTracker;private ViewConfiguration mViewConfiguration;private int mScaledMaximumFlingVelocity;private int mScaledMinimumFlingVelocity;private static final String TAG = "ScrollTextView";public ScrollTextView(Context context) {super(context);init(context);}public ScrollTextView(Context context, @Nullable AttributeSet attrs) {super(context, attrs);init(context);}public ScrollTextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);init(context);}private void init(Context context) {mScroller = new Scroller(context);mOverScroller = new OverScroller(context);mViewConfiguration = ViewConfiguration.get(context);mScaledMinimumFlingVelocity = mViewConfiguration.getScaledMinimumFlingVelocity();mScaledMaximumFlingVelocity = mViewConfiguration.getScaledMaximumFlingVelocity();Log.d("ScrollTextView", "mScaledMinimumFlingVelocity:" + mScaledMinimumFlingVelocity);Log.d("ScrollTextView", "mScaledMaximumFlingVelocity:" + mScaledMaximumFlingVelocity);}@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);}@Overridepublic boolean onTouchEvent(MotionEvent event) {if (mVelocityTracker == null) {mVelocityTracker = VelocityTracker.obtain();}switch (event.getAction()) {case MotionEvent.ACTION_DOWN:if (!mScroller.isFinished()) {mScroller.abortAnimation();}mLastY = (int) event.getY();break;case MotionEvent.ACTION_UP:mVelocityTracker.computeCurrentVelocity(1000,mScaledMaximumFlingVelocity);int yVelocity = (int) mVelocityTracker.getYVelocity();Log.d("ScrollTextView", "yVelocity:" + yVelocity);if (Math.abs(yVelocity) > mScaledMinimumFlingVelocity) {if (!mScroller.isFinished()) {mScroller.abortAnimation();}mScroller.fling(0,getScrollY(),0, -yVelocity,0,0,0,getLayout().getHeight() - getHeight());//之前老是掉帧的感觉   原来是因为fling 之后 并没有去执行invalidate 有时候可以是因为可能刚好到了60帧刷新的节点postInvalidateOnAnimation();}mVelocityTracker.recycle();mVelocityTracker = null;mLastY = (int) event.getY();break;case MotionEvent.ACTION_MOVE:boolean b = overScrollBy(0, (int) (mLastY - event.getY()), getScrollX(), getScrollY(), 0, getLayout().getHeight() - getHeight(), 0, 0, true);if (b) {mVelocityTracker.clear();}mLastY = (int) event.getY();break;}if (mVelocityTracker != null) {mVelocityTracker.addMovement(event);}return true;}@Overrideprotected void onOverScrolled(int scrollX, int scrollY, boolean clampedX, boolean clampedY) {super.onOverScrolled(scrollX, scrollY, clampedX, clampedY);if (!mScroller.isFinished()) {if (clampedY) {
//                mScroller.springBack(mScrollX, mScrollY, 0, 0, 0, getScrollRange());}}else {super.scrollTo(scrollX, scrollY);Log.d("ScrollTextView", "onOverScrolled");}}@Overridepublic void computeScroll() {super.computeScroll();if (mScroller.computeScrollOffset()) {scrollBy(mScroller.getCurrX() - getScrollX() ,mScroller.getCurrY() - getScrollY());Log.d("ScrollTextView", "computescorll");}if (mOverScroller.computeScrollOffset()) {scrollTo(mOverScroller.getCurrX(),mOverScroller.getCurrY());}}/**** @param startx* @param starty* @param xDistance* @param yDistance* @param time*///如果time 很大，那么你的距离要相应的大 不然可能不会滑动public void startScroll(int startx,int starty,int xDistance,int yDistance,int time ){//startScroll 要调用invalidate 请求重新绘制mScroller.startScroll(startx,starty,xDistance,yDistance,time);invalidate();}public void startScroll(int startx,int starty,int xDistance,int yDistance){//startScroll 要调用invalidate 请求重新绘制
//        mScroller.startScroll(startx,starty,xDistance,yDistance);mOverScroller.startScroll(startx,starty,xDistance,yDistance);ViewCompat.postInvalidateOnAnimation(this);}@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {super.onMeasure(widthMeasureSpec, heightMeasureSpec);}
}

加油，慢慢积累，万物始于简单。one day,i will be 大神。

参考：http://www.linuxidc.com/Linux/2016-01/127276.htm