什么是Scroller
我们知道View中有两个方法可以实现滚动/位置变化，scrollTo/scrollBy
/**
* Set the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the x position to scroll to
* @param y the y position to scroll to
*/
public void scrollTo(int x, int y) `
/**
* Move the scrolled position of your view. This will cause a call to
* {@link #onScrollChanged(int, int, int, int)} and the view will be
* invalidated.
* @param x the amount of pixels to scroll by horizontally
* @param y the amount of pixels to scroll by vertically
*/
public void scrollBy(int x, int y) {
scrollTo(mScrollX + x, mScrollY + y);
}
那为什么view已经有了滚动的能力还要一个Scroller去单独实现一套：
- 自带的效果比较生硬（瞬间完成），没有中间的过度效果，所以需要scroller
Sroller 使用
郭霖老师文章里的使用：
public class ScrollerLayout extends ViewGroup {
private static final String TAG = "ScrollerLayout";
/**
* 用于完成滚动操作的实例
*/
private Scroller mScroller;
/**
* 判定为拖动的最小移动像素数
*/
private int mTouchSlop;
/**
* 手机按下时的屏幕坐标
*/
private float mXDown;
/**
* 手机当时所处的屏幕坐标
*/
private float mXMove;
/**
* 上次触发ACTION_MOVE事件时的屏幕坐标
*/
private float mXLastMove;
/**
* 界面可滚动的左边界
*/
private int leftBorder;
/**
* 界面可滚动的右边界
*/
private int rightBorder;
public ScrollerLayout(Context context, AttributeSet attrs) {
super(context, attrs);
// 第一步，创建Scroller的实例
mScroller = new Scroller(context);
ViewConfiguration configuration = ViewConfiguration.get(context);
// 获取TouchSlop值
mTouchSlop = ViewConfigurationCompat.getScaledPagingTouchSlop(configuration);
Log.d(TAG, "ScrollerLayout: default=" + ViewConfigurationCompat.getScaledPagingTouchSlop(configuration)
+ " mTouchSlop=" + mTouchSlop);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
Log.d(TAG, "onMeasure: widthMeasureSpec=" + widthMeasureSpec + " heightMeasureSpec=" + heightMeasureSpec);
int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
View childView = getChildAt(i);
// 为ScrollerLayout中的每一个子控件测量大小
measureChild(childView, widthMeasureSpec, heightMeasureSpec);
}
}
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
if (changed) {
int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
View childView = getChildAt(i);
// 为ScrollerLayout中的每一个子控件在水平方向上进行布局
childView.layout(i * childView.getMeasuredWidth(), 0, (i + 1) * childView.getMeasuredWidth(), childView.getMeasuredHeight());
}
// 初始化左右边界值
leftBorder = getChildAt(0).getLeft();
rightBorder = getChildAt(getChildCount() - 1).getRight();
Log.d(TAG, "onLayout: changed=" + changed + " l=" + l + " t=" + t + " r=" + r + " b=" + b
+ " leftBorder=" + leftBorder + " rightBorder=" + rightBorder);
}
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
switch (ev.getAction()) {
case MotionEvent.ACTION_DOWN:
mXDown = ev.getRawX();
mXLastMove = mXDown;
break;
case MotionEvent.ACTION_MOVE:
mXMove = ev.getRawX();
float diff = Math.abs(mXMove - mXDown);
mXLastMove = mXMove;
// 当手指拖动值大于TouchSlop值时，认为应该进行滚动，拦截子控件的事件
if (diff > mTouchSlop) {
return true;
}
break;
}
return super.onInterceptTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
Log.d(TAG, "onTouchEvent: event=" + event.getAction());
switch (event.getAction()) {
case MotionEvent.ACTION_MOVE:
mXMove = event.getRawX();
int scrolledX = (int) (mXLastMove - mXMove);
if (getScrollX() + scrolledX < leftBorder) {
scrollTo(leftBorder, 0);
return true;
} else if (getScrollX() + getWidth() + scrolledX > rightBorder) {
scrollTo(rightBorder - getWidth(), 0);
return true;
}
scrollBy(scrolledX, 0);
mXLastMove = mXMove;
break;
case MotionEvent.ACTION_UP:
// 当手指抬起时，根据当前的滚动值来判定应该滚动到哪个子控件的界面
int targetIndex = (getScrollX() + getWidth() / 2) / getWidth();
int dx = targetIndex * getWidth() - getScrollX();
// 第二步，调用startScroll()方法来初始化滚动数据并刷新界面
mScroller.startScroll(getScrollX(), 0, dx, 0);
invalidate();
break;
}
return super.onTouchEvent(event);
}
@Override
public void computeScroll() {
Log.d(TAG, "computeScroll: ");
// 第三步，重写computeScroll()方法，并在其内部完成平滑滚动的逻辑
if (mScroller.computeScrollOffset()) {
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
invalidate();
}
}
}
- TouchSlop是做什么用的