最近做的demo里面用到scrollto 和scroller 发现传递负值的时候 view才往坐标轴的正方向滚动
很不理解，因为scrollto的注释写的是
* @param x the x position to scroll to
* @param y the y position to scroll to
查了半天view的代码
找到一个可能是原因的地方：代码如下（只是个人猜的）
/**
* Mark the area defined by the rect (l,t,r,b) as needing to be drawn.* The coordinates of the dirty rect are relative to the view.
* If the view is visible, {@link #onDraw(android.graphics.Canvas)}
* will be called at some point in the future. This must be called from
* a UI thread. To call from a non-UI thread, call {@link #postInvalidate()}.
* @param l the left position of the dirty region
* @param t the top position of the dirty region
* @param r the right position of the dirty region
* @param b the bottom position of the dirty region
*/
public void invalidate(int l, int t, int r, int b) {
if (skipInvalidate()) {
return;
}
if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS) ||
(mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID ||
(mPrivateFlags & INVALIDATED) != INVALIDATED) {
mPrivateFlags &= ~DRAWING_CACHE_VALID;
mPrivateFlags |= INVALIDATED;
mPrivateFlags |= DIRTY;
final ViewParent p = mParent;
final AttachInfo ai = mAttachInfo;
//noinspection PointlessBooleanExpression,ConstantConditions
if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
if (p != null && ai != null && ai.mHardwareAccelerated) {
// fast-track for GL-enabled applications; just invalidate the whole hierarchy
// with a null dirty rect, which tells the ViewAncestor to redraw everything
p.invalidateChild(this, null);
return;
}
}
if (p != null && ai != null && l < r && t < b) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;
final Rect tmpr = ai.mTmpInvalRect;
tmpr.set(l - scrollX, t - scrollY, r - scrollX, b - scrollY);
p.invalidateChild(this, tmpr);
}
}
}
If the view is visible, onDraw(android.graphics.Canvas) will be called at some point in the future
就是说在ondraw之前调用了这个方法 而且给view了一个指定的矩形框
而矩形框的值是这样设置的
final int scrollX = mScrollX;
final int scrollY = mScrollY;
final Rect tmpr = ai.mTmpInvalRect;
tmpr.set(l - scrollX, t - scrollY, r - scrollX, b - scrollY);
p.invalidateChild(this, tmpr);
所以scrollto需要传递负值 要不然这里的矩形框区域的 坐标系会往负方向移动
所以view的scrollto方法的注释写的我有点不明白