android scrollTo 方法為什麼傳負值往正方向走

最近做的demo裡面用到scrollto 和scroller 發現傳遞負值的時候 view才往坐標軸的正方向滾動

很不理解，因為scrollto的注釋寫的是

* @param x the x position to scroll to

* @param y the y position to scroll to

查了半天view的代碼

找到一個可能是原因的地方：代碼如下（只是個人猜的）

/**

* 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

就是說在ondraw之前調用了這個方法 而且給view了一個指定的矩形框

而矩形框的值是這樣設置的

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需要傳遞負值 要不然這裡的矩形框區域的 坐標系會往負方向移動

所以view的scrollto方法的注釋寫的我有點不明白