RecyclerView.ItemDecoration
这个类包含三个方法 1:
•onDraw(Canvas c, RecyclerView parent, State state)
•onDrawOver(Canvas c, RecyclerView parent, State state)
•getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)
getItemOffsets 中为 outRect 设置的4个方向的值,将对child的大小有影响。
outRect.set(left, top, right, bottom);
Rect getItemDecorInsetsForChild(View child) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (!lp.mInsetsDirty) {
return lp.mDecorInsets;
}
final Rect insets = lp.mDecorInsets;
insets.set(0, 0, 0, 0);
final int decorCount = mItemDecorations.size();
for (int i = 0; i < decorCount; i++) {
mTempRect.set(0, 0, 0, 0);
mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);
insets.left += mTempRect.left;
insets.top += mTempRect.top;
insets.right += mTempRect.right;
insets.bottom += mTempRect.bottom;
}
lp.mInsetsDirty = false;
return insets;
}
public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
widthUsed += insets.left + insets.right;
heightUsed += insets.top + insets.bottom;
final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
getPaddingLeft() + getPaddingRight() +
lp.leftMargin + lp.rightMargin + widthUsed, lp.width,
canScrollHorizontally());
final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
getPaddingTop() + getPaddingBottom() +
lp.topMargin + lp.bottomMargin + heightUsed, lp.height,
canScrollVertically());
if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
child.measure(widthSpec, heightSpec);
}
}
在计算child的大小的时候,将outRect设置的四边减去了。
1、当RecyclerView可以水平滑动的时候
outRect设置的top,bottom将影响item的高度大小
2、当RecyclerView可以竖直滑动的时候
outRect设置的top,bottom将影响item的宽度大小
public static int getChildMeasureSpec(int parentSize, int parentMode, int padding,
int childDimension, boolean canScroll) {
int size = Math.max(0, parentSize - padding);
int resultSize = 0;
int resultMode = 0;
if (canScroll) {
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
switch (parentMode) {
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
resultSize = size;
resultMode = parentMode;
break;
case MeasureSpec.UNSPECIFIED:
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
break;
}
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
} else {
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = size;
resultMode = parentMode;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = size;
if (parentMode == MeasureSpec.AT_MOST || parentMode == MeasureSpec.EXACTLY) {
resultMode = MeasureSpec.AT_MOST;
} else {
resultMode = MeasureSpec.UNSPECIFIED;
}
}
}
//noinspection WrongConstant
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
onDraw和onDrawOver画的区域不受outRect影响。绘制时,先执行ItemDecoration的onDraw,然后执行绘制child,最后执行ItemDecoration的onDrawOver.