1.看过第一篇文章的都会注意到在设置RecyclerView的GridLayoutManager的Item时设置的是正方形的布局。可是当设置自定义分割线的时候出现了一些问题。如果对GridLayoutManager添加分割线有疑惑的可以查看上一篇文章。
RecyclerView添加自定义ItemDecoration实现(1)
具体情况如下图所示:
GridLayotManager_bug_5dp.png
问题:从图中可能不能很好的看出存在的问题。我们不妨加大分割线的宽度之后再观察。所以我们将之前分割线的宽度改为20dp。效果如下:
drawable_item_decoration_20dp.png
GridLayoutManager_bug_20dp.png
现在可以很明显的看出之前遗留下来的bug：第一列和第二列的形状和第三列的不同。那么为什么会出现这种情况呢？我们下面从源码分析一下添加ItemDecoration的原理。
2.在RecyclerView中有如下一部分代码
/**
* Measure a child view using standard measurement policy, taking the padding
* of the parent RecyclerView and any added item decorations into account.
*
* <p>If the RecyclerView can be scrolled in either dimension the caller may
* pass 0 as the widthUsed or heightUsed parameters as they will be irrelevant.</p>
*
* @param child Child view to measure
* @param widthUsed Width in pixels currently consumed by other views, if relevant
* @param heightUsed Height in pixels currently consumed by other views, if relevant
*/
public void measureChild(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() + widthUsed, lp.width,
canScrollHorizontally());
final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
getPaddingTop() + getPaddingBottom() + heightUsed, lp.height,
canScrollVertically());
if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
child.measure(widthSpec, heightSpec);
}
}
/**
* Calculate a MeasureSpec value for measuring a child view in one dimension.
*
* @param parentSize Size of the parent view where the child will be placed
* @param parentMode The measurement spec mode of the parent
* @param padding Total space currently consumed by other elements of parent
* @param childDimension Desired size of the child view, or MATCH_PARENT/WRAP_CONTENT.
* Generally obtained from the child view's LayoutParams
* @param canScroll true if the parent RecyclerView can scroll in this dimension
*
* @return a MeasureSpec value for the child view
*/
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 (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (canScroll) {
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 == 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);
}
Rect getItemDecorInsetsForChild(View child) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
if (!lp.mInsetsDirty) {
return lp.mDecorInsets;
}
if (mState.isPreLayout() && (lp.isItemChanged() || lp.isViewInvalid())) {
// changed/invalid items should not be updated until they are rebound.
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;
}
- 首先我们先看 getChildMeasureSpec 方法。 当 childDimension >= 0 时让结果为 childDimension，否则为 Math.max(0, parentSize – padding)。这个的意思就是如果子view的尺寸如果是精确值的话，那么就是精确值的结果，如果是 MATCH_PARENT 或者 WRAP_CONTENT 的话就开始计算结果。
其次我们从 measureChild 方法中可以看出，widthSpec 的 getChildMeasureSpec 中可以得到
paddingg = getPaddingLeft() + getPaddingRight() + widthUsed;而
final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child); widthUsed += insets.left + insets.right;最后我们可以从 getItemDecorInsetsForChild 方法中可以看出 Rect insets就是在自定义 ItemDecoration 时
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {} 中的outRect。所以计算 widthUsed 时候，与 outRect 有关，而我们在 getItemOffsets 方法中有这么一段代码if (!isLastColumn(currentItemPosition, mSpanCount)) { outRect.right = mDrawable.getIntrinsicWidth(); } else { outRect.right = 0; }如果是最后一列的话，outRect.right = 0；这使得第三列子view的 widthUsed 与前两列的不同，所以第三列子view的 padding 就会比前两列的少一个分割线的宽度。
3.解决
那我们应该怎么解决这个问题呢？问题的关键就是分割线的宽度分配不均导致的，我们只要均匀分配宽度即可。
statement.png
如图所示，我们对于有三列的情况来说，averWidth = (2 * dividerWidth)/3, 将两个分割线的宽度，均分给3个子 view 的 outRect。所以为了适配其他情况得到如下式子 :
averWidth = [(spanCount - 1 ) * dividerWidth ]/spanCount ;
现在是最重要的步骤，如果将averWidth分给每个子 view，就会存在如下情况：
leftN + rightN = averWidth;
left(N+1) + rigthN = dividerWidth;
(注:leftN为第N个view的左偏移值，rightN为第N个View的右偏移值,计数从0开始)
对于 rightN 存在如下的情况:
right0 = averWidth - left0;
right1 = averWidth - left1;
right2 = averWidth - left2;
.
.
.
rightN = averWidth - leftN;
对于 leftN 存在如下的情况:
left0 = 0;
left1 = dividerWidth - right0;
left2 = dividerWidth - right1;
.
.
.
leftN = dividerWidth - right(N-1);
从上面的式子可以看出，我们只要得到 leftN 的就能很快的得到 rightN，所以将 rightN 式子合并到 leftN 中得到如下的结果：
left0 = 0;
left1 = dividerWidth - (averWidth - left0)
= dividerWidth - averWidth + left0
= dividerWidth - averWidth
left2 = dividerWidth - (averWidth - left1)
= dividerWidth -[averWidth - (dividerWidth - averWidth)]
= 2 * (dividerWidth - averWidth)
.
.
.
leftN = N * (dividerWidth - averWidth)
所以，根据化简后的结果更改代码，修改完后的 getItemOffsets 方法具体如下:
/**
* @param outRect 用于规定分割线的范围
* @param view 进行分割线操作的子view
* @param parent 父view
* @param state (这里暂时不使用)
*/
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
//notification的时候要获取
mTotalItems = parent.getAdapter().getItemCount();
if (0 == mSpanCount) {
RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
//判断是否为GridLayoutManager
if (layoutManager instanceof GridLayoutManager) {
GridLayoutManager gridLayoutManager = (GridLayoutManager) layoutManager;
mSpanCount = gridLayoutManager.getSpanCount();
} else {
mSpanCount = 1;
}
}
//在源码中有一个过时的方法，里面有获取当前ItemPosition
int currentItemPosition = ((RecyclerView.LayoutParams) view.getLayoutParams()).getViewLayoutPosition();
//
if (!isLastRow(currentItemPosition, mTotalItems, mSpanCount))
outRect.bottom = mDrawable.getIntrinsicWidth();
else
outRect.bottom = 0;
//将isLastColumn注释，添加下面代码
// if (!isLastColumn(currentItemPosition, mSpanCount)) {
// outRect.right = mDrawable.getIntrinsicWidth();
// } else {
// outRect.right = 0;
// }
int averWidth = (mSpanCount - 1) * mDrawable.getIntrinsicWidth() / mSpanCount;
int dX = mDrawable.getIntrinsicWidth() - averWidth;
outRect.left = (currentItemPosition % mSpanCount) * dX;
outRect.right = averWidth - outRect.left;
}
结果截图:
final.png
