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如果GridLayoutManager使用item的布局都是wrap_cotent 那么会在布局更改时会出现一些出人意料的情况。#xff08;本文完全不具备可读性和说教性#xff0c;仅为博主方便查找问题#xff09;
布局item:
!--layout_item.xml--
?xml version1.0本文完全不具备可读性和说教性仅为博主方便查找问题
布局item:
!--layout_item.xml--
?xml version1.0 encodingutf-8?
com.vb.rerdemo.MyConstraintLayout xmlns:androidhttp://schemas.android.com/apk/res/androidxmlns:apphttp://schemas.android.com/apk/res-autoandroid:layout_widthmatch_parentandroid:layout_heightwrap_contentandroid:layout_marginTop10dpandroid:background#f0fcom.google.android.material.card.MaterialCardViewandroid:layout_widthmatch_parentapp:layout_constraintTop_toTopOfparentapp:cardCornerRadius10dpapp:cardBackgroundColor#908000android:layout_height240dpTextViewandroid:idid/tvandroid:layout_gravitycenterandroid:layout_widthwrap_contentandroid:texthello worldandroid:layout_heightwrap_contentapp:layout_constraintBottom_toBottomOfparentapp:layout_constraintEnd_toEndOfparentapp:layout_constraintStart_toStartOfparentapp:layout_constraintTop_toTopOfparent //com.google.android.material.card.MaterialCardView
/com.vb.rerdemo.MyConstraintLayout//LastGapDecoration.kt
//给最后一行的item添加一个高度
class LastGapDecoration : ItemDecoration() {override fun getItemOffsets(outRect: Rect,view: View,parent: RecyclerView,state: RecyclerView.State) {super.getItemOffsets(outRect, view, parent, state)super.getItemOffsets(outRect, view, parent, state)val itemPosition parent.getChildAdapterPosition(view)val gridLayoutManager parent.layoutManager as? GridLayoutManager ?: returnval spanCount gridLayoutManager.spanCountval itemCount gridLayoutManager.itemCountif (spanCount 0) {return}val lastRowItemCount itemCount % spanCountval lastRow isLastRow(itemPosition, itemCount, spanCount, lastRowItemCount)Log.d(fmy,lastRow ${lastRow} itemPosition ${itemPosition} lastRowItemCount ${lastRowItemCount} itemCount ${itemCount} viewid ${view.hashCode()})if (lastRow) {outRect.bottom ScreenUtil.dp2px(40f,App.myapp)} else {outRect.bottom 0}}private fun isLastRow(itemPosition: Int,itemCount: Int,spanCount: Int,lastRowItemCount: Int): Boolean {// 如果最后一行的数量不足一整行则直接判断位置if (lastRowItemCount ! 0 itemPosition itemCount - lastRowItemCount) {return true}// 如果最后一行的数量足够一整行则需要计算val rowIndex itemPosition / spanCountval totalRow ceil(itemCount.toDouble() / spanCount).toInt()return rowIndex totalRow - 1}
}当我们填充6个布局后的效果
红色区域和4和5之间的间距通过LastGapDecoration完成。
此时我们移除3后 根本原因在于GridLayoutManager#layoutChunk函数中 public class GridLayoutManager extends LinearLayoutManager {View[] mSet;//layoutChunk 每次调用只拿取当前行view进行对比计算//比如GridLayoutManager一行两个那么每次会拿取每行的对应view进行计算void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,LayoutState layoutState, LayoutChunkResult result) {int count 0;while (count mSpanCount layoutState.hasMore(state) remainingSpan 0) {//略... 经过一些的计算mSet放入本次要进行摆放的view//count 一般为GridLayoutManager的spanCount数量mSet[count] view;count;}//maxSize是指在本次layoutChunk中所有view里面最大的高度数据。包含view自身和ItemDecorations得到的int maxSize 0;// we should assign spans before item decor offsets are calculatedfor (int i 0; i count; i) {//计算ItemDecorationscalculateItemDecorationsForChild(view, mDecorInsets);//调用measure计算view宽高 核心//核心代码点注意这里调用子view的measure参数为layoutparameter高度//我们把这里称为操作AmeasureChild(view, otherDirSpecMode, false);//核心代码点这里这里会得到这个view的宽高和ItemDecorations填充的高度和final int size mOrientationHelper.getDecoratedMeasurement(view);//核心代码点 记录最大数值if (size maxSize) {maxSize size;}}//我们把这里称为操作B//取出当前行中的所有view。保证行高度一致for (int i 0; i count; i) {final View view mSet[i];if (mOrientationHelper.getDecoratedMeasurement(view) ! maxSize) {final LayoutParams lp (LayoutParams) view.getLayoutParams();final Rect decorInsets lp.mDecorInsets;final int verticalInsets decorInsets.top decorInsets.bottom lp.topMargin lp.bottomMargin;final int horizontalInsets decorInsets.left decorInsets.right lp.leftMargin lp.rightMargin;final int totalSpaceInOther getSpaceForSpanRange(lp.mSpanIndex, lp.mSpanSize);final int wSpec;final int hSpec;if (mOrientation VERTICAL) {wSpec getChildMeasureSpec(totalSpaceInOther, View.MeasureSpec.EXACTLY,horizontalInsets, lp.width, false);//核心代码点 这里会强制当前行所有view的高度与最高的view保持一致。 hSpec View.MeasureSpec.makeMeasureSpec(maxSize - verticalInsets,View.MeasureSpec.EXACTLY);} else {//略}//执行测量measureChildWithDecorationsAndMargin(view, wSpec, hSpec, true);}} }
} 上面的代码可以总结为
取出当前的所有view对所有view执行一次高度测量并记录当前最高的view数据在此执行一次测量保证当前行的所有view高度一致
我们重点再看一眼measureChild函数 private void measureChild(View view, int otherDirParentSpecMode, boolean alreadyMeasured) {final LayoutParams lp (LayoutParams) view.getLayoutParams();final Rect decorInsets lp.mDecorInsets;final int verticalInsets decorInsets.top decorInsets.bottom lp.topMargin lp.bottomMargin;final int horizontalInsets decorInsets.left decorInsets.right lp.leftMargin lp.rightMargin;final int availableSpaceInOther getSpaceForSpanRange(lp.mSpanIndex, lp.mSpanSize);final int wSpec;final int hSpec;if (mOrientation VERTICAL) {wSpec getChildMeasureSpec(availableSpaceInOther, otherDirParentSpecMode,horizontalInsets, lp.width, false);//mOrientationHelper.getTotalSpace()可以先忽略//verticalInsets 就是decorate中的高度和一些margin等数值//lp.height如果是wrapcontent那么一返回高度为0的MeasureSpec.UNSPECIFIED//lp.height如果不是wrapcontent那么一返回高度为父亲高度减去verticalInsets的MeasureSpec.EXACTLY//lp.height如果是一个明确数值那么一返回高度为设置的高度的MeasureSpec.EXACTLY//总结getChildMeasureSpec传入布局参数高度和decorate高度hSpec getChildMeasureSpec(mOrientationHelper.getTotalSpace(), getHeightMode(),verticalInsets, lp.height, true);} else {//略}//透传给子view测量measureChildWithDecorationsAndMargin(view, wSpec, hSpec, alreadyMeasured);}measureChildWithDecorationsAndMargin函数会根据必要性确定是否要执行子view的测量操作。
private void measureChildWithDecorationsAndMargin(View child, int widthSpec, int heightSpec,boolean alreadyMeasured) {RecyclerView.LayoutParams lp (RecyclerView.LayoutParams) child.getLayoutParams();final boolean measure;if (alreadyMeasured) {measure shouldReMeasureChild(child, widthSpec, heightSpec, lp);} else {measure shouldMeasureChild(child, widthSpec, heightSpec, lp);}//根据情况是否执行if (measure) {child.measure(widthSpec, heightSpec);}}boolean shouldReMeasureChild(View child, int widthSpec, int heightSpec, LayoutParams lp) {return !mMeasurementCacheEnabled|| !isMeasurementUpToDate(child.getMeasuredWidth(), widthSpec, lp.width)|| !isMeasurementUpToDate(child.getMeasuredHeight(), heightSpec, lp.height);}boolean shouldMeasureChild(View child, int widthSpec, int heightSpec, LayoutParams lp) {return child.isLayoutRequested()|| !mMeasurementCacheEnabled|| !isMeasurementUpToDate(child.getWidth(), widthSpec, lp.width)|| !isMeasurementUpToDate(child.getHeight(), heightSpec, lp.height);} shouldReMeasureChild可以总结为
如果没有开启缓存那么一定执行测绘如果开启了缓存那么判断之前是否执行过相同参数测量 在了解上面的信息我们可以总结一下流程发现问题 我们假设假设wrapcotent计算的高度为50 decorate插入的高度为10
插入0时0执行操作A不执行操作B
插入1时
0和1同时执行操作A不执行操作B。0由于之前测绘过不会触发onmeasure。 1触发onmeasure
插入2时
0和1同时执行操作A,不执行操作B 0和1不会触发onmeasure。 2执行操作A并触发onmeasure
插入3时
0和1同时执行操作A,不执行操作B 0和1不会触发onmeasure。 3和2执行操作A 2不会触发onmeasure3触发onmeasure。
移除1时
0 和 1 同时执行操作A 0 和1不会触发onmeasure操作B不会执行(虽然1被移除 但是由于预布局存在还需要进行一次比较)2 和 3 同时执行操作A 2 和3不会触发onmeasure. 由于2移动第一行不会有decorate高度因此2执行操作B并触发onmeasure。2 高度为60移除后2和3虽然不在一行但需要执行预布局0和2进行同时执行操作A 0 和2不会触发onmeasure同时0会被执行操作B把高度填充到60. 虽然2没有decorate的高度 但是上一次预布局引起了2高度错误3 同时执行操作A 不会触发onmeasure 不会触发操作B
解决方案
val manager GridLayoutManager(this, 2)
manager.isMeasurementCacheEnabled false
