看完打造属于你的LayoutManager - huachao1001的专栏 - 博客频道 - CSDN.NET
编写的，再次感谢改博主。
public classMyGridLayoutextendsRecyclerView.LayoutManager {
privateActivitymActivity;
private inthorizontallyOffset=0;
private inttotalWidth=0;//控件的总宽度
//保存所有的Item的上下左右的偏移量信息
privateSparseArrayallItemFrames=newSparseArray<>();
//记录Item是否出现过屏幕且还没有回收。true表示出现过屏幕上，并且还没被回收
privateSparseBooleanArrayhasAttachedItems=newSparseBooleanArray();
publicMyGridLayout(Activity activity) {
this.mActivity= activity;
}
@Override
public voidonLayoutChildren(RecyclerView.Recycler recycler,RecyclerView.State state) {
//如果没有item，直接返回
if(getItemCount() <=0)return;
//跳过preLayout，preLayout主要用于支持动画
if(state.isPreLayout()) {
return;
}
//在布局之前，将所有的子View先Detach掉，放入到Scrap缓存中
detachAndScrapAttachedViews(recycler);
/**
*定义横、竖方向的偏移量
*/
intoffsetX =0;
intoffsetY =0;
totalWidth=0;//初始化宽度
intlins =1;//行
intcolumn =0;//页
intlinNumber =0;
intnumber =0;
DisplayMetrics metric =newDisplayMetrics();
mActivity.getWindowManager().getDefaultDisplay().getMetrics(metric);
intmaxWidth = metric.widthPixels;//屏幕宽度（像素）
Log.d("[MyGridLayout]","maxWidth:"+ maxWidth);
Log.d("[MyGridLayout]","ItemCount:"+ getItemCount());
for(inti =0;i < getItemCount();i++) {
View scrap = recycler.getViewForPosition(i);//根据position获取一个碎片view，可以从回收的view中获取，也可能新构造一个
addView(scrap);
measureChildWithMargins(scrap,0,0);//计算此碎片view包含边距的尺寸
intwidth = getDecoratedMeasuredWidth(scrap);//获取此碎片view包含边距和装饰的宽度width
intheight = getDecoratedMeasuredHeight(scrap);//获取此碎片view包含边距和装饰的高度height
linNumber = maxWidth / width;
/**
* view复用
*/
Rect frame =allItemFrames.get(i);
if(frame ==null) {
frame =newRect();
}
if(number >= linNumber) {//第二行
if(number == linNumber) offsetX = offsetX - linNumber * width;
//最后，将View布局到RecyclerView容器中
// layoutDecorated(scrap, offsetX, height, offsetX + width, height * 2);
frame.set(offsetX,height,offsetX + width,height *2);
offsetX += width;
if(number == (linNumber *2-1)) {
number =0;
column++;
}else{
number++;
}
}else{//第一行
//最后，将View布局到RecyclerView容器中
// layoutDecorated(scrap, offsetX, offsetY, offsetX + width, height);
frame.set(offsetX,offsetY,offsetX + width,height);
offsetX += width;
number++;
}
/**
* view复用
*/
//将当前的Item的Rect边界数据保存
allItemFrames.put(i,frame);
//由于已经调用了detachAndScrapAttachedViews，因此需要将当前的Item设置为未出现过
hasAttachedItems.put(i, false);
//第一行 第二行
totalWidth= number < linNumber ? offsetX : column * width * linNumber - (maxWidth - linNumber * width);
}
Log.d("[MyGridLayout]","totalWidth-1:"+totalWidth);
//如果所有子View的高度和没有填满RecyclerView的高度，
//则将高度设置为RecyclerView的高度
totalWidth= Math.max(totalWidth,getHorizontallySpace());
Log.d("[MyGridLayout]","totalWidth-2:"+totalWidth);
/**
* view复用
*/
recycleAndFillItems(recycler,state);
}
@Override
publicRecyclerView.LayoutParamsgenerateDefaultLayoutParams() {
return newRecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,
ViewGroup.LayoutParams.WRAP_CONTENT);
}
@Override
public booleancanScrollHorizontally() {
return true;
}
@Override
public intscrollHorizontallyBy(intdx,RecyclerView.Recycler recycler,RecyclerView.State state) {
//先detach掉所有的子View
detachAndScrapAttachedViews(recycler);
//实际要滑动的距离
inttravel = dx;
Log.d("[MyGridLayout]","travel:"+ travel);
//如果滑动到最顶部
if(horizontallyOffset+ dx <0) {
travel = -horizontallyOffset;
}else if(horizontallyOffset+ dx >totalWidth) {//如果滑动到最底部horizontallyOffset + dx > totalWidth - getHorizontallySpace()
// travel = totalWidth - getHorizontallySpace() - horizontallyOffset;
travel =totalWidth-horizontallyOffset;
}
//将横向的偏移量+travel
horizontallyOffset+= travel;
Log.d("[MyGridLayout]","travel-1:"+ travel);
//平移容器内的item
offsetChildrenHorizontal(-travel);
/**
* view复用
*/
recycleAndFillItems(recycler,state);
returntravel;
}
/**
*回收不需要的Item，并且将需要显示的Item从缓存中取出
*/
private voidrecycleAndFillItems(RecyclerView.Recycler recycler,RecyclerView.State state) {
if(state.isPreLayout()) {//跳过preLayout，preLayout主要用于支持动画
return;
}
//当前scroll offset状态下的显示区域
Rect displayFrame =newRect(horizontallyOffset,0,horizontallyOffset+ getHorizontallySpace(),getVerticalSpace());
/**
*将滑出屏幕的Items回收到Recycle缓存中
*/
Rect childFrame =newRect();
for(inti =0;i < getChildCount();i++) {
View child = getChildAt(i);
childFrame.left= getDecoratedLeft(child);
childFrame.top= getDecoratedTop(child);
childFrame.right= getDecoratedRight(child);
childFrame.bottom= getDecoratedBottom(child);
//如果Item没有在显示区域，就说明需要回收
if(!Rect.intersects(displayFrame,childFrame)) {
//回收掉滑出屏幕的View
removeAndRecycleView(child,recycler);
}
}
//重新显示需要出现在屏幕的子View
for(inti =0;i < getItemCount();i++) {
if(Rect.intersects(displayFrame,allItemFrames.get(i))) {
View scrap = recycler.getViewForPosition(i);
measureChildWithMargins(scrap,0,0);
addView(scrap);
Rect frame =allItemFrames.get(i);// -horizontallyOffset
//将这个item布局出来
layoutDecorated(scrap,
frame.left-horizontallyOffset,
frame.top,
frame.right-horizontallyOffset,
frame.bottom);
}
}
}
/**
*获取RecyclerView在水平方向上的可用空间，
*即去除了padding后的高度
*/
private intgetHorizontallySpace() {
returngetWidth() - getPaddingLeft() - getPaddingRight();
}
/**
*获取RecyclerView在竖直方向上的可用空间，
*即去除了padding后的高度
*/
private intgetVerticalSpace() {
returngetHeight() - getPaddingBottom() - getPaddingTop();
}
}
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