参考博客 https://blog.csdn.net/zxt0601/article/details/52956504
目标
学习写个简陋版的LinearLayoutManager
流程
- 继承RecyclerView.LayoutManager
- 重写onLayoutChildren来添加子View
- 重写scrollVerticallyBy来实现竖向滚动
继承RecyclerView.LayoutManager
就一个抽象方法,重写就行了
public class CustomLayoutManager extends RecyclerView.LayoutManager {
@Override
public RecyclerView.LayoutParams generateDefaultLayoutParams() {
return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,ViewGroup.LayoutParams.WRAP_CONTENT);
}
}
onLayoutChildren
相当于ViewGroup的 onLayout.一开始的界面构建就是这个入口
1 在RecyclerView初始化时，会被调用两次。
2 在调用adapter.notifyDataSetChanged()时，会被调用。
3 在调用setAdapter替换Adapter时,会被调用。
4 在RecyclerView执行动画时，它也会被调用。
onLayoutChildren中的流程
- 报废当前的View
- 获取对应位置的子view
- 添加进RecyclerView
- 测量子View的宽高
- 根据测量的宽高,给他们排列好位置
注意,这一版本是没考虑缓存,全一股脑添加进RecyclerView的,目的是熟悉代码为后面铺垫
其中的函数
- detachAndScrapAttachedViews : 是将当前Recycler中的view全部移除并放到报废缓存里,之后优先重用缓存里的view
- getDecoratedMeasuredWidth/getDecoratedMeasuredHeight 获取宽高,这个是加上了DecorateView
的,现在没有 RecyclerView.addItemDecoration();直接理解为宽高就行 - layoutDecorated就是用来个子View排位置的
- actualHeight记录了目前的高度,为了实现LinearLayoutManager的垂直排序来的
然后看看具体代码吧
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
if (getItemCount() == 0){
detachAndScrapAttachedViews(recycler);
return;
}
//state.isPreLayout()是支持动画的
if (getItemCount() == 0 && state.isPreLayout()){
return;
}
detachAndScrapAttachedViews(recycler);
actualHeight = 0;
for (int i = 0 ;i < getItemCount() ; i++){
View scrap = recycler.getViewForPosition(i);
addView(scrap);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,actualHeight,width,actualHeight+height);
actualHeight+=height;
}
}
测试的Activity
public class LayoutManagerActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_layout_manager);
CustomLayoutManager customLayoutManager = new CustomLayoutManager();
RecyclerView rv = findViewById(R.id.rv);
rv.setLayoutManager(customLayoutManager);
rv.setAdapter(new RecyclerView.Adapter<VH>() {
@NonNull
@Override
public VH onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.item_custom,viewGroup,false);
Log.d("LayoutManagerActivity","onCreateViewHolder " + i);
return new VH(view);
}
@Override
public void onBindViewHolder(@NonNull VH viewHolder, int i) {
viewHolder.tv.setText(i+"");
if (i%2 == 0){
viewHolder.tv.setBackgroundColor(Color.RED);
}else {
viewHolder.tv.setBackgroundColor(Color.BLUE);
}
Log.d("LayoutManagerActivity","onBindViewHolder " + i);
}
@Override
public int getItemCount() {
return 20;
}
});
}
private static class VH extends RecyclerView.ViewHolder{
TextView tv ;
public VH(@NonNull View itemView) {
super(itemView);
tv = itemView.findViewById(R.id.tv);
}
}
}
就可以看到布局排列好了.但是
- onCreateViewHolder调用了20次,onBindViewHolder也调用了20.
- 不能滑动
滑动
canScrollVertically返回true就是可以垂直滑动
scrollVerticallyBy是滑动具体的逻辑
scrollVerticallyBy的参数要说明下
参数:
dy : 是当前滑动的距离,界面向下滚动的时候,dy为正,向上滚动的时候dy为负
返回的值: 如果Math.abs(返回值)小于dy,说明到达边界了,这里简单的处理下,如果到达边界了直接返回0
逻辑
- 通过totalScrollY来记录已经滑动的总距离
- 向下滚动的时候,如果总距离超过了子view的总高度-屏幕高度,说明到达下边界了
- 向上滚动的时候,如果总距离小于等于0,就是到达了上边界
- 其他就是正常情况了,使用offsetChildrenVertical来滚动界面
具体如下
@Override
public boolean canScrollVertically() {
return true;
}
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
//界面向下滚动的时候,dy为正,向上滚动的时候dy为负
//向下滚动的时候,最下面的值不能超过总值,
//向上滚动的时候,最上面的值不能小于0
int willScrollTo = totalScrollY + dy;
Log.d(TAG,"scrollVerticallyBy " + dy + " totalScrollY " + totalScrollY);
if (willScrollTo >= actualHeight-getHeight()){
offsetChildrenVertical(-1*(actualHeight - getHeight() - totalScrollY));
totalScrollY = actualHeight- getHeight();
return 0;
}
if (willScrollTo <= 0){
offsetChildrenVertical(totalScrollY);
totalScrollY = 0;
return 0;
}
offsetChildrenVertical(dy*-1);
totalScrollY +=dy;
return dy;
}
这样就完成了简陋版的LinearLayoutManager.完整代码
/**
* 只有填充,滑动,没有回收
*/
public class CustomLayoutManager extends RecyclerView.LayoutManager {
private final String TAG = "feifeifei";
private int actualHeight = 0;
private int totalScrollY = 0;
@Override
public RecyclerView.LayoutParams generateDefaultLayoutParams() {
return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,ViewGroup.LayoutParams.WRAP_CONTENT);
}
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
if (getItemCount() == 0){
detachAndScrapAttachedViews(recycler);
return;
}
//state.isPreLayout()是支持动画的
if (getItemCount() == 0 && state.isPreLayout()){
return;
}
detachAndScrapAttachedViews(recycler);
actualHeight = 0;
for (int i = 0 ;i < getItemCount() ; i++){
View scrap = recycler.getViewForPosition(i);
addView(scrap);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,actualHeight,width,actualHeight+height);
actualHeight+=height;
}
}
@Override
public boolean canScrollVertically() {
return true;
}
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
//界面向下滚动的时候,dy为正,向上滚动的时候dy为负
//向下滚动的时候,最下面的值不能超过总值,
//向上滚动的时候,最上面的值不能小于0
int willScrollTo = totalScrollY + dy;
Log.d(TAG,"scrollVerticallyBy " + dy + " totalScrollY " + totalScrollY);
if (willScrollTo >= actualHeight-getHeight()){
offsetChildrenVertical(-1*(actualHeight - getHeight() - totalScrollY));
totalScrollY = actualHeight- getHeight();
return 0;
}
if (willScrollTo <= 0){
offsetChildrenVertical(totalScrollY);
totalScrollY = 0;
return 0;
}
offsetChildrenVertical(dy*-1);
totalScrollY +=dy;
return dy;
}
}
加入回收功能
其实就是基于上边的简陋版本进行扩展
- onLayoutChildren的时候不添加全部view,只添加可视范围内的View
- 滑动的时候要更复杂一点
- 如果向下滚动,先往RecyclerView下面添加即将展示的View
- 如果往上滚动,就往RecyclerView上面添加即将展示的View
- 添加完View后就调用offsetChildrenVertical进行滚动
- 完了后检查是否有子View离开了可视界面,如果不可见了,就是用removeAndRecycleView来移除掉
onLayoutChildren
与之前不同的就是最后几句,如果超过RecyclerView的高度了,就不Add了
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
Log.d(TAG,"onLayoutChildren ");
if (getItemCount() == 0){
detachAndScrapAttachedViews(recycler);
return;
}
//state.isPreLayout()是支持动画的
if (getItemCount() == 0 && state.isPreLayout()){
return;
}
//将当前Recycler中的view全部移除并放到报废缓存里,之后优先重用缓存里的view
detachAndScrapAttachedViews(recycler);
int actualHeight = 0;
for (int i = 0 ;i < getItemCount() ; i++){
View scrap = recycler.getViewForPosition(i);
addView(scrap);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,actualHeight,width,actualHeight+height);
actualHeight+=height;
//超出界面的就不画了,也不add了
if (actualHeight > getHeight()){
break;
}
}
}
scrollVerticallyBy
之前说了,分为填充,滚动,回收
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
Log.d("feifeifei","getChildCount() " + getChildCount() + " recycler.getScrapList().size() " + recycler.getScrapList().size());
//界面向下滚动的时候,dy为正,向上滚动的时候dy为负
//向下滚动的时候,最下面的值不能超过总值,
//向上滚动的时候,最上面的值不能小于0
//填充
fill(dy,recycler,state);
//滚动
offsetChildrenVertical(dy*-1);
//回收已经离开界面的
recycleOut(dy,recycler,state);
return dy;
}
填充
例如向下滚动
- 通过getChildAt获取最后一个View
- 再通过getPosition获取这个View的Adapter中的位置,最后一个了,就不要继续填充了,因为没有了,如果有下一个,就继续
- 这里还没有滑动,但是即将滑动的距离dy传进来了,如果最后一个View滑动dy后小于RecyclerView的高度了说明最后一个View已经全部出现在界面上了,之后就是空白了,需要添加新的子View
- 那就做获取,测量,添加操作
向上滚动是一样的逻辑
private void fill(int dy, RecyclerView.Recycler recycler, RecyclerView.State state){
//向下滚动
if (dy > 0){
//先在底部填充
View lastView = getChildAt(getChildCount() -1);
int lastPos = getPosition(lastView);
if (lastPos == getChildCount()-1){
return;
}
Log.d("feifeifei","lastView top" + lastView.getTop() + " bottom " + lastView.getBottom());
if (lastView.getBottom() - dy < getHeight()){
View scrap = recycler.getViewForPosition(lastPos+1);
addView(scrap);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,lastView.getBottom(),width,lastView.getBottom()+height);
}
}else {
//向上滚动
//现在顶部填充
View firstView = getChildAt(0);
Log.d("feifeifei","firstView top" + firstView.getTop() + " bottom " + firstView.getBottom());
int layoutPostion = getPosition(firstView);
if (layoutPostion == 0){
return;
}
if (firstView.getTop() >= 0 ){
View scrap = recycler.getViewForPosition(layoutPostion -1);
addView(scrap,0);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,firstView.getTop() - height,width,firstView.getTop());
}
}
}
滚动
滚动就是直接用offsetChildrenVertical(dy*-1);但是需要和简陋版一样,需要搞定边界问题
例如: 到达上边界后,滑动的距离不是dy,而是第一个View还剩下多少距离可以滑动,代码如下
int canScroll = dy;
if (dy>0){
View lastView = getChildAt(getChildCount() -1);
int lastPos = getPosition(lastView);
if (lastPos >= getItemCount()-1){
if (lastView.getBottom() - dy < getHeight()){
canScroll = lastView.getBottom() - getHeight();
offsetChildrenVertical(canScroll*-1);
return 0;
}
}
}else {
View firView = getChildAt(0);
int firstPos = getPosition(firView);
if (firstPos <= 0){
if (firView.getTop() - dy >= 0){
canScroll = firView.getTop();
offsetChildrenVertical(canScroll*-1);
return 0;
}
}
}
回收
通过getChildCount() 获取当前所有的子View
例如向上滚动,name就回收最下面的,最下面的View的top滑动后超出了RecyclerView的高度,说明这个View全部在界面外了,可以回收了,使用removeAndRecycleView移除并回收
向下滚动就判断顶部的Bottom是否小于0
private void recycleOut(int dy, RecyclerView.Recycler recycler, RecyclerView.State state){
for (int i = 0 ; i <getChildCount() ;i++){
View view = getChildAt(i);
Log.d("feifeifei","recycleOut position "+ i + " top " + view.getTop() + " bottom " + view.getBottom());
if (dy >0){
if (view.getBottom()-dy <0){
Log.d("feifeifei","recycleOut " + i);
removeAndRecycleView(view,recycler);
}
}else {
if (view.getTop()-dy > getHeight()){
Log.d("feifeifei","recycleOut " + i);
removeAndRecycleView(view,recycler);
}
}
}
}
这样带回收的LayoutManager也完成了,全部代码如下
/**
* 填充,滑动,回收
*/
public class CustomLayoutManager2 extends RecyclerView.LayoutManager {
private final String TAG = CustomLayoutManager2.class.getSimpleName();
@Override
public RecyclerView.LayoutParams generateDefaultLayoutParams() {
return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT,ViewGroup.LayoutParams.WRAP_CONTENT);
}
// 1 在RecyclerView初始化时，会被调用两次。
// 2 在调用adapter.notifyDataSetChanged()时，会被调用。
// 3 在调用setAdapter替换Adapter时,会被调用。
// 4 在RecyclerView执行动画时，它也会被调用。
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
Log.d(TAG,"onLayoutChildren ");
if (getItemCount() == 0){
detachAndScrapAttachedViews(recycler);
return;
}
//state.isPreLayout()是支持动画的
if (getItemCount() == 0 && state.isPreLayout()){
return;
}
//将当前Recycler中的view全部移除并放到报废缓存里,之后优先重用缓存里的view
detachAndScrapAttachedViews(recycler);
int actualHeight = 0;
for (int i = 0 ;i < getItemCount() ; i++){
View scrap = recycler.getViewForPosition(i);
addView(scrap);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,actualHeight,width,actualHeight+height);
actualHeight+=height;
//超出界面的就不画了,也不add了
if (actualHeight > getHeight()){
break;
}
}
}
@Override
public boolean canScrollVertically() {
return true;
}
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
Log.d("feifeifei","getChildCount() " + getChildCount() + " recycler.getScrapList().size() " + recycler.getScrapList().size());
//界面向下滚动的时候,dy为正,向上滚动的时候dy为负
//向下滚动的时候,最下面的值不能超过总值,
//向上滚动的时候,最上面的值不能小于0
int canScroll = dy;
if (dy>0){
View lastView = getChildAt(getChildCount() -1);
int lastPos = getPosition(lastView);
if (lastPos >= getItemCount()-1){
if (lastView.getBottom() - dy < getHeight()){
canScroll = lastView.getBottom() - getHeight();
offsetChildrenVertical(canScroll*-1);
return 0;
}
}
}else {
View firView = getChildAt(0);
int firstPos = getPosition(firView);
if (firstPos <= 0){
if (firView.getTop() - dy >= 0){
canScroll = firView.getTop();
offsetChildrenVertical(canScroll*-1);
return 0;
}
}
}
//底部填充
fill(dy,recycler,state);
//滚动
offsetChildrenVertical(dy*-1);
//回收已经离开界面的
recycleOut(dy,recycler,state);
return dy;
}
private void fill(int dy, RecyclerView.Recycler recycler, RecyclerView.State state){
//向下滚动
if (dy > 0){
//先在底部填充
View lastView = getChildAt(getChildCount() -1);
int lastPos = getPosition(lastView);
if (lastPos == getChildCount()-1){
return;
}
Log.d("feifeifei","lastView top" + lastView.getTop() + " bottom " + lastView.getBottom());
if (lastView.getBottom() - dy < getHeight()){
View scrap = recycler.getViewForPosition(lastPos+1);
addView(scrap);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,lastView.getBottom(),width,lastView.getBottom()+height);
// bottomItemPos++;
}
}else {
//向上滚动
//现在顶部填充
View firstView = getChildAt(0);
Log.d("feifeifei","firstView top" + firstView.getTop() + " bottom " + firstView.getBottom());
int layoutPostion = getPosition(firstView);
if (layoutPostion == 0){
return;
}
if (firstView.getTop() >= 0 ){
View scrap = recycler.getViewForPosition(layoutPostion -1);
addView(scrap,0);
measureChildWithMargins(scrap,0,0);
int width = getDecoratedMeasuredWidth(scrap);
int height = getDecoratedMeasuredHeight(scrap);
layoutDecorated(scrap,0,firstView.getTop() - height,width,firstView.getTop());
}
}
}
private void recycleOut(int dy, RecyclerView.Recycler recycler, RecyclerView.State state){
for (int i = 0 ; i <getChildCount() ;i++){
View view = getChildAt(i);
// Log.d("feifeifei","recycleOut position "+ i + " getDecoratedTop " + getDecoratedTop(view) + " getDecoratedBottom " + getDecoratedBottom(view));
Log.d("feifeifei","recycleOut position "+ i + " top " + view.getTop() + " bottom " + view.getBottom());
if (dy >0){
if (view.getBottom()-dy <0){
Log.d("feifeifei","recycleOut " + i);
removeAndRecycleView(view,recycler);
}
}else {
if (view.getTop()-dy > getHeight()){
Log.d("feifeifei","recycleOut " + i);
removeAndRecycleView(view,recycler);
}
}
}
}
}
然后通过adapter打印日志,onCreateViewHolder只打印了7次(我的界面上显示的7个item),然后滚动界面的时候,onBindViewHolder依次打印.看来回收还是成功的.这样一个简单版的带回收的LinearLayoutManager就好了
问题:
- RecyclerView的缓存机制有点复杂
- detachAndScrapAttachedViews貌似就是报废当前子View并放到缓存里,removeAndRecycleView是直接把子View移除了并放到缓存里
- 再对比系统的LinearLayoutManager,这个有缓存的版本还是简陋的一塌糊涂.对于LayoutManger的学习还是需要再接再厉
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