From here https://blog.csdn.net/zxt0601/article/details/52956504
照着写一遍，熟悉代码逻辑,修正那块看着脑袋晕，暂不考虑，复制作者代码就行。
下边是效果图，增加了一个boolean，默认为false，如果为true的话就是下图效果，每行控件都居中显示。
image.png
代码如下：
package com.charliesong.demo0327;
import android.graphics.Rect;
import android.support.v7.widget.OrientationHelper;
import android.support.v7.widget.RecyclerView;
import android.util.SparseArray;
import android.view.View;
import android.view.ViewGroup;
/**
* Created by charlie.song on 2018/5/8.
*/
public class FlowGravityLayoutManager extends RecyclerView.LayoutManager {
private int mVerticalOffset;//竖直偏移量 每次换行时，要根据这个offset判断
private int mFirstVisiPos;//屏幕可见的第一个View的Position
private int mLastVisiPos;//屏幕可见的最后一个View的Position
private OrientationHelper orientationHelper;//系统自带的类，这里选择的是垂直方向的，用来获取top,bottom，以及child的宽高等
private SparseArray<Rect> mItemRects;//key 是position，保存的是view的bound属性
private boolean horizontalCenter=false;//同一行的控件是否居中显示
public FlowGravityLayoutManager() {
orientationHelper = OrientationHelper.createOrientationHelper(this, OrientationHelper.VERTICAL);
mItemRects = new SparseArray<>()；
setAutoMeasureEnabled(true);
}
public void setHorizontalCenter(boolean horizontalCenter) {
this.horizontalCenter = horizontalCenter;
}
@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);
oneRowViews.clear();
return;
}
if (getChildCount() == 0 && state.isPreLayout()) {
return;
}
detachAndScrapAttachedViews(recycler);
mVerticalOffset = 0;
mFirstVisiPos = 0;
mLastVisiPos = getItemCount() ;
fill(recycler, state);
}
private void fill(RecyclerView.Recycler recycler, RecyclerView.State state) {
fill(recycler, state, 0);
}
int leftOffset = getPaddingLeft();
int lineMaxHeight = 0;//每行的最大高度，因为同一行的控件可能高度不一样，取最大值，好决定下一行的位置
int topOffset;
SparseArray<View> oneRowViews=new SparseArray<>();//保存在同一行的view，方便在这一行控件加载完以后平移，只对居中对齐的情况下处理
/**
* 填充childView的核心方法,应该先填充，再移动。
* 在填充时，预先计算dy的在内，如果View越界，回收掉。
* 一般情况是返回dy，如果出现View数量不足，则返回修正后的dy.
*
* @param dy RecyclerView给我们的位移量,+,显示底端， -，显示头部,手指往上滑是正的，往下滑是负的
* @return 修正以后真正的dy（可能剩余空间不够移动那么多了 所以return <|dy|）
*/
private int fill(RecyclerView.Recycler recycler, RecyclerView.State state, int dy) {
topOffset = orientationHelper.getStartAfterPadding();
//回收越界子View
if (getChildCount() > 0) {//滑动时进来的
for (int i = getChildCount() - 1; i >= 0; i--) {
View child = getChildAt(i);
if (dy > 0) {//上滑，顶部的view可能消失
if (getDecoratedBottom(child) - dy < topOffset) {
removeAndRecycleView(child, recycler);
mFirstVisiPos++;
}
} else if (dy < 0) {//手指往下滑，底部的view 可能消失
if (getDecoratedTop(child) - dy > orientationHelper.getEndAfterPadding()) {
removeAndRecycleView(child, recycler);
mLastVisiPos--;
}
}
}
}
//
leftOffset = getPaddingLeft();
lineMaxHeight = 0;
//布局子View阶段
if (dy >= 0) {
int minPos = mFirstVisiPos;
mLastVisiPos = getItemCount() - 1;
if (getChildCount() > 0) {
View lastVisibleView = getChildAt(getChildCount() - 1);
minPos = getPosition(lastVisibleView) + 1;
topOffset = getDecoratedTop(lastVisibleView);
leftOffset = getDecoratedRight(lastVisibleView);
lineMaxHeight = Math.max(lineMaxHeight, orientationHelper.getDecoratedMeasurement(lastVisibleView));
}
for (int i = minPos; i <=mLastVisiPos; i++) {
//找recycler要一个childItemView,我们不管它是从scrap里取，还是从RecyclerViewPool里取，亦或是onCreateViewHolder里拿。
View child = recycler.getViewForPosition(i);
addView(child);
measureChildWithMargins(child, 0, 0);
if (leftOffset + orientationHelper.getDecoratedMeasurementInOther(child) <= getWidth() - getPaddingRight()) {//这一行可以装的下，不用换行
layoutChild(child,i);
} else {
transViewX();
leftOffset = getPaddingLeft();
topOffset += lineMaxHeight;
lineMaxHeight = 0;
//新起一行的时候要判断一下边界
if (topOffset - dy > orientationHelper.getEndAfterPadding()) {
removeAndRecycleView(child, recycler);
mLastVisiPos=i-1;//已经跑到recyclerview最底部，不可见了，修改mLastVisPos,循环结束
} else {
layoutChild(child,i);
}
}
}
//添加完后，判断是否已经没有更多的ItemView，并且此时屏幕仍有空白，则需要修正dy
View lastVisibleView = getChildAt(getChildCount() - 1);
int lastPosition = getPosition(lastVisibleView);
if (lastPosition == getItemCount() - 1) {
int gap = orientationHelper.getEndAfterPadding() - getDecoratedBottom(lastVisibleView);
if (gap > 0) {
dy -= gap;
}
}
} else {
int maxPosition = getItemCount() - 1;
mFirstVisiPos = 0;
if (getChildCount() > 0) {
View firstView = getChildAt(0);
maxPosition = getPosition(firstView) - 1;
}
for (int i = maxPosition; i >= mFirstVisiPos; i--) {
Rect rect = mItemRects.get(i);
if (rect.bottom - mVerticalOffset - dy < getPaddingTop()) {
mFirstVisiPos = i + 1;
break;
} else {
View child = recycler.getViewForPosition(i);
addView(child, 0);//将View添加至RecyclerView中，childIndex为1，但是View的位置还是由layout的位置决定
measureChildWithMargins(child, 0, 0);
layoutDecoratedWithMargins(child, rect.left, rect.top - mVerticalOffset, rect.right, rect.bottom - mVerticalOffset);
}
}
}
return dy;
}
//换行或者add最后一个view的时候，平移下最终的位置，
private void transViewX(){
if(!horizontalCenter){
return;
}
int transX=(getWidth()-getPaddingRight()-leftOffset)/2;
for(int i=0;i<oneRowViews.size();i++){
int key=oneRowViews.keyAt(i);
View transChild= oneRowViews.get(key);
transChild.offsetLeftAndRight(transX);
Rect rect = mItemRects.get(key);
rect.offset(transX,0);
}
oneRowViews.clear();
}
private void layoutChild(View child,int i){
oneRowViews.put(i,child);
layoutDecoratedWithMargins(child, leftOffset, topOffset, leftOffset + orientationHelper.getDecoratedMeasurementInOther(child),
topOffset + orientationHelper.getDecoratedMeasurement(child));
//保存Rect供逆序layout用
Rect rect = new Rect(leftOffset, topOffset + mVerticalOffset, leftOffset + orientationHelper.getDecoratedMeasurementInOther(child),
topOffset + orientationHelper.getDecoratedMeasurement(child) + mVerticalOffset);
mItemRects.put(i, rect);
leftOffset += orientationHelper.getDecoratedMeasurementInOther(child);
lineMaxHeight = Math.max(lineMaxHeight, orientationHelper.getDecoratedMeasurement(child));
if(i==mLastVisiPos){
transViewX();
}
}
@Override
public boolean canScrollVertically() {
return true;
}
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
if (dy == 0 || getChildCount() == 0) {
return 0;
}
int realOffset = dy;
//边界修复代码
if (mVerticalOffset + realOffset < 0) {//上边界
realOffset = -mVerticalOffset;
}else if (realOffset > 0){
//利用最后一个子View比较修正
View lastView=getChildAt(getChildCount()-1);
if(getPosition(lastView)==getItemCount()-1){
int gap = getHeight() - getPaddingBottom() - getDecoratedBottom(lastView);
if (gap > 0) {
realOffset = -gap;
} else if (gap == 0) {
realOffset = 0;
} else {
realOffset = Math.min(realOffset, -gap);
}
}
}
realOffset= fill(recycler, state, realOffset);//先填充，再位移。
mVerticalOffset += realOffset;//累加实际滑动距离
offsetChildrenVertical(-realOffset);//滑动
return realOffset;
}
}
简单记录下修改代码碰到的问题：
居中显示
最开始我的思路是用个tag【默认是0】，然后每次换行，平移完控件以后，tag就修改为换行后的position
结果我发现没有效果，offsetLeftAndRight不行我换成layoutDecoratedWithMargins也不行，这就奇怪了。
后来想着可能这个child不是我们要的child，也就是说通过recycler.getViewForPosition(start)，虽然里边的pisition是一样的，可获取到的child不是一个东西，所以操作没反应。
最后就是上边的代码了，将每行的child，都放到oneRowViews里，换行的时候取出来进行平移，果然没问题。
//这里的代码最早就是写在换行的逻辑里的，结果无效
int transX=(getWidth()-getPaddingRight()-leftOffset)/2;
for(int start=tag;start<i;start++){
Rect rect = mItemRects.get(start);
rect.offset(transX,0);
View transChild = recycler.getViewForPosition(start);
transChild.offsetLeftAndRight(transX);
// layoutDecoratedWithMargins(transChild, rect.left, rect.top , rect.right, rect.bottom );
System.out.println("138=============="+start+"=="+rect.toShortString()+"==="+transChild.getRight());
}
tag=i;
