先看效果
capture.gif
一、recyclerview依赖
//RecyclerView
implementation 'com.android.support:recyclerview-v7:28.0.0'
二、工具类
1、VegaLayoutManager
import android.graphics.Rect;
import android.support.v4.util.ArrayMap;
import android.support.v7.widget.RecyclerView;
import android.util.SparseArray;
import android.util.SparseBooleanArray;
import android.view.View;
import android.view.ViewGroup;
/**
* Created by zhanghui on 2020/9/5.
*/
public class VegaLayoutManager extends RecyclerView.LayoutManager {
private int scroll = 0;
private SparseArray locationRects = new SparseArray<>();
private SparseBooleanArray attachedItems = new SparseBooleanArray();
private ArrayMap viewTypeHeightMap = new ArrayMap<>();
private boolean needSnap = false;
private int lastDy = 0;
private int maxScroll = -1;
private RecyclerView.Adapter adapter;
private RecyclerView.Recycler recycler;
public VegaLayoutManager() {
setAutoMeasureEnabled(true);
}
@Override
public RecyclerView.LayoutParams generateDefaultLayoutParams() {
return new RecyclerView.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
}
@Override
public void onAdapterChanged(RecyclerView.Adapter oldAdapter, RecyclerView.Adapter newAdapter) {
super.onAdapterChanged(oldAdapter, newAdapter);
this.adapter = newAdapter;
}
@Override
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
this.recycler = recycler; // 二话不说，先把recycler保存了
if (state.isPreLayout()) {
return;
}
buildLocationRects();
// 先回收放到缓存，后面会再次统一layout
detachAndScrapAttachedViews(recycler);
layoutItemsOnCreate(recycler);
}
private void buildLocationRects() {
locationRects.clear();
attachedItems.clear();
int tempPosition = getPaddingTop();
int itemCount = getItemCount();
for (int i = 0; i < itemCount; i++) {
// 1. 先计算出itemWidth和itemHeight
int viewType = adapter.getItemViewType(i);
int itemHeight;
if (viewTypeHeightMap.containsKey(viewType)) {
itemHeight = viewTypeHeightMap.get(viewType);
} else {
View itemView = recycler.getViewForPosition(i);
addView(itemView);
measureChildWithMargins(itemView, View.MeasureSpec.UNSPECIFIED, View.MeasureSpec.UNSPECIFIED);
itemHeight = getDecoratedMeasuredHeight(itemView);
viewTypeHeightMap.put(viewType, itemHeight);
}
// 2. 组装Rect并保存
Rect rect = new Rect();
rect.left = getPaddingLeft();
rect.top = tempPosition;
rect.right = getWidth() - getPaddingRight();
rect.bottom = rect.top + itemHeight;
locationRects.put(i, rect);
attachedItems.put(i, false);
tempPosition = tempPosition + itemHeight;
}
if (itemCount == 0) {
maxScroll = 0;
} else {
computeMaxScroll();
}
}
/**
* 对外提供接口，找到第一个可视view的index
*/
public int findFirstVisibleItemPosition() {
int count = locationRects.size();
Rect displayRect = new Rect(0, scroll, getWidth(), getHeight() + scroll);
for (int i = 0; i < count; i++) {
if (Rect.intersects(displayRect, locationRects.get(i)) &&
attachedItems.get(i)) {
return i;
}
}
return 0;
}
/**
* 计算可滑动的最大值
*/
private void computeMaxScroll() {
maxScroll = locationRects.get(locationRects.size() - 1).bottom - getHeight();
if (maxScroll < 0) {
maxScroll = 0;
return;
}
int itemCount = getItemCount();
int screenFilledHeight = 0;
for (int i = itemCount - 1; i >= 0; i--) {
Rect rect = locationRects.get(i);
screenFilledHeight = screenFilledHeight + (rect.bottom - rect.top);
if (screenFilledHeight > getHeight()) {
int extraSnapHeight = getHeight() - (screenFilledHeight - (rect.bottom - rect.top));
maxScroll = maxScroll + extraSnapHeight;
break;
}
}
}
/**
* 初始化的时候，layout子View
*/
private void layoutItemsOnCreate(RecyclerView.Recycler recycler) {
int itemCount = getItemCount();
Rect displayRect = new Rect(0, scroll, getWidth(), getHeight() + scroll);
for (int i = 0; i < itemCount; i++) {
Rect thisRect = locationRects.get(i);
if (Rect.intersects(displayRect, thisRect)) {
View childView = recycler.getViewForPosition(i);
addView(childView);
measureChildWithMargins(childView, View.MeasureSpec.UNSPECIFIED, View.MeasureSpec.UNSPECIFIED);
layoutItem(childView, locationRects.get(i));
attachedItems.put(i, true);
childView.setPivotY(0);
childView.setPivotX(childView.getMeasuredWidth() / 2);
if (thisRect.top - scroll > getHeight()) {
break;
}
}
}
}
/**
* 初始化的时候，layout子View
*/
private void layoutItemsOnScroll() {
int childCount = getChildCount();
// 1. 已经在屏幕上显示的child
int itemCount = getItemCount();
Rect displayRect = new Rect(0, scroll, getWidth(), getHeight() + scroll);
int firstVisiblePosition = -1;
int lastVisiblePosition = -1;
for (int i = childCount - 1; i >= 0; i--) {
View child = getChildAt(i);
if (child == null) {
continue;
}
int position = getPosition(child);
if (!Rect.intersects(displayRect, locationRects.get(position))) {
// 回收滑出屏幕的View
removeAndRecycleView(child, recycler);
attachedItems.put(position, false);
} else {
// Item还在显示区域内，更新滑动后Item的位置
if (lastVisiblePosition < 0) {
lastVisiblePosition = position;
}
if (firstVisiblePosition < 0) {
firstVisiblePosition = position;
} else {
firstVisiblePosition = Math.min(firstVisiblePosition, position);
}
layoutItem(child, locationRects.get(position)); //更新Item位置
}
}
// 2. 复用View处理
if (firstVisiblePosition > 0) {
// 往前搜索复用
for (int i = firstVisiblePosition - 1; i >= 0; i--) {
if (Rect.intersects(displayRect, locationRects.get(i)) &&
!attachedItems.get(i)) {
reuseItemOnSroll(i, true);
} else {
break;
}
}
}
// 往后搜索复用
for (int i = lastVisiblePosition + 1; i < itemCount; i++) {
if (Rect.intersects(displayRect, locationRects.get(i)) &&
!attachedItems.get(i)) {
reuseItemOnSroll(i, false);
} else {
break;
}
}
}
/**
* 复用position对应的View
*/
private void reuseItemOnSroll(int position, boolean addViewFromTop) {
View scrap = recycler.getViewForPosition(position);
measureChildWithMargins(scrap, 0, 0);
scrap.setPivotY(0);
scrap.setPivotX(scrap.getMeasuredWidth() / 2);
if (addViewFromTop) {
addView(scrap, 0);
} else {
addView(scrap);
}
// 将这个Item布局出来
layoutItem(scrap, locationRects.get(position));
attachedItems.put(position, true);
}
private void layoutItem(View child, Rect rect) {
int topDistance = scroll - rect.top;
int layoutTop, layoutBottom;
int itemHeight = rect.bottom - rect.top;
if (topDistance < itemHeight && topDistance > 0) {
float rate1 = (float) topDistance / itemHeight;
float rate2 = 1 - rate1 * rate1 / 3;
float rate3 = 1 - rate1 * rate1;
child.setScaleX(rate2);
child.setScaleY(rate2);
child.setAlpha(rate3);
layoutTop = 0;
layoutBottom = itemHeight;
} else {
child.setScaleX(1);
child.setScaleY(1);
child.setAlpha(1);
layoutTop = rect.top - scroll;
layoutBottom = rect.bottom - scroll;
}
layoutDecorated(child, rect.left, layoutTop, rect.right, layoutBottom);
}
@Override
public boolean canScrollVertically() {
return true;
}
@Override
public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
if (getItemCount() == 0 || dy == 0) {
return 0;
}
int travel = dy;
if (dy + scroll < 0) {
travel = -scroll;
} else if (dy + scroll > maxScroll) {
travel = maxScroll - scroll;
}
scroll += travel; //累计偏移量
lastDy = dy;
if (!state.isPreLayout() && getChildCount() > 0) {
layoutItemsOnScroll();
}
return travel;
}
@Override
public void onAttachedToWindow(RecyclerView view) {
super.onAttachedToWindow(view);
new StartSnapHelper().attachToRecyclerView(view);
}
@Override
public void onScrollStateChanged(int state) {
if (state == RecyclerView.SCROLL_STATE_DRAGGING) {
needSnap = true;
}
super.onScrollStateChanged(state);
}
public int getSnapHeight() {
if (!needSnap) {
return 0;
}
needSnap = false;
Rect displayRect = new Rect(0, scroll, getWidth(), getHeight() + scroll);
int itemCount = getItemCount();
for (int i = 0; i < itemCount; i++) {
Rect itemRect = locationRects.get(i);
if (displayRect.intersect(itemRect)) {
if (lastDy > 0) {
// scroll变大，属于列表往下走，往下找下一个为snapView
if (i < itemCount - 1) {
Rect nextRect = locationRects.get(i + 1);
return nextRect.top - displayRect.top;
}
}
return itemRect.top - displayRect.top;
}
}
return 0;
}
public View findSnapView() {
if (getChildCount() > 0) {
return getChildAt(0);
}
return null;
}
}
2、StartSnapHelper
import android.support.annotation.NonNull;
import android.support.v7.widget.LinearSnapHelper;
import android.support.v7.widget.RecyclerView;
import android.view.View;
/**
* 定位到第一个子View的SnapHelper
* Created by zhanghui on 2020/9/5.
*/
public class StartSnapHelper extends LinearSnapHelper {
@Override
public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,
@NonNull View targetView) {
int[] out = new int[2];
out[0] = 0;
out[1] = ((VegaLayoutManager) layoutManager).getSnapHeight();
return out;
}
@Override
public View findSnapView(RecyclerView.LayoutManager layoutManager) {
VegaLayoutManager custLayoutManager = (VegaLayoutManager) layoutManager;
return custLayoutManager.findSnapView();
}
}
三、activity使用
private void initView() {
recycler = (RecyclerView) findViewById(R.id.recyclerview);
//管理器
recycler.setLayoutManager(new VegaLayoutManager());
list = new ArrayList<>();
bannerlist = new ArrayList<>();
recyclerAdapter = new RecyclerAdapter(this,list,bannerlist);
recycler.setAdapter(recyclerAdapter);
for (int i = 0; i < 3; i++) {
BannerBean bannerBean = new BannerBean(R.mipmap.ic_launcher);
bannerlist.add(bannerBean);
}
for (int i = 0; i < 25; i++) {
DemoBean demoBean = new DemoBean("张三" + i, 1 + i, R.mipmap.ic_launcher);
list.add(demoBean);
}
recyclerAdapter.notifyDataSetChanged();
}
//管理器 （直接引用即可）
recycler.setLayoutManager(new VegaLayoutManager());
本文章仅限实现效果