宣告：原創作品，轉載請註明出處：www.jianshu.com/p/c197e3bf8…
前一篇文字我講解了ItemDecoration的使用方式，這篇文章預設大家已經讀過RecyclerView使用指南（四）—— 使用ItemDecoration，所以，不熟悉ItemDecoration的同學請先去看前一篇文章。 OK，我們先來看一下我們將要實現的效果：
一、實現帶有Section的樣式
我們先重寫getItemOffsets()方法，增加outRect的高度，然後重寫onDraw()方法，畫出一個rectangle。程式碼如下：
public class DemoItemDecoration extends RecyclerView.ItemDecoration {
private int mSectionHeight = 80;
private Paint mPaint;
public DemoItemDecoration() {
mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaint.setColor(Color.BLUE);
}
@Override
public void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
outRect.top = mSectionHeight;
}
@Override
public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.onDraw(c, parent, state);
int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
View child = parent.getChildAt(i);
float sectionLeft = parent.getLeft();
float sectionTop = child.getTop() - mSectionHeight;
float sectionRight = parent.getWidth();
float sectionBottom = child.getTop();
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaint);
}
}
@Override
public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.onDrawOver(c, parent, state);
}
}
複製程式碼實現了這樣一個效果：
二、實現分組
剛剛實現的每一個Item都有section，這與實際需求時不符的，那麼我們要把資料進行分組，每一組的第一條Item上面才有section，這裡，為了讓ItemDecoration不與資料來源發生直接關係，我們新增一個GroupBean類來描述是否需要增加section。如下：
public class GroupBean {
private int mGroupId;
private int mGroupPosition;
private boolean mIsFirst;
private boolean mIsLast;
public GroupBean(int groupId, int groupPosition, boolean isFirst, boolean isLast) {
mGroupId = groupId;
mGroupPosition = groupPosition;
this.mIsFirst = isFirst;
this.mIsLast = isLast;
}
public int getGroupId() {
return mGroupId;
}
public int getGroupPosition() {
return mGroupPosition;
}
public boolean isFirst() {
return mIsFirst;
}
public boolean isLast() {
return mIsLast;
}
}
複製程式碼然後改寫我們的getItemOffsets()方法和onDraw()方法，只有每一個分組的第一條Item才顯示section。程式碼如下：
public class DemoItemDecoration extends RecyclerView.ItemDecoration {
private int mSectionHeight = 80;
private Paint mPaintSection;
private Paint mPaintText;
private List<GroupBean> mGroupBeans;
public DemoItemDecoration(List<GroupBean> groupBeans) {
//資料
mGroupBeans = groupBeans;
//畫筆
mPaintSection = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaintSection.setColor(Color.BLUE);
mPaintText = new Paint(Paint.ANTI_ALIAS_FLAG);
mPaintText.setTextSize(60);
mPaintText.setColor(Color.YELLOW);
}
@Override
public void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.getItemOffsets(outRect, view, parent, state);
int position = parent.getChildAdapterPosition(view);
if (mGroupBeans.get(position).isFirst()) {
outRect.top = mSectionHeight;
}
}
@Override
public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.onDraw(c, parent, state);
}
@Override
public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.onDrawOver(c, parent, state);
int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
View child = parent.getChildAt(i);
int position = parent.getChildAdapterPosition(child);
GroupBean groupBean = mGroupBeans.get(position);
if (mGroupBeans.get(position).isFirst()) {
float sectionLeft = parent.getLeft();
float sectionTop = child.getTop() - mSectionHeight;
float sectionRight = parent.getWidth();
float sectionBottom = child.getTop();
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
}
}
}
}
複製程式碼最後，在Activity中進行ItemDecoration與GroupBean列表的繫結，以及ItemDecoration與RecyclerView的繫結：
private void initRv() {
List<GroupBean> groupBeans = new ArrayList<>();
//根據RecyclerView的資料來源，設定需要增加section的item
for (Data data : mList) {
//這裡就是模擬一下，所以我取4的倍數增加section
int i = mList.indexOf(data);
int groupId = i / 4;
int groupPosition = i % 4;
GroupBean groupBean = null;
//這裡是假資料嘛，4的倍數有section，那餘數是3的時候肯定是分組的最後一個啦
if (groupPosition == 0) {
groupBean = new GroupBean(groupId, groupPosition, true, false);
}
if (groupPosition == 3) {
groupBean = new GroupBean(groupId, groupPosition, false, true);
}
groupBeans.add(groupBean);
}
RecyclerView recyclerView = findViewById(R.id.rv);
recyclerView.setAdapter(new SingleItemAdapter(mList));
recyclerView.addItemDecoration(new DemoItemDecoration(groupBeans));
}
複製程式碼我們來看下效果：
三、實現section在列表頂部懸浮
實現吸頂效果，我們還需要做到讓我們的section在列表頂部懸浮，來分析一下邏輯：
- 每個分組的第一條資料需要有section
- 列表的最上方必須顯示一個section 現在來修改一下onDrawOver()，實現一下：
@Override
public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.onDrawOver(c, parent, state);
int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
View child = parent.getChildAt(i);
int position = parent.getChildAdapterPosition(child);
GroupBean groupBean = mGroupBeans.get(position);
//所有分組的第一條資料有section
if (groupBean.isFirst()) {
float sectionLeft = parent.getLeft();
float sectionTop = child.getTop() - mSectionHeight;
float sectionRight = parent.getWidth();
float sectionBottom = child.getTop();
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
}
//列表的最上方顯示section資訊（這裡section是第一條顯示的條目所對應的groupId）
LinearLayoutManager layoutManager = (LinearLayoutManager) parent.getLayoutManager();
int firstVisibleItemPosition = layoutManager.findFirstVisibleItemPosition();
if (position == firstVisibleItemPosition) {
float sectionLeft = parent.getLeft();
float sectionTop = parent.getTop();
float sectionRight = parent.getWidth();
float sectionBottom = parent.getTop() + mSectionHeight;
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
}
}
}
複製程式碼來看下效果：
四、優化section更替的特效
我們仔細觀察上面的效果圖，當下面的section向上移動的時候，上面的section沒有移動，所以，看起來下面的section直接覆蓋到了它的上面。 那麼上面的section應該在什麼時機進行移動呢？它的底邊應該是在該分組中最後一個Item的底部的上方，所以，我們更改，當section的底部低於“分組中最後一個Item”時，section整體上移，移動的距離就是section的高度與條目底部的差。 我們只需更改onDrawOver()方法的程式碼：
@Override
public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
super.onDrawOver(c, parent, state);
int childCount = parent.getChildCount();
for (int i = 0; i < childCount; i++) {
View child = parent.getChildAt(i);
int position = parent.getChildAdapterPosition(child);
GroupBean groupBean = mGroupBeans.get(position);
//所有分組的第一條資料有section
if (groupBean.isFirst()) {
float sectionLeft = parent.getLeft();
float sectionTop = child.getTop() - mSectionHeight;
float sectionRight = parent.getWidth();
float sectionBottom = child.getTop();
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
}
//列表的最上方顯示section資訊（這裡section是第一條顯示的條目所對應的groupId）
LinearLayoutManager layoutManager = (LinearLayoutManager) parent.getLayoutManager();
int firstVisibleItemPosition = layoutManager.findFirstVisibleItemPosition();
if (position == firstVisibleItemPosition) {
//如果是本組的最後一條，section的底部就不能低於這個條目的底部
if (groupBean.isLast()) {
//當條目的底部已經高於section的時候，section應該隨著條目的底部往上移動
if (child.getBottom() < mSectionHeight) {
float sectionLeft = parent.getLeft();
float sectionTop = parent.getTop() - (mSectionHeight - child.getBottom());
float sectionRight = parent.getWidth();
float sectionBottom = parent.getTop() + mSectionHeight - (mSectionHeight - child.getBottom());
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
} else {
float sectionLeft = parent.getLeft();
float sectionTop = parent.getTop();
float sectionRight = parent.getWidth();
float sectionBottom = parent.getTop() + mSectionHeight;
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
}
} else {
float sectionLeft = parent.getLeft();
float sectionTop = parent.getTop();
float sectionRight = parent.getWidth();
float sectionBottom = parent.getTop() + mSectionHeight;
c.drawRect(sectionLeft, sectionTop, sectionRight, sectionBottom, mPaintSection);
c.drawText(String.valueOf(groupBean.getGroupId()), sectionLeft, sectionBottom - 5, mPaintText);
}
}
}
}
複製程式碼在Activity中新增資料：
private void initRv() {
List<GroupBean> groupBeans = new ArrayList<>();
//根據RecyclerView的資料來源，設定需要增加section的item
for (Data data : mList) {
//這裡就是模擬一下，所以我取4的倍數增加section
int i = mList.indexOf(data);
int groupId = i / 4;
int groupPosition = i % 4;
GroupBean groupBean;
//這裡是假資料嘛，4的倍數有section，那餘數是3的時候肯定是分組的最後一個啦
if (groupPosition == 0) {
groupBean = new GroupBean(groupId, groupPosition, true, false);
} else if (groupPosition == 3) {
groupBean = new GroupBean(groupId, groupPosition, false, true);
} else {
groupBean = new GroupBean(groupId, groupPosition, false, false);
}
groupBeans.add(groupBean);
}
RecyclerView recyclerView = findViewById(R.id.rv);
recyclerView.setAdapter(new SingleItemAdapter(mList));
recyclerView.addItemDecoration(new DemoItemDecoration(groupBeans));
}
複製程式碼最後看一下效果：
總結
這篇文章我們實現了一個吸頂效果的特效，是屬於比較高階的用法了，關於ItemDecoration的用法也用它進行收尾了。另外，示例程式碼中的冗餘程式碼比較多，主要是為了看起來容易理解，請小朋友們在使用過程中合理地優化程式碼。
參考文獻
系列文章
《RecyclerView使用指南（一）—— 基本使用》
《RecyclerView使用指南（二）—— 多種ItemLayout》
《RecyclerView使用指南（三）—— 新增分割線和點選事件》
《RecyclerView使用指南（四）—— 使用ItemDecoration》
《RecyclerView使用指南（五）—— 實現吸頂效果》