RecyclerView使用指南（五）—— 實現吸頂效果

RecyclerView · 發表 2018-12-03 23:11:24

摘要：宣告：原創作品，轉載請註明出處： https://www.jianshu.com/p/c197e3bf8329 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...

宣告：原創作品，轉載請註明出處： ofollow,noindex">https://www.jianshu.com/p/c197e3bf8329

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);
}
}

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;
}
}

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);
}
}
}
}

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));
}

我們來看下效果：

好，這樣，我們就實現了分組的效果，但是我們想要的吸頂效果，section是應該顯示到Item圖層的上方的，那麼我們使用onDraw()方法來實現，顯然是不合理的，既然如此，我們就將onDraw()方法中的內容剪下到onDrawOver()中好了~

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);
}
}
}

來看下效果：

嗯，我們實現了一個吸頂效果，但是兩個section進行更替的特效顯得比較粗糙啊，我們想要的是下面的section將上面的section頂上去，OK，我們再進行優化一下。

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);
}
}
}
}

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));
}

最後看一下效果：