RecyclerView之ItemDecoration由浅入深

tech2025-02-12 15

RecyclerView没有像之前ListView提供divider属性，而是提供了方法

recyclerView.addItemDecoration()

其中ItemDecoration需要我们自己去定制重写，一开始可能有人会觉得麻烦不好用，最后你会发现这种可插拔设计不仅好用，而且功能强大。

ItemDecoration类主要是三个方法：

public void onDraw(Canvas c, RecyclerView parent, State state) public void onDrawOver(Canvas c, RecyclerView parent, State state) public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state)

官方源码虽然都写的很清楚，但还不少小伙伴不知道怎么理解，怎么用或用哪个方法，下面我画个简单的图来帮你们理解一下。

图画的丑请见谅，首先我们假设绿色区域代表的是我们的内容，红色区域代表我们自己绘制的装饰，可以看到：

图1：代表了getItemOffsets(),可以实现类似padding的效果

图2：代表了onDraw(),可以实现类似绘制背景的效果，内容在上面

图3：代表了onDrawOver()，可以绘制在内容的上面，覆盖内容

注意上面是我个人从应用角度的看法，事实上实现上面的效果可能三个方法每个方法都可以实现。只不过这种方法更好理解。

下面是我们没有添加任何ItemDecoration的界面主页布局界面很简单，背景设成灰色

<?xml version="1.0" encoding="utf-8"?> <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@color/gray">//灰色背景 <android.support.design.widget.AppBarLayout android:layout_width="match_parent" android:layout_height="wrap_content"> <android.support.v7.widget.Toolbar android:id="@+id/toolbar" android:layout_width="match_parent" android:layout_height="?attr/actionBarSize" android:background="?attr/colorPrimary" app:layout_scrollFlags="scroll|enterAlways"/> </android.support.design.widget.AppBarLayout> <android.support.v7.widget.RecyclerView android:id="@+id/recycler_view" android:layout_width="match_parent" android:layout_height="match_parent" app:layout_behavior="@string/appbar_scrolling_view_behavior" /> </android.support.design.widget.CoordinatorLayout>

ok 接下来，让我们来实现实际开发中常遇到的场景。

padding

从前面的图可以看到实现这个效果，需要重写getItemOffsets方法。

public class SimplePaddingDecoration extends RecyclerView.ItemDecoration { private int dividerHeight; public SimplePaddingDecoration(Context context) { dividerHeight = context.getResources().getDimensionPixelSize(R.dimen.divider_height); } @Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) { super.getItemOffsets(outRect, view, parent, state); outRect.bottom = dividerHeight;//类似加了一个bottom padding } }

没错，就这么2行代码，然后添加到RecyclerView

recyclerView.addItemDecoration(new SimplePaddingDecoration(this));

实现效果：

分割线

分割线在app中是经常用到的，用ItemDecoration怎么实现呢，其实上面padding改成1dp就实现了分割线的效果，但是分割线的颜色只能是背景灰色，所以不能用这种方法。

要实现分割线效果需要 getItemOffsets()和 onDraw()2个方法，首先用 getItemOffsets给item下方空出一定高度的空间（例子中是1dp），然后用onDraw绘制这个空间

public class SimpleDividerDecoration extends RecyclerView.ItemDecoration { private int dividerHeight; private Paint dividerPaint; public SimpleDividerDecoration(Context context) { dividerPaint = new Paint(); dividerPaint.setColor(context.getResources().getColor(R.color.colorAccent)); dividerHeight = context.getResources().getDimensionPixelSize(R.dimen.divider_height); } @Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) { super.getItemOffsets(outRect, view, parent, state); outRect.bottom = dividerHeight; } @Override public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) { int childCount = parent.getChildCount(); int left = parent.getPaddingLeft(); int right = parent.getWidth() - parent.getPaddingRight(); for (int i = 0; i < childCount - 1; i++) { View view = parent.getChildAt(i); float top = view.getBottom(); float bottom = view.getBottom() + dividerHeight; c.drawRect(left, top, right, bottom, dividerPaint); } } }

实现效果：

public class LeftAndRightTagDecoration extends RecyclerView.ItemDecoration { private int tagWidth; private Paint leftPaint; private Paint rightPaint; public LeftAndRightTagDecoration(Context context) { leftPaint = new Paint(); leftPaint.setColor(context.getResources().getColor(R.color.colorAccent)); rightPaint = new Paint(); rightPaint.setColor(context.getResources().getColor(R.color.colorPrimary)); tagWidth = context.getResources().getDimensionPixelSize(R.dimen.tag_width); } @Override public void onDrawOver(Canvas c, RecyclerView parent, 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 pos = parent.getChildAdapterPosition(child); boolean isLeft = pos % 2 == 0; if (isLeft) { float left = child.getLeft(); float right = left + tagWidth; float top = child.getTop(); float bottom = child.getBottom(); c.drawRect(left, top, right, bottom, leftPaint); } else { float right = child.getRight(); float left = right - tagWidth; float top = child.getTop(); float bottom = child.getBottom(); c.drawRect(left, top, right, bottom, rightPaint); } } } }

实现效果：

组合

不要忘记的是ItemDecoration是可以叠加的

recyclerView.addItemDecoration(new LeftAndRightTagDecoration(this)); recyclerView.addItemDecoration(new SimpleDividerDecoration(this));

我们把上面2个ItemDecoration同时添加到RecyclerView看下什么效果

是不是有种狂拽炫酷吊炸天的赶脚。。。

三个方法都用了一遍，你以为这就结束了？呵呵并没有

section

这个是什么呢，先看下我们实现的效果

一看这个就很熟悉吧，手机上面的通讯录联系人，知乎日报都是这样效果，可以叫分组，也可以叫section分块先不管它叫什么。

这个怎么实现呢？其实和实现分割线是一样的道理，只是不是所有的item都需要分割线，只有同组的第一个需要。

我们首先定义一个接口给activity进行回调用来进行数据分组和获取首字母

public interface DecorationCallback { long getGroupId(int position); String getGroupFirstLine(int position); }

然后再来看我们的ItemDecoration

public class SectionDecoration extends RecyclerView.ItemDecoration { private static final String TAG = "SectionDecoration"; private DecorationCallback callback; private TextPaint textPaint; private Paint paint; private int topGap; private Paint.FontMetrics fontMetrics; public SectionDecoration(Context context, DecorationCallback decorationCallback) { Resources res = context.getResources(); this.callback = decorationCallback; paint = new Paint(); paint.setColor(res.getColor(R.color.colorAccent)); textPaint = new TextPaint(); textPaint.setTypeface(Typeface.DEFAULT_BOLD); textPaint.setAntiAlias(true); textPaint.setTextSize(80); textPaint.setColor(Color.BLACK); textPaint.getFontMetrics(fontMetrics); textPaint.setTextAlign(Paint.Align.LEFT); fontMetrics = new Paint.FontMetrics(); topGap = res.getDimensionPixelSize(R.dimen.sectioned_top);//32dp } @Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) { super.getItemOffsets(outRect, view, parent, state); int pos = parent.getChildAdapterPosition(view); Log.i(TAG, "getItemOffsets：" + pos); long groupId = callback.getGroupId(pos); if (groupId < 0) return; if (pos == 0 || isFirstInGroup(pos)) {//同组的第一个才添加padding outRect.top = topGap; } else { outRect.top = 0; } } @Override public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) { super.onDraw(c, parent, state); int left = parent.getPaddingLeft(); int right = parent.getWidth() - parent.getPaddingRight(); int childCount = parent.getChildCount(); for (int i = 0; i < childCount; i++) { View view = parent.getChildAt(i); int position = parent.getChildAdapterPosition(view); long groupId = callback.getGroupId(position); if (groupId < 0) return; String textLine = callback.getGroupFirstLine(position).toUpperCase(); if (position == 0 || isFirstInGroup(position)) { float top = view.getTop() - topGap; float bottom = view.getTop(); c.drawRect(left, top, right, bottom, paint);//绘制红色矩形 c.drawText(textLine, left, bottom, textPaint);//绘制文本 } } } private boolean isFirstInGroup(int pos) { if (pos == 0) { return true; } else { long prevGroupId = callback.getGroupId(pos - 1); long groupId = callback.getGroupId(pos); return prevGroupId != groupId; } } public interface DecorationCallback { long getGroupId(int position); String getGroupFirstLine(int position); } }

可以看到和divider实现一样，都是重写getItemOffsets()和onDraw()2个方法，不同的是根据数据做了处理。在Activity中使用

recyclerView.addItemDecoration(new SectionDecoration(this, new SectionDecoration.DecorationCallback() { @Override public long getGroupId(int position) { return Character.toUpperCase(dataList.get(position).getName().charAt(0)); } @Override public String getGroupFirstLine(int position) { return dataList.get(position).getName().substring(0, 1).toUpperCase(); } }));

干净舒服，不少github类似的库都是去adapter进行处理侵入性太强或许ItemDecoration是个更好的选择，可插拔，可替换。

到这里细心的人就会发现了，header不会动啊，我手机上的通讯录可是会随的滑动而变动呢，这个可以实现么？

StickyHeader

这个东西怎么叫我也不知道啊粘性头部？英文也有叫 pinned section 取名字真是个麻烦事。先看下我们简单实现的效果

首先一看到图，我们就应该想到header不动肯定是要绘制item内容之上的，需要重写onDrawOver()方法，其他地方和section实现一样。

public class PinnedSectionDecoration extends RecyclerView.ItemDecoration { private static final String TAG = "PinnedSectionDecoration"; private DecorationCallback callback; private TextPaint textPaint; private Paint paint; private int topGap; private Paint.FontMetrics fontMetrics; public PinnedSectionDecoration(Context context, DecorationCallback decorationCallback) { Resources res = context.getResources(); this.callback = decorationCallback; paint = new Paint(); paint.setColor(res.getColor(R.color.colorAccent)); textPaint = new TextPaint(); textPaint.setTypeface(Typeface.DEFAULT_BOLD); textPaint.setAntiAlias(true); textPaint.setTextSize(80); textPaint.setColor(Color.BLACK); textPaint.getFontMetrics(fontMetrics); textPaint.setTextAlign(Paint.Align.LEFT); fontMetrics = new Paint.FontMetrics(); topGap = res.getDimensionPixelSize(R.dimen.sectioned_top); } @Override public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) { super.getItemOffsets(outRect, view, parent, state); int pos = parent.getChildAdapterPosition(view); long groupId = callback.getGroupId(pos); if (groupId < 0) return; if (pos == 0 || isFirstInGroup(pos)) { outRect.top = topGap; } else { outRect.top = 0; } } @Override public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) { super.onDrawOver(c, parent, state); int itemCount = state.getItemCount(); int childCount = parent.getChildCount(); int left = parent.getPaddingLeft(); int right = parent.getWidth() - parent.getPaddingRight(); float lineHeight = textPaint.getTextSize() + fontMetrics.descent; long preGroupId, groupId = -1; for (int i = 0; i < childCount; i++) { View view = parent.getChildAt(i); int position = parent.getChildAdapterPosition(view); preGroupId = groupId; groupId = callback.getGroupId(position); if (groupId < 0 || groupId == preGroupId) continue; String textLine = callback.getGroupFirstLine(position).toUpperCase(); if (TextUtils.isEmpty(textLine)) continue; int viewBottom = view.getBottom(); float textY = Math.max(topGap, view.getTop()); if (position + 1 < itemCount) { //下一个和当前不一样移动当前 long nextGroupId = callback.getGroupId(position + 1); if (nextGroupId != groupId && viewBottom < textY ) {//组内最后一个view进入了header textY = viewBottom; } } c.drawRect(left, textY - topGap, right, textY, paint); c.drawText(textLine, left, textY, textPaint); } } }

好了，现在发现ItemDecoration有多强大了吧！当然还有更多就需要你自己去发现了。

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RecyclerView之ItemDecoration由浅入深

padding

分割线

标签

组合

section

StickyHeader