RecyclerView之ItemDecoration由浅入深(转载)

向右看齐 2022-06-11 00:55 157阅读 0赞

> 译文的GitHub地址：[RecyclerView之ItemDecoration由浅入深][RecyclerView_ItemDecoration]

> 译者注：RecyclerView第一篇，希望后面坚持下来

声明:此博客为转载博客，非出自本人，博客来源地址为:

https://github.com/thinkSky1206/android-blog/blob/master/RecyclerView%E4%B9%8BItemDecoration%E7%94%B1%E6%B5%85%E5%85%A5%E6%B7%B1.md

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。

[![ItemDecoration][]][ItemDecoration]

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

[![activity][]][activity]

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：

[![Padding ItemDecoration.png][]][Padding ItemDecoration.png]

\#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：

[![Divider ItemDecoration.png][]][Divider ItemDecoration.png]

\#标签

现在很多电商app会给商品加上一个标签，比如“推荐”，“热卖”，“秒杀”等等，可以看到这些标签都是覆盖在内容之上的，这就可以用onDrawOver()来实现，我们这里简单实现一个有趣的标签

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：

[![Tag  ItemDecoration][Tag ItemDecoration]][Tag ItemDecoration]

\#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

[![ItemDecoration][ItemDecoration 1]][ItemDecoration 1]

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

[![Section ItemDecoration][]][Section ItemDecoration]

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

[![stickyheader][]][stickyheader]

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。

[RecyclerView_ItemDecoration]: https://github.com/thinkSky1206/android-blog/blob/master/RecyclerView%E4%B9%8BItemDecoration%E7%94%B1%E6%B5%85%E5%85%A5%E6%B7%B1.md
[ItemDecoration]: /images/20220611/4e2d025640104c32837f4fc409062c4e.png
[activity]: /images/20220611/f250f4d66c2b4bfb97ed9b4ff789b814.png
[Padding ItemDecoration.png]: /images/20220611/9c030e638d1547018e60af1ba6a39052.png
[Divider ItemDecoration.png]: /images/20220611/60732d4c15574e8ebe6b93bc1a67c1f7.png
[Tag ItemDecoration]: /images/20220611/907c5b5c1e474500ab06de11b6088391.png
[ItemDecoration 1]: /images/20220611/1fc032e63290459795d46d024fcb298a.png
[Section ItemDecoration]: /images/20220611/cd6111d872344e37ae86e8ca50f91be1.png
[stickyheader]: /images/20220611/36903651111042bc9968ae94df7b3ae9.png