译文的GitHub地址：RecyclerView之ItemDecoration由浅入深

译者注：RecyclerView第一篇，希望后面坚持下来

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。

RecyclerView之ItemDecoration使用教程

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

RecyclerView之ItemDecoration使用教程

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：

RecyclerView之ItemDecoration使用教程

Padding ItemDecoration

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：

RecyclerView之ItemDecoration使用教程

Divider ItemDecoration

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：

RecyclerView之ItemDecoration使用教程

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

RecyclerView之ItemDecoration使用教程

ItemDecoration

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

RecyclerView之ItemDecoration使用教程

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

RecyclerView之ItemDecoration使用教程

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。