FloatingDecoration：五行代码实现悬浮导航栏

0.前言

FloatingDecoration本质上就是一个自定义ItemDecoration，FloatingDecoration的效果图如下：

1.使用方法

伸手党福利：

compile ‘com.chauncey.view:floatingdecoration:1.0.1’

FloatingDecoration decoration = new FloatingDecoration(this, new FloatingDecoration.DecorationCallback() {            @Override            public String getGroupLabel(int position) {                return items.get(position).getTitle();            }        });

//设置分隔线，参数为Drawabledecoration.setDividingLine(mDividingLine);

别忘了

recyclerView.addItemDecoration(decoration);

轻松实现悬浮导航栏效果

2.ItemDecoration

ItemDecoration类主要有三种方法：

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)

三种方法作用如下：
onDraw：绘制背景，会被Item的内容所覆盖。适合用来绘制每个分组的标识
onDrawOver：绘制前景，会覆盖Item的内容。适合用来绘制悬浮的顶部。
getItemOffsets：获取Item的距离，也可以给Item添加边距。绘制之前需要先空出一些空间。

3.FloatingDecoration源码解析

1.getItemOffsets

    @Override    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {        super.getItemOffsets(outRect, view, parent, state);        int pos = parent.getChildAdapterPosition(view);        String groupLabel = mDecorationCallback.getGroupLabel(pos);        if (groupLabel == null) return;        //只有每个分组的第一个item需要label        if (isFirstInGroup(pos))            outRect.top = mLabelHeight;    }    private boolean isFirstInGroup(int pos) {        if (pos == 0) {            return true;        } else {            String prevLabel = mDecorationCallback.getGroupLabel(pos - 1);            String label = mDecorationCallback.getGroupLabel(pos);            //与上一个item的label进行对比，若不一致则当前item是新一组的第一个元素            if (prevLabel.equals(label)) {                return false;            } else {                return true;            }        }    }

放两张设置前后的效果对比图

2.onDraw

    //绘制分组的label    @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();        if (mDividingLine != null) {        //绘制分隔线。ps：可通过setDividingLine(Drawable drawable)设置分隔线            drawDividingLine(c, parent, left, right, childCount);        }        //绘制分组标识        drawLabelText(c, parent, left, right, childCount);    }    private void drawDividingLine(Canvas canvas, RecyclerView parent, int left, int right, int childCount) {        for (int i = 0; i < childCount; i++) {            View view = parent.getChildAt(i);            RecyclerView.LayoutParams lp = (RecyclerView.LayoutParams) view.getLayoutParams();            int top = view.getBottom() + lp.bottomMargin;            int bottom = top + mDividingLine.getIntrinsicHeight();            mDividingLine.setBounds(left, top, right, bottom);            mDividingLine.draw(canvas);        }    }    private void drawLabelText(Canvas canvas, RecyclerView parent, int left, int right, int childCount) {        for (int i = 0; i < childCount; i++) {            View view = parent.getChildAt(i);            int position = parent.getChildAdapterPosition(view);            if (isFirstInGroup(position)) {                String groupLabel = mDecorationCallback.getGroupLabel(position);                if (groupLabel == null) return;                int bottom = view.getTop();                int top = bottom  - mLabelHeight;                //绘制矩形                canvas.drawRect(left, top, right, bottom, mPaint);                //将文本绘制在矩形的竖直中间位置                canvas.drawText(groupLabel , left + 30, bottom - mLabelHeight / 2 + (float) getLabelHeight() / 4, mTextPaint);            }        }    }

    //获取label文本的高度    private Paint.FontMetrics mMetrics = mTextPaint.getFontMetrics();    private double getLabelHeight() {        return Math.ceil(mMetrics.bottom - mMetrics.top);    }

设置之后整体效果如下

3.onDrawOver

@Override    public void onDrawOver(Canvas canvas, RecyclerView parent, RecyclerView.State state) {        super.onDrawOver(canvas, parent, state);        //获取当前可见的第一个item，意思就是在当前屏幕中的列表的第一个可见item。（已经划出屏幕外的item不算）        int position = ((LinearLayoutManager) (parent.getLayoutManager())).findFirstVisibleItemPosition();        int left = parent.getPaddingLeft();        int right = parent.getWidth() - parent.getPaddingRight();        String label = mDecorationCallback.getGroupLabel(position);        canvas.drawRect(left, 0, right, mLabelHeight, mPaint);        canvas.drawText(label, 30, mLabelHeight / 2 + (float) getLabelHeight() / 4, mTextPaint);    }

效果如图：

不过可以看得出来，顶部label的切换很突兀，于是想做一个慢慢把上一个label顶出屏幕的效果。

思路大致如下：

正常情况下悬浮顶部一直绘制在最上方

当分组的最后一个Item离开屏幕时，该分组的Label也随之跟着离开，而此时下面又接着下一个分组的label，就会产生下个label把上个label顶出的效果：

把代码改成：

    //绘制悬浮顶部    @Override    public void onDrawOver(Canvas canvas, RecyclerView parent, RecyclerView.State state) {        super.onDrawOver(canvas, parent, state);        int position = ((LinearLayoutManager) (parent.getLayoutManager())).findFirstVisibleItemPosition();        int left = parent.getPaddingLeft();        int right = parent.getWidth() - parent.getPaddingRight();        String label = mDecorationCallback.getGroupLabel(position);        //此处开始不同        for (int i = 0; i < parent.getChildCount(); i++) {            View child = parent.getChildAt(i);            //判断item是不是该组的最后一个元素            if (isLastInGroup(position)) {                int bottom = child.getBottom();                //滑动过程中，若分组中最后一个item的bottom小于label的高度，就把label的绘制位置往上提。                if (bottom <= mLabelHeight) {                    canvas.drawRect(left, 0, right, bottom, mPaint);                    canvas.drawText(label, 30, mLabelHeight / 2 + (float) getLabelHeight() / 4 - (mLabelHeight - bottom), mTextPaint);                    //以下代码不再执行，免得出现两次绘制。                    return;                }            }        }        canvas.drawRect(left, 0, right, mLabelHeight, mPaint);        canvas.drawText(label, 30, mLabelHeight / 2 + (float) getLabelHeight() / 4, mTextPaint);    }    private boolean isLastInGroup(int pos) {        String label = mDecorationCallback.getGroupLabel(pos);        String nextLabel;        try {            //若item是列表中所有item的最后一个，则pos+1会导致角标越界            nextLabel = mDecorationCallback.getGroupLabel(pos + 1);        } catch (ArrayIndexOutOfBoundsException exception) {            return true;        }        if (!label.equals(nextLabel)) return true;        return false;    }

大功告成，再来看看效果：

源码已经上传至Github，喜欢的话记得给我点个star。