Android itemDecoration接口实现吸顶悬浮标题

public static abstract class ItemDecoration {
        /**
         * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
         * Any content drawn by this method will be drawn before the item views are drawn,
         * and will thus appear underneath the views.
         *
         * @param c Canvas to draw into
         * @param parent RecyclerView this ItemDecoration is drawing into
         * @param state The current state of RecyclerView
         */
        public void onDraw(Canvas c, RecyclerView parent, State state) {
            onDraw(c, parent);
        }
        /**
         * Draw any appropriate decorations into the Canvas supplied to the RecyclerView.
         * Any content drawn by this method will be drawn after the item views are drawn
         * and will thus appear over the views.
         *
         * @param c Canvas to draw into
         * @param parent RecyclerView this ItemDecoration is drawing into
         * @param state The current state of RecyclerView.
         */
        public void onDrawOver(Canvas c, RecyclerView parent, State state) {
            onDrawOver(c, parent);
        }
        /**
         * Retrieve any offsets for the given item. Each field of <code>outRect</code> specifies
         * the number of pixels that the item view should be inset by, similar to padding or margin.
         * The default implementation sets the bounds of outRect to 0 and returns.
         *
         * <p>
         * If this ItemDecoration does not affect the positioning of item views, it should set
         * all four fields of <code>outRect</code> (left, top, right, bottom) to zero
         * before returning.
         *
         * <p>
         * If you need to access Adapter for additional data, you can call
         * {@link RecyclerView#getChildAdapterPosition(View)} to get the adapter position of the
         * View.
         *
         * @param outRect Rect to receive the output.
         * @param view    The child view to decorate
         * @param parent  RecyclerView this ItemDecoration is decorating
         * @param state   The current state of RecyclerView.
         */
        public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
            getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
                    parent);
        }
        /**
         * @deprecated
         * Override {@link #onDrawOver(Canvas, RecyclerView, RecyclerView.State)}
         */
        @Deprecated
        public void onDrawOver(Canvas c, RecyclerView parent) {
        }
        /**
         * @deprecated
         * Override {@link #onDraw(Canvas, RecyclerView, RecyclerView.State)}
         */
        @Deprecated
        public void onDraw(Canvas c, RecyclerView parent) {
        }
        /**
         * @deprecated
         * Use {@link #getItemOffsets(Rect, View, RecyclerView, State)}
         */
        @Deprecated
        public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {
            outRect.set(0, 0, 0, 0);
        }
    }

	public void onDraw(Canvas c, RecyclerView parent, State state) {
		onDraw(c, parent);
	}
	public void onDrawOver(Canvas c, RecyclerView parent, State state) {
    	onDrawOver(c, parent);
    }
	public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {
		getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),
	                parent);
	}

class MyDecoration(context: Context): RecyclerView.ItemDecoration() {
        var mPaint:Paint? = null
        var mPaint2:Paint? = null
        var mTextPaint:Paint? = null
        var mTitleHeight:Int? = null
        var mTitleHeight2:Int? = null
        var mTitleTextSize:Float? = null
        init {
            mTitleHeight =
                TypedValue.applyDimension(
                    TypedValue.COMPLEX_UNIT_DIP,
                    30f,
                    context.getResources().getDisplayMetrics()
                ).toInt()
            mTitleHeight2 =
                TypedValue.applyDimension(
                    TypedValue.COMPLEX_UNIT_DIP,
                    3f,
                    context.getResources().getDisplayMetrics()
                ).toInt()
            mTitleTextSize =
                TypedValue.applyDimension(
                    TypedValue.COMPLEX_UNIT_SP,
                    16f,
                    context.getResources().getDisplayMetrics()
                )
            mTextPaint = Paint()
            mTextPaint?.let {
                it.setTextSize(mTitleTextSize!!)
                it.setAntiAlias(true)
                it.setColor(Color.WHITE)
            }
            mPaint = Paint()
            mPaint?.let {
                it.setAntiAlias(true)
                it.setColor(Color.RED)
            }
            mPaint2 = Paint()
            mPaint2?.let {
                it.setAntiAlias(true)
                it.setColor(Color.BLUE)
            }
        }
        /**
         * recyclerView绘制onDraw -> item.onDraw -> onDrawOver
         */
        override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
            for (index in 0 until parent.childCount) {
                val childView = parent.getChildAt(index)
                childView?.let {
                    val rect = Rect()
                    val position = parent.getChildAdapterPosition(it)
                    if (isTitleItem(position)) {
                        rect.top = childView.top - mTitleHeight!!
                        rect.bottom = childView.top
                    } else {
                        rect.top = childView.top - mTitleHeight2!!
                        rect.bottom = childView.top
                    }
                    rect.left = parent.paddingLeft
                    rect.right = parent.width - parent.paddingRight
                    if (isTitleItem(position)) {
                        mPaint?.let { it1 -> c.drawRect(rect, it1) }
                        mTextPaint?.let { it3 ->
                            c.drawText(
                                getTitleStr(position),
                                0f,
                                rect.top.toFloat() + (mTitleHeight?.div(2.00f)?:0f),
                                it3)}
                    } else {
                        mPaint2?.let { it1 -> c.drawRect(rect, it1) }
                    }
                }
            }
        }
        /**
         * recyclerView绘制onDraw -> item.onDraw -> onDrawOver
         * 绘制覆盖在item上面的分割线效果
         */
        override fun onDrawOver(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
            val childView = parent.getChildAt(0)
            var nextView:View? = null;
            if (1 < parent.childCount) {
                nextView = parent.getChildAt(1)
            }
            childView?.let {
                val rect = Rect()
                val position = parent.getChildAdapterPosition(it)
                mTitleHeight?.let { height ->
                    if (nextView != null
                        && it.bottom - height < parent.paddingTop
                        && isTitleItem(parent.getChildAdapterPosition(nextView))
                        && !isSameTitle(parent.getChildAdapterPosition(nextView),position)) {
                        rect.top = it.bottom - height
                        rect.bottom = it.bottom
                    } else {
                        rect.top = parent.paddingTop
                        rect.bottom = rect.top + height
                    }
                }
                rect.left = parent.paddingLeft
                rect.right = parent.width - parent.paddingRight
                mPaint?.let { it1 -> c.drawRect(rect, it1) }
                mTextPaint?.let { it3 ->
                    c.drawText(
                        getTitleStr(position),
                        0f,
                        rect.top + (mTitleHeight?.div(2.00f)?:0f),
                        it3)}
            }
        }
        /**
         * 用于设置item周围的偏移量的，类似于设置padding喝margin效果，
         * 空出的效果用于绘制分割线
         */
        override fun getItemOffsets(
            outRect: Rect,
            view: View,
            parent: RecyclerView,
            state: RecyclerView.State
        ) {
            val position:Int = parent.getChildAdapterPosition(view)
            if (position % 4 == 0) {
                outRect.top = mTitleHeight!!
            } else{
                outRect.top = mTitleHeight2!!
            }
        }
        fun isTitleItem(position: Int):Boolean {
            return position % 4 == 0
        }
        fun getTitleStr(position: Int):String {
            return "标题：${position / 4}"
        }
        fun isSameTitle(position1: Int,position2: Int):Boolean {
            return (position1 / 4) == (position2 / 4)
        }
    }