Android中自定义RecyclerView.ItemDecoration

在Android应用开发中，往往显示列表都是需要用到RecyclerView的，而RecyclerView默认是没有item的分隔线的，需要调用addItemDecoration添加分隔线，你可以用它自带的DividerItemDecoration，也可以自己去继承RecyclerView.ItemDecoration来实现分隔线功能，下面我就具体来讲解下怎么自己绘制分隔线。

需要重写的方法有：

onDraw和getItemOffsets，还有可能重写onDrawOver。

onDraw():绘制分隔线的具体绘制代码

onDrawOver():在item绘制完成之后，添加绘制一点东西，它和onDraw的区别在于onDraw在绘制itemView之前，而它在绘制itemView之后

getItemOffsets():设置绘制item的偏移位置，通过改变outRect的left、top、right、bottom的值来指定偏移位置，默认这4个值都为0，表示不偏移，即根据原item的宽度和高度绘制，如果不预留出位置的话，下一个item就会覆盖掉当前的分隔线的绘制。而要预留出位置，如果是垂直排列的条目，你可以这样设置outRect。

outRect.bottom = mDividerHeight;

下面看一下RecyclerView的draw()和onDraw()方法，分析过View绘制流程的就知道，View会在draw()方法里面调用onDraw()，在RecyclerView的onDraw()方法中会绘制itemDecoration的onDraw()方法和itemView的内容，绘制完成后才会绘制onDrawOver()中需要绘制的内容。
@Override
public void draw(Canvas c) {
    super.draw(c);

    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDrawOver(c, this, mState);
    }
    // TODO If padding is not 0 and clipChildrenToPadding is false, to draw glows properly, we
    // need find children closest to edges. Not sure if it is worth the effort.
    boolean needsInvalidate = false;
    if (mLeftGlow != null && !mLeftGlow.isFinished()) {
        final int restore = c.save();
        final int padding = mClipToPadding ? getPaddingBottom() : 0;
        c.rotate(270);
        c.translate(-getHeight() + padding, 0);
        needsInvalidate = mLeftGlow != null && mLeftGlow.draw(c);
        c.restoreToCount(restore);
    }
    if (mTopGlow != null && !mTopGlow.isFinished()) {
        final int restore = c.save();
        if (mClipToPadding) {
            c.translate(getPaddingLeft(), getPaddingTop());
        }
        needsInvalidate |= mTopGlow != null && mTopGlow.draw(c);
        c.restoreToCount(restore);
    }
    if (mRightGlow != null && !mRightGlow.isFinished()) {
        final int restore = c.save();
        final int width = getWidth();
        final int padding = mClipToPadding ? getPaddingTop() : 0;
        c.rotate(90);
        c.translate(-padding, -width);
        needsInvalidate |= mRightGlow != null && mRightGlow.draw(c);
        c.restoreToCount(restore);
    }
    if (mBottomGlow != null && !mBottomGlow.isFinished()) {
        final int restore = c.save();
        c.rotate(180);
        if (mClipToPadding) {
            c.translate(-getWidth() + getPaddingRight(), -getHeight() + getPaddingBottom());
        } else {
            c.translate(-getWidth(), -getHeight());
        }
        needsInvalidate |= mBottomGlow != null && mBottomGlow.draw(c);
        c.restoreToCount(restore);
    }

    // If some views are animating, ItemDecorators are likely to move/change with them.
    // Invalidate RecyclerView to re-draw decorators. This is still efficient because children's
    // display lists are not invalidated.
    if (!needsInvalidate && mItemAnimator != null && mItemDecorations.size() > 0
            && mItemAnimator.isRunning()) {
        needsInvalidate = true;
    }

    if (needsInvalidate) {
        ViewCompat.postInvalidateOnAnimation(this);
    }
}

@Override
public void onDraw(Canvas c) {
    super.onDraw(c);

    final int count = mItemDecorations.size();
    for (int i = 0; i < count; i++) {
        mItemDecorations.get(i).onDraw(c, this, mState);
    }
}

本文地址：https://blog.csdn.net/a_lwh____/article/details/107378053