Android控件——RecyclerView系列三（Item Decoration）_android additemdecoration-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_43499030/article/details/95094036

本文介绍了如何在Android中使用RecyclerView的ItemDecoration来添加分割线和其他装饰效果。通过自定义ItemDecoration类并重写onDraw()或onDrawOver()方法，可以实现绘制在Item上方或覆盖内容的装饰。此外，讨论了如何根据布局方向调整分割线的绘制，并给出了绘制表格线的示例。最后，提到了如何避免在表格的最后一列和最后一行绘制边框。

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1 ItemDecoration

1.1 ItemDecoration简单使用

前述文章的例子item间并没有分割线，RecyclerView并没有支持divider这样的属性，我们可以自己定制分割线。

RecyclerView添加分割线的方法是：mRecyclerView.addItemDecoration()，该方法的参数为RecyclerView.ItemDecoration，该类为抽象类

该类的源码为：

public abstract static class ItemDecoration {
   
        public ItemDecoration() {
   
        }

        public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
   
            this.onDraw(c, parent);
        }

		/** @deprecated */
        @Deprecated
        public void onDraw(@NonNull Canvas c, @NonNull RecyclerView parent) {
   
        }

        public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
   
            this.onDrawOver(c, parent);
        }

		/** @deprecated */
        @Deprecated
        public void onDrawOver(@NonNull Canvas c, @NonNull RecyclerView parent) {
   
        }

        /** @deprecated */
        @Deprecated
        public void getItemOffsets(@NonNull Rect outRect, int itemPosition, @NonNull RecyclerView parent) {
   
            outRect.set(0, 0, 0, 0);
        }

        public void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {
   
            this.getItemOffsets(outRect, ((RecyclerView.LayoutParams)view.getLayoutParams()).getViewLayoutPosition(), parent);
        }
    }

当我们调用addItemDecoration()方法添加decoration的时候，RecyclerView在绘制的时候，去会绘制decorator，即调用该类的onDraw()和onDrawOver()方法，

onDraw方法先于drawChildren
onDrawOver在drawChildren之后，一般我们选择复写其中一个即可。
getItemOffsets为每一个item设置一定的偏移量，主要用于绘制Decorator。

简单示例（这里使用LayoutManager为LinearLayoutManager时）

public class DividerItemDecoration extends RecyclerView.ItemDecoration {
   

    private static final int[] ATTRS = new int[]{
   
            android.R.attr.listDivider
    };

    public static final int HORIZONTAL_LIST = LinearLayoutManager.HORIZONTAL;

    public static final int VERTICAL_LIST = LinearLayoutManager.VERTICAL;

    private Drawable mDivider;

    private int mOrientation;

    public DividerItemDecoration(Context context, int orientation) {
   
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        a.recycle();
        setOrientation(orientation);
    }

    public void setOrientation(int orientation) {
   
        if (orientation != HORIZONTAL_LIST && orientation != VERTICAL_LIST) {
   
            throw new IllegalArgumentException("invalid orientation");
        }
        mOrientation = orientation;
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent) {
   

        if (mOrientation == VERTICAL_LIST) {
   
            drawVertical(c, parent);
        } else {
   
            drawHorizontal(c, parent);
        }

    }


    public void drawVertical(Canvas c, RecyclerView parent) {
   
        final int left = parent.getPaddingLeft();
        final int right = parent.getWidth() - parent.getPaddingRight();

        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
   
            final View child = parent.getChildAt(i);
            android.support.v7.widget.RecyclerView v = new android.support.v7.widget.RecyclerView(parent.getContext());
            final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
                    .getLayoutParams();
            final int top = child.getBottom() + params.bottomMargin;
            final int bottom = top + mDivider