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Google官方解释

• An ItemDecoration allows the application to add a special drawing and layout offset to specific item views from the adapter's data set. This can be useful for drawing dividers between items, highlights, visual grouping boundaries and more.

• All ItemDecorations are drawn in the order they were added, before the item views (in onDraw() and after the items (in onDrawOver(Canvas, RecyclerView, RecyclerView.State).

个人理解：

大致意思是：

• ItemDecoration允许应用程序从适配器的数据集中为制定的view添加制定的图形和布局偏移量。该特性一般被用于在两个item之间绘制分割线，高亮度以及视觉分组等等。

• 所有的ItemDecorations都按照它们被添加的顺序在item被绘制之前（在onDraw方法中）和在Items被绘制之后（在onDrawOver(Canvas,RecyclerView,RecyclerView.State)）进行绘制。

可以看到，ItemDecoration是相当强大和灵活的。

method学习：

[getItemOffsets](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#getItemOffsets(android.graphics.Rect, int, android.support.v7.widget.RecyclerView))(Rect outRect, int itemPosition, RecyclerView parent)
This method was deprecated in API level 22.0.0. Use [getItemOffsets(Rect, View, RecyclerView, State)](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#getItemOffsets(android.graphics.Rect, android.view.View, android.support.v7.widget.RecyclerView, android.support.v7.widget.RecyclerView.State))

• 该方法在API 22.0.0之后已被废弃，我们可以看代替的方法

[getItemOffsets](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#getItemOffsets(android.graphics.Rect, android.view.View, android.support.v7.widget.RecyclerView, android.support.v7.widget.RecyclerView.State))(Rect outRect, View view, RecyclerView parent, RecyclerView.State state)
Retrieve any offsets for the given item.

• 我们可以通过该方法中的outRect来设置item的padding值。比如你要在item底部添加一条分割线，此时为了不影响item原来的布局参数，我们一般会返回一个地步padding为某个pd的outRect，在recyclerview绘制item的时候会讲该布局数据加入，我们原来的item就会多出一个底部padding，是不是解耦的很完美呢？

[onDraw](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#onDraw(android.graphics.Canvas, android.support.v7.widget.RecyclerView))(Canvas c, RecyclerView parent)
*This method was deprecated in API level 22.0.0. Override [onDraw(Canvas, RecyclerView, RecyclerView.State)](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#onDraw(android.graphics.Canvas, android.support.v7.widget.RecyclerView, android.support.v7.widget.RecyclerView.State))

• 该方法也已经过期了，看下面的

[onDraw](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#onDraw(android.graphics.Canvas, android.support.v7.widget.RecyclerView, android.support.v7.widget.RecyclerView.State))(Canvas c, RecyclerView parent, RecyclerView.State state)
Draw any appropriate decorations into the Canvas supplied to the RecyclerView.

• 该方法会在绘制item之前调用，也就是说他的层级是在item之下的，通过该方法，我们可以爱绘制item之前绘制我们需要的内容。

[onDrawOver](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#onDrawOver(android.graphics.Canvas, android.support.v7.widget.RecyclerView, android.support.v7.widget.RecyclerView.State))(Canvas c, RecyclerView parent, RecyclerView.State state)
Draw any appropriate decorations into the Canvas supplied to the RecyclerView.

• 该方法已过期，看下面的

[onDrawOver](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#onDrawOver(android.graphics.Canvas, android.support.v7.widget.RecyclerView))(Canvas c, RecyclerView parent)
*This method was deprecated in API level 22.0.0. Override [onDrawOver(Canvas, RecyclerView, RecyclerView.State)](https://developer.android.com/reference/android/support/v7/widget/RecyclerView.ItemDecoration.html#onDrawOver(android.graphics.Canvas, android.support.v7.widget.RecyclerView, android.support.v7.widget.RecyclerView.State))

• 该方法于onDrawOver类似，在绘制item之后会调用该方法。

此时，也许你会疑问，他真的是这样执行的么？为了一探究竟，我们来看下源码吧。

 @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);
    }...}

• 从recyclerview的源码中我们可以看到，在draw方法中后会遍历recyclerview里面的itemDecoration然后调用itemdecoration的onDrawOver方法；而recyclerview调用了super.draw(c)之后会先，父类会先调用recyclerview的onDraw方法；

   @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);
    } }
  

• 在recyclerview的onDraw里又会调用itemDecoration的onDraw方法,当recyclerview的onDraw方法执行完之后，recyclerview的draw方法中super.draw(c)；后面的代码才会继续执行，而recyclerview是在绘制了自己之后才会去绘制item。

• 结论：itemDecoration的onDraw方法在item绘制之前调用，itemDecoration的onDrawOver方法在绘制item之后调用。

接下来我们在看下getItemOffsets这个方法。他真的把我们的outRect加到item的布局参数里面了么？预知真相，看源码。

   Rect getItemDecorInsetsForChild(View child) {
    final LayoutParams lp = (LayoutParams) child.getLayoutParams();
    if (!lp.mInsetsDirty) {
        return lp.mDecorInsets;
    }

    if (mState.isPreLayout() && (lp.isItemChanged() || lp.isViewInvalid())) {
        // changed/invalid items should not be updated until they are rebound.
        return lp.mDecorInsets;
    }
    final Rect insets = lp.mDecorInsets;
    insets.set(0, 0, 0, 0);
    final int decorCount = mItemDecorations.size();
    for (int i = 0; i < decorCount; i++) {
        mTempRect.set(0, 0, 0, 0);
        mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);
        insets.left += mTempRect.left;
        insets.top += mTempRect.top;
        insets.right += mTempRect.right;
        insets.bottom += mTempRect.bottom;
    }
    lp.mInsetsDirty = false;
    return insets;
}

• 首先我们可以看到，getItemOffsets这个方法在recyclerview的getItemDecorInsetsForChild 中被调用，该方法会把所有的itemDecortion中的rect累加后返回；我们再看下getItemDecorInsetsForChild在哪被调用的。

    public void measureChild(View child, int widthUsed, int heightUsed) {
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
  
        final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
  

• 在measureChild方法中被调用，也就是recyclerview在测量childView的时候
  public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
  final LayoutParams lp = (LayoutParams) child.getLayoutParams();

        final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
  

• 使用margins测量childView时会用到

• 结论，在getItemOffsets方法中outRect会影响到recyclerview中childView的布局。

使用ItemDecoration实现分割线的都调用过addItemDecoration方法。发现，只要调用一次addItemDecoration将自定义的分割线ItemDecoration添加进去就可以实现分割线效果了，如果我们添加多次会如何呢？

public void addItemDecoration(ItemDecoration decor, int index) {
    if (mLayout != null) {
        mLayout.assertNotInLayoutOrScroll("Cannot add item decoration during a scroll  or"
                + " layout");
    }
    if (mItemDecorations.isEmpty()) {
        setWillNotDraw(false);
    }
    if (index < 0) {
        mItemDecorations.add(decor);
    } else {
        mItemDecorations.add(index, decor);
    }
    markItemDecorInsetsDirty();
    requestLayout();
}

• 从RecyclerView.addItemDecoration方法源码可以看到，内部使用了一个ArrayList类型的mItemDecorations存储我们添加的所有ItemDecoration。markItemDecorInsetsDirty方法有什么用呢？我们看下源码

  void markItemDecorInsetsDirty() {
    final int childCount = mChildHelper.getUnfilteredChildCount();
    for (int i = 0; i < childCount; i++) {
        final View child = mChildHelper.getUnfilteredChildAt(i);
        ((LayoutParams) child.getLayoutParams()).mInsetsDirty = true;
    }
    mRecycler.markItemDecorInsetsDirty();
  }
  

• 里面有一个mInsetsDirty被重置为true，最终调用mRecycler.markItemDecorInsetsDirty();我们继续看mRecycler.markItemDecorInsetsDirty();方法源码：

void markItemDecorInsetsDirty() {
final int cachedCount = mCachedViews.size();
for (int i = 0; i < cachedCount; i++) {
final ViewHolder holder = mCachedViews.get(i);
LayoutParams layoutParams = (LayoutParams) holder.itemView.getLayoutParams();
if (layoutParams != null) {
layoutParams.mInsetsDirty = true;
}
}
}

• 里面也是将layoutParams的mInsetsDirty重置为true，这个mInsetsDirty有什么用呢 ？我们继续看源码：

  Rect getItemDecorInsetsForChild(View child) {
    final LayoutParams lp = (LayoutParams) child.getLayoutParams();
    if (!lp.mInsetsDirty) {
        return lp.mDecorInsets;
    }
  
    if (mState.isPreLayout() && (lp.isItemChanged() || lp.isViewInvalid())) {
        // changed/invalid items should not be updated until they are rebound.
        return lp.mDecorInsets;
    }
    final Rect insets = lp.mDecorInsets;
    insets.set(0, 0, 0, 0);
    final int decorCount = mItemDecorations.size();
    for (int i = 0; i < decorCount; i++) {
        mTempRect.set(0, 0, 0, 0);
        mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);
        insets.left += mTempRect.left;
        insets.top += mTempRect.top;
        insets.right += mTempRect.right;
        insets.bottom += mTempRect.bottom;
    }
    lp.mInsetsDirty = false;
    return insets;
  }
  

• 看到这段代码感觉应该是它了，可以看到，

  • 判断childView的layoutParams的mInsetsDirty是不是false 是false直接返回mDecorInsets。

• 判断itemDecoration是否已改变或者已不可用，mState.isPreLayout是recyclerview用来处理动画的。

• 如果前面的都不是，就会从新调用itemDecoration的getItemOffsets方法，重新计算layout偏离值之后返回。

• 出于性能的考虑，如果之前为ChildView生成过DecorInsets，那么会缓存在ChildView的LayoutParam中(mDecorInsets), 同时为了保证mDecorInsets的时效性，还同步维护了一个mInsetsDirty标记在LayoutParam中

• 在获取ChidlView的DecorInsets时，如果其mInsetsDirty为false，那么代表缓存没有过期，直接返回缓存的mDecorInsets。

• 如果mInsetsDirty为true，表示缓存已过期，需要根据ItemDecoration集合重新生成

  • 添加或者删除ItemDecoration的时候，会将所有ChildView包括Recycler中的mInsetsDirty设置为true来使DecorInsets缓存失效

总结：其实getItemDecorInsetsForChild方法我们之前在本章前面有分析到。他就是在测量childView的时候会调用，所以如果我们的itemDecortion中途需要更新，我们需要调用markItemDecorInsetsDirty方法，然后调用requestLayout请求重新绘制，这样在重新绘制childView的时候，就会重新计算ItemDecortion中返回的layout偏离值。达到我们想要的效果。

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