为何PopupWindow一定要设置Background才能实现点击其他地方自动消失的效果

    好久没写博客了，来一篇有问题搞不懂就要看！源！码！系列
    之前一直搞不懂为什么PopupWindow一定要设置Background才能实现点击其他地方消失的效果，后来在老师的带领下看了下源码，豁然开朗（虽然还有些地方搞不懂）。

在PopupWindow类里有一个方法叫preparePopup()，对mBackground做了判断，若其不为空，则将mContentView嵌入到一个PopupViewContainer中。否则不执行此操作。具体代码如下：

/**
     * <p>Prepare the popup by embedding in into a new ViewGroup if the
     * background drawable is not null. If embedding is required, the layout
     * parameters' height is modified to take into account the background's
     * padding.</p>
     *
     * @param p the layout parameters of the popup's content view
     */
    private void preparePopup(WindowManager.LayoutParams p) {
        if (mContentView == null || mContext == null || mWindowManager == null) {
            throw new IllegalStateException("You must specify a valid content view by "
                    + "calling setContentView() before attempting to show the popup.");
        }

        if (mBackground != null) {
            final ViewGroup.LayoutParams layoutParams = mContentView.getLayoutParams();
            int height = ViewGroup.LayoutParams.MATCH_PARENT;
            if (layoutParams != null &&
                    layoutParams.height == ViewGroup.LayoutParams.WRAP_CONTENT) {
                height = ViewGroup.LayoutParams.WRAP_CONTENT;
            }

            // when a background is available, we embed the content view
            // within another view that owns the background drawable
            PopupViewContainer popupViewContainer = new PopupViewContainer(mContext);
            PopupViewContainer.LayoutParams listParams = new PopupViewContainer.LayoutParams(
                    ViewGroup.LayoutParams.MATCH_PARENT, height
            );
            popupViewContainer.setBackground(mBackground);
            popupViewContainer.addView(mContentView, listParams);

            mPopupView = popupViewContainer;
        } else {
            mPopupView = mContentView;
        }

        mPopupView.setElevation(mElevation);
        mPopupViewInitialLayoutDirectionInherited =
                (mPopupView.getRawLayoutDirection() == View.LAYOUT_DIRECTION_INHERIT);
        mPopupWidth = p.width;
        mPopupHeight = p.height;
    }

再看下PopupViewContainer的代码：

它里面注册了Touch事件，这下就大概明白了。如果我们不设置background的话，mContentView就不会被放入PopupViewContainer， 自然响应不了触摸事件，也就不会消失了。具体还是看代码吧：

所以说，源码才是王道。但我搞不懂的地方是，为啥一定要设置了背景才把mContentView加到PopupViewContainer中，有了解的朋友们请指教。