PopupWindow为什么要设置setBackgroundDrawable(new BitmapDrawable());

这个可以控制popupwindow的隐藏，可以实现点击popupwindow以外的区域时隐藏popupwindow，还有一个功能就是响应back键。实现按back键可以隐藏popupwindow的功能。

设置后点击旁边区域可以关闭PopupWindow

点击空白处 PopupWindow 消失 的作用

源码中有下面一段：
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.setBackgroundDrawable(mBackground);
popupViewContainer.addView(mContentView, listParams);

mPopupView = popupViewContainer;
}

当mBackground不为空时设置PopupViewContainer，在PopupViewContainer中处理了点击事件，
private class PopupViewContainer extends FrameLayout {
private static final String TAG = "PopupWindow.PopupViewContainer";

public PopupViewContainer(Context context) {
super(context);
}

@Override
protected int[] onCreateDrawableState(int extraSpace) {
if (mAboveAnchor) {
// 1 more needed for the above anchor state
final int[] drawableState = super.onCreateDrawableState(extraSpace + 1);
View.mergeDrawableStates(drawableState, ABOVE_ANCHOR_STATE_SET);
return drawableState;
} else {
return super.onCreateDrawableState(extraSpace);
}
}

@Override
public boolean dispatchKeyEvent(KeyEvent event) {
if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
if (getKeyDispatcherState() == null) {
return super.dispatchKeyEvent(event);
}

if (event.getAction() == KeyEvent.ACTION_DOWN
&& event.getRepeatCount() == 0) {
KeyEvent.DispatcherState state = getKeyDispatcherState();
if (state != null) {
state.startTracking(event, this);
}
return true;
} else if (event.getAction() == KeyEvent.ACTION_UP) {
KeyEvent.DispatcherState state = getKeyDispatcherState();
if (state != null && state.isTracking(event) && !event.isCanceled()) {
dismiss();
return true;
}
}
return super.dispatchKeyEvent(event);
} else {
return super.dispatchKeyEvent(event);
}
}

哈哈，今天也遇到了这个问题。