[Android]从源码的角度理解为什么Dialog不能在Application中展示(1)

throw e;
}
}
}

这里创建了viewRootImpl并执行了ViewRootImpl#setView方法：

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
…
try {
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(),
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mInputChannel);
} catch (RemoteException e) {
} finally {

}
…
if (res < WindowManagerGlobal.ADD_OKAY) {
mAttachInfo.mRootView = null;
mAdded = false;
mFallbackEventHandler.setView(null);
unscheduleTraversals();
setAccessibilityFocus(null, null);
switch (res) {
case WindowManagerGlobal.ADD_BAD_APP_TOKEN:
case WindowManagerGlobal.ADD_BAD_SUBWINDOW_TOKEN:
throw new WindowManager.BadTokenException(
"Unable to add window – token " + attrs.token

• " is not valid; is your activity running?");
  …
  }
  throw new RuntimeException(
  "Unable to add window – unknown error code " + res);
  }
  …
  }
  }
  }

在这个方法中，我们找到了抛出异常的地方，当res的值为WindowManagerGlobal.ADD_BAD_APP_TOKEN或则WindowManagerGlobal.ADD_BAD_SUBWINDOW_TOKEN时，会抛出文章开头处的错误。 这个res是IWindowSession#addToDisplay方法的返回值。IWindowSession是一个Binder接口，负责ViewRootImpl和WindowManagerService的通信，在ViewRootImpl对象创建时获取。在这里IWindowSession#addToDisplay最终会调用WindowManagerService#addWindow方法：

// com.android.server.wm.WindowManagerService
public int addWindow(Session session, IWindow client, int seq,
WindowManager.LayoutParams attrs, int viewVisibility, int displayId,
Rect outContentInsets, Rect outStableInsets, Rect outOutsets,
InputChannel outInputChannel) {
…
synchronized(mWindowMap) {
…
if (type >= FIRST_SUB_WINDOW && type <= LAST_SUB_WINDOW) {
parentWindow = w