在我们看来两者效果都是一样的，其实看下源码就知道cancel肯定会去调dismiss的，如果调用的cancel的话就可以监听DialogInterface.OnCancelListener。
/**
* Cancel the dialog. This is essentially the same as calling {@link #dismiss()}, but it will
* also call your {@link DialogInterface.OnCancelListener} (if registered).
*/
public void cancel() {
if (!mCanceled && mCancelMessage != null) {
mCanceled = true;
// Obtain a new message so this dialog can be re-used
Message.obtain(mCancelMessage).sendToTarget();
}
dismiss();
}
dismiss可以在任何线程调用，但是最好不要覆写dismiss方法，实在需要就在onStop里去override。
/**
* Dismiss this dialog, removing it from the screen. This method can be
* invoked safely from any thread. Note that you should not override this
* method to do cleanup when the dialog is dismissed, instead implement
* that in {@link #onStop}.
*/
