React Native: alert : tried to show an alert while not attached to an activity

今天使用alert来弹出消息时候，发现下方一直弹出一个黄色的错误提示：tried to show an alert while not attached to an activity，使用的是Android 4.x的机子。

经谷歌，发现是因为android->app->src->main->java下的MainActivity少了几个方法，将其补上就可以正常弹出Alert。另外尽量使用Alert.alert()来弹出alert（要先import Alert）：

@Overrideprotected void onPause() {    super.onPause();    if (mReactInstanceManager != null) {        mReactInstanceManager.onHostPause(this);    }}@Overrideprotected void onResume() {    super.onResume();    if (mReactInstanceManager != null) {        mReactInstanceManager.onHostResume(this, this);    }}@Overrideprotected void onDestroy() {    super.onDestroy();    if (mReactInstanceManager != null) {        mReactInstanceManager.onHostDestroy();    }}@Override public void onBackPressed() {    if (mReactInstanceManager != null) {        mReactInstanceManager.onBackPressed();    } else {        super.onBackPressed();    }}