Android SwipeRefreshLayout返回null

Question

我想使用SwipeRefreshLayout替换数据，但当我这样做时

mSwipeRefreshLayout = (SwipeRefreshLayout)findViewById(R.id.refresh);

mSwipeRefreshLayout为null。代码示例：

<androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="5dp">
 <androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="5dp">

<LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_margin="10dp"
            android:orientation="vertical">

  </LinearLayout>

    </androidx.cardview.widget.CardView>
</androidx.swiperefreshlayout.widget.SwipeRefreshLayout>


import androidx.swiperefreshlayout.widget.SwipeRefreshLayout;


private SwipeRefreshLayout mSwipeRefreshLayout ;

@Override
    protected void onCreate(Bundle savedInstanceState) {
        mSwipeRefreshLayout = (SwipeRefreshLayout)findViewById(R.id.refresh);
        mSwipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                Toast.makeText(SpisokActivity.this, "Обновление", Toast.LENGTH_SHORT).show();
                mSwipeRefreshLayout.setRefreshing(false);
            }
        });

所以，在mSwipeRefreshLayout = (SwipeRefreshLayout)findViewById(R.id.refresh)之后，我得到了一个错误。我认为原因是mSwipeRefreshLayout有null。

Answer 1

在findViewById之前使用此命令

setContentView(R.layout.main)