view.findViewById(R.id ....)和findViewById(R.id ...)之间的区别

Question

我正在尝试实现自定义数组适配器,

问题是我使用时代码崩溃了

 ImageView imageView=(ImageView)findViewById(R.id.imageView);

而不是

ImageView imageView=(ImageView)rowView.findViewById(R.id.imageView);

其中rowView是我已经实施的ListView的布局文件.

为什么会发生这种情况,我认为rowView.findViewById(R.id ..)只会让它更快地通过id找到元素,但没有它应用程序崩溃,你能解释一下吗？

这是我的活动代码

公共类MainActivity扩展AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //Now for the list view

    ListView listView=(ListView)findViewById(R.id.listView);

    String[] values =new String[]{"iOS","android","firefoxOs","Ubuntu"};

   MySimpleAdapter simpleAdapter=new MySimpleAdapter(this,values);

   listView.setAdapter(simpleAdapter);
    //

    }

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

class MySimpleAdapter extends ArrayAdapter<String>{

    private Context context;
    private String[] values;

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
    //listView calls this on its adapter for each row
        Log.v("list","getView");
        LayoutInflater inflater = (LayoutInflater) context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        View rowView=inflater.inflate(R.layout.custom_row,parent,false);

        TextView textView1= (TextView) rowView.findViewById(R.id.textView1);

        textView1.setText(values[position]);
        //for the android row alone set an image , others leave it as blank
        if(values[position].startsWith("and"))
        {
            ImageView imageView=(ImageView)rowView.findViewById(R.id.imageView);
            imageView.setImageResource(R.drawable.ic_launcher);//Ok R.drawale....is also an id
        }


        return rowView;
    }

    public MySimpleAdapter(Context context,String[] values) {
        super(context, R.layout.custom_row,values);

        this.context=context;
        this.values=values;
        Log.v("list","constructor");
    }
}

}

Answer 1

findViewById总是需要一个上下文来查找视图.

如果你从扩展Activity类的类调用它,你可以使用findViewById,因为Activity是上下文.

当你从一个扩展的类中调用它时fragment,你必须使用它getActivity().findViewById

在你的情况下,因为你在一个中调用它adapter,你需要在其中找到该视图listview row.所以你用view.findViewById

Answer 2

findViewById()用于在您的Activity. 例如，这是不可能的Fragment。view.findViewById()用于在特定的其他视图中查找视图。例如，ListView在行布局中查找视图。没有它，您的应用程序会崩溃，因为您要搜索的视图在rowView. 通过使用法线，findViewById()您将找不到视图。