android获取ImageView的宽高为什么是0？

public class MainActivity extends Activity {
    private ImageView iv;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv = (ImageView)findViewById(R.id.iv);
        System.out.println(iv.getWidth()+"------"+iv.getHeight());
    }

你给ImageView分配src资源了吗