java - 绘图应用程序 - 将某些功能添加到 onCreateView 时出错

Question

我是 Android 新手，我正在遵循此处的教程 http://code.tutsplus.com/tutorials/android-sdk-create-a-drawing-app-touch-interaction--mobile-19202为了制作一个简单的绘图应用程序。一切正常，直到我在 onCreateView 方法中添加这两个函数

currPaint = (ImageButton)paintLayout.getChildAt(0);



currPaint.setImageDrawable(getResources().getDrawable(R.drawable.paint_pressed));

这是错误的代码:

        package com.example.tastycorpse;

    import android.support.v7.app.ActionBarActivity;
    import android.support.v7.app.ActionBar;
    import android.support.v4.app.Fragment;
    import android.annotation.SuppressLint;
    import android.os.Bundle;
    import android.view.LayoutInflater;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.view.ViewGroup;
    import android.os.Build;
    import android.widget.ImageButton;
    import android.widget.LinearLayout;

    @SuppressLint("ValidFragment")
    public class MainActivity extends ActionBarActivity {
        private DrawingView drawView;
        private ImageButton currPaint;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            if (savedInstanceState == null) {
                getSupportFragmentManager().beginTransaction()
                        .add(R.id.container, new PlaceholderFragment()).commit();
            }
        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {

            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.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();
            if (id == R.id.action_settings) {
                return true;
            }
            return super.onOptionsItemSelected(item);
        }

        /**
         * A placeholder fragment containing a simple view.
         */
        public class PlaceholderFragment extends Fragment {

            public PlaceholderFragment() {
            }

            @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup container,
                    Bundle savedInstanceState) {

                View rootView = inflater.inflate(R.layout.fragment_main, container,
                        false);
                drawView = (DrawingView)findViewById(R.id.drawing);
                //get the palette and first color button
                LinearLayout paintLayout = (LinearLayout)findViewById(R.id.paint_colors);
                currPaint = (ImageButton)paintLayout.getChildAt(0);
                currPaint.setImageDrawable(getResources().getDrawable(R.drawable.paint_pressed));

                return rootView;
            }





    }
    //user clicked paint
    public void paintClicked(View view){
        //use chosen color      
        if(view!=currPaint){
            ImageButton imgView = (ImageButton)view;
            String color = view.getTag().toString();
            drawView.setColor(color);
            //update ui
            imgView.setImageDrawable(getResources().getDrawable(R.drawable.paint_pressed));
            currPaint.setImageDrawable(getResources().getDrawable(R.drawable.paint));
            currPaint=(ImageButton)view;
        }
    }

}

这是日志猫

05-04 12:40:24.224: E/AndroidRuntime(2887): Caused by: java.lang.NullPointerException
05-04 12:43:14.489: E/AndroidRuntime(3048): Caused by: java.lang.NullPointerException
05-04 12:43:47.919: E/AndroidRuntime(3187): Caused by: java.lang.NullPointerException
05-04 12:43:54.607: E/AndroidRuntime(3232): Caused by: java.lang.NullPointerException

我试图遵循并理解教程的所有步骤，但我被困在这里......(不幸的是-app-已停止)。我什至找不到有关旧问题的任何内容，因为错误太笼统了。有人可以向我解释为什么我会收到此错误吗？

Answer 1

在刚刚膨胀的 rootView 上调用 findViewById()，而不是 Activity.findViewById()。 fragment View 层次结构尚未成为 Activity View 层次结构的一部分，尝试从 Activity 中查找 fragment View 将返回 null。在 null 上调用诸如 getChildAt() 之类的方法会导致 NPE。

例如，更改调用，例如

drawView = (DrawingView)findViewById(R.id.drawing);

至

drawView = (DrawingView)rootView.findViewById(R.id.drawing);