android - Picasso android Target 不能为空

Question

我是 android studio 的新手，仍在从视频教程中学习。我找到了 Picasso 并想将它用于我的项目，但是当我尝试在 ListView 中应用/练习时，我似乎没有得到它，我遇到了一个错误，提示 Target must not be null。我尝试了周围的解决方案，但仍然一无所获。对不起，我的英语不好。这是我的代码，请帮助我。

class MyAdapter extends ArrayAdapter<String> {

Context context;

public MyAdapter(Context context, int row_layout, String[] values) {
    super(context, R.layout.row_layout2, values);
}



@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ImageHolder holder = null; 
    holder = new ImageHolder();

    LayoutInflater theInflater = LayoutInflater.from(getContext());
    View theView = theInflater.inflate(R.layout.row_layout2, parent, false);
    String tvShow = getItem(position);

    TextView theTextView = (TextView) theView.findViewById(R.id.textView1);

    theTextView.setText(tvShow);

    holder.imageIcon = (ImageView) convertView.findViewById(R.id.imageView1);

    convertView.setTag(holder);

    Picasso.with(this.context).load(getItem(R.drawable.sample4)).into(holder.imageIcon); //ERROR HERE

    return theView;
}

static class ImageHolder
{
    ImageView imageIcon;
}
}

主要 Activity

public class MainActivity extends AppCompatActivity {

static class ImageHolder {
    ImageView imageIcon;
}

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

    String[] favoriteShows = {
            "Hannibal", "Sherlock", "Supernatural"
    };

    //ImageHolder holder = new ImageHolder();

    ListAdapter theAdapter = new MyAdapter(this, R.layout.row_layout, favoriteShows);

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

    theListView.setAdapter(theAdapter);

    theListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
            String tvShowPicked= "You selected " +
                    String.valueOf(adapterView.getItemAtPosition(position));

            Toast.makeText(MainActivity.this, tvShowPicked, Toast.LENGTH_SHORT).show();
        }
    });

我还尝试将 Picasso 行放在 MainActivity 中以查看是否有任何变化。

 holder.imageIcon = (ImageView) findViewById(R.id.imageView1);

    ImageView imageView = (ImageView) findViewById(R.id.imageView1);

    Picasso.with(this)
            .load(R.drawable.sample4)
            .into(holder.imageIcon);

我的 row_layout2.xml

<ImageView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginLeft="1dp"
    android:layout_marginTop="1dp"
    android:layout_marginRight="1dp"
    android:src="@drawable/sample3"
    android:id="@+id/imageView1"
    android:adjustViewBounds="true"
    />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/textView1"
    android:textSize="10sp"
    android:textStyle="bold"
    android:textColor="#000000"
    android:gravity="left"
    android:paddingLeft="15dp">
</TextView>
<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/textView2"
    android:textSize="10sp"
    android:textStyle="bold"
    android:textColor="#000000"
    android:gravity="left|bottom"
    android:padding="15dp"
    android:text="ADDRESS HERE">
</TextView>

我的activity_main.xml

<ListView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/theListView">

    </ListView>

错误日志:

06-29 21:06:13.990    1288-1288/com.erlawkward.albarol.listviewexample E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.erlawkward.albarol.listviewexample/com.erlawkward.albarol.listviewexample.MainActivity}: java.lang.IllegalArgumentException: Target must not be null.

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)

at android.app.ActivityThread.access$600(ActivityThread.java:141)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)

at android.os.Handler.dispatchMessage(Handler.java:99)

at android.os.Looper.loop(Looper.java:137)

at android.app.ActivityThread.main(ActivityThread.java:5041)

at java.lang.reflect.Method.invokeNative(Native Method)

at java.lang.reflect.Method.invoke(Method.java:511)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)

at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.IllegalArgumentException: Target must not be null.

at com.squareup.picasso.RequestCreator.into(RequestCreator.java:618)

at com.squareup.picasso.RequestCreator.into(RequestCreator.java:601)

at com.erlawkward.albarol.listviewexample.MainActivity.onCreate(MainActivity.java:59)

at android.app.Activity.performCreate(Activity.java:5104)

at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
            
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            
at android.app.ActivityThread.access$600(ActivityThread.java:141)
            
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            
at android.os.Handler.dispatchMessage(Handler.java:99)
            
at android.os.Looper.loop(Looper.java:137)
            
at android.app.ActivityThread.main(ActivityThread.java:5041)
            
at java.lang.reflect.Method.invokeNative(Native Method)
            
at java.lang.reflect.Method.invoke(Method.java:511)
            
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            
at dalvik.system.NativeStart.main(Native 

方法)

Answer 1

在你的适配器中替换

Picasso.with(this.context).load(getItem(R.drawable.sample4)).into(holder.imageIcon);

与

Picasso.with(getContext()).load(R.drawable.sample4).into(holder.imageIcon); 

Edit

此外，您还以错误的方式使用了 ViewHolder 模式。使用它的全部意义在于避免每次调用 getView 时都会膨胀布局。我按如下方式编辑方法

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    ImageHolder holder;

    if (convertView == null){
        LayoutInflater theInflater = LayoutInflater.from(getContext());
        convertView = theInflater.inflate(R.layout.row_layout2, parent, false);

        holder = new ImageHolder();
        holder.textView = (TextView) convertView.findViewById(R.id.textView1);
        holder.imageIcon = (ImageView) convertView.findViewById(R.id.imageView1);

        convertView.setTag(holder);
    }
    else{
        holder = (ImageHolder) convertView.getTag();
    }

    holder.textView.setText(getItem(position));

    //ERROR FIXED
    Picasso.with(getContext()).load(R.drawable.sample4).into(holder.imageIcon); 

    return convertView;
}

持有人

static class ImageHolder
{
    ImageView imageIcon;
    TextView textView;
}

和onCreate在Activity中

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

    String[] favoriteShows = {
            "Hannibal", "Sherlock", "Supernatural"
    };

    ListAdapter theAdapter = new MyAdapter(this, R.layout.row_layout2, favoriteShows);

    ListView theListView = (ListView) findViewById(R.id.theListView);
    theListView.setAdapter(theAdapter);

    theListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> adapterView, View view, int position, long id) {
            String tvShowPicked = "You selected " +
                    String.valueOf(adapterView.getItemAtPosition(position));

            Toast.makeText(MainActivity.this, tvShowPicked, Toast.LENGTH_SHORT).show();
        }
    });
}