Android 添加 ImageLoader 时 getInstance(this) 中的 Volley 错误

Question

我正在跟进 image cashing tutorial using Volley Android 开发人员，我在请求图像请求和缓存它时遇到问题，我想是因为我创建的 singelton（从教程中复制） .

我的 Eclipse 在 getInstance(this) 中给出了错误，因为这是上下文，我正在请求一个我猜想的图像。

ImageRequest request = new ImageRequest(
    url,
    new Response.Listener<Bitmap>() {
        @Override
        public void onResponse(Bitmap bitmap) {
            mNetworkImageView = (NetworkImageView) findViewById(R.id.ImageView);
            mImageLoader = MySingleton.getInstance(this).getImageLoader();
            mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);
        //  mImageLoader = MySingleton.getInstance(this).getImageLoader();
        //  mImageLoader.get(IMAGE_URL, ImageLoader.getImageListener(mImageView,
        //      R.drawable.ic_launcher, R.drawable.ic_launcher));
            }
        },
    0,
    0,
    null,
    new Response.ErrorListener() {
        public void onErrorResponse(VolleyError error) {
        //  mImageView.setImageResource(R.drawable.ic_launcher);
        }
    });
MySingleton.getInstance(this).addToRequestQueue(request);

这是单例：

package com.example.p;

import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.toolbox.ImageLoader;
import com.android.volley.toolbox.Volley;

import android.content.Context;
import android.graphics.Bitmap;
import android.support.v4.util.LruCache;

public class MySingleton {
    private static MySingleton mInstance;
    private RequestQueue mRequestQueue;
    private ImageLoader mImageLoader;
    private static Context mCtx;

    private MySingleton(Context context) {
        mCtx = context;
        mRequestQueue = getRequestQueue();

        mImageLoader = new ImageLoader(
            mRequestQueue,
            new ImageLoader.ImageCache() {
                private final LruCache<String, Bitmap>
                    cache = new LruCache<String, Bitmap>(20);

                @Override
                public Bitmap getBitmap(String url) {
                    return cache.get(url);
                }

                @Override
                public void putBitmap(String url, Bitmap bitmap) {
                    cache.put(url, bitmap);
                }
            });
    }

    public static synchronized MySingleton getInstance(Context context) {
        if (mInstance == null) {
            mInstance = new MySingleton(context);
        }
        return mInstance;
    }

    public RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            // getApplicationContext() is key, it keeps you from leaking the
            // Activity or BroadcastReceiver if someone passes one in.
            mRequestQueue = Volley.newRequestQueue(mCtx.getApplicationContext());
        }
        return mRequestQueue;
    }

    public <T> void addToRequestQueue(Request<T> req) {
        getRequestQueue().add(req);
    }

    public ImageLoader getImageLoader() {
        return mImageLoader;
    }
}

我能够获取图像并以这种方式显示它，但我需要缓存它，所以我猜它是将它添加到请求中，对吗？ .. 有帮助吗？

mNetworkImageView = (NetworkImageView) findViewById(R.id.ImageView);
mImageLoader = MySingleton.getInstance(this).getImageLoader();
mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);

Answer 1

这是我的工作示例代码。希望对您有所帮助：

MainActivity.java:

import ...

public class MainActivity extends Activity {

    final Context mContext = this;   

    @Override

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

        NetworkImageView mNetworkImageView = (NetworkImageView) findViewById(R.id.networkImageView);

        String mUrl = "http://192.168.0.100/api/getimage";
        mNetworkImageView.setImageUrl(mUrl, VolleySingleton.getInstance(mContext).getImageLoader());       
    }

    ...  

}

VolleySingleton.java:

public class VolleySingleton {
    private static VolleySingleton mInstance;
    private RequestQueue mRequestQueue;
    private ImageLoader mImageLoader;
    private static Context mContext;

    private VolleySingleton(Context context) {
        mContext = context;
        mRequestQueue = getRequestQueue();

        mImageLoader = new ImageLoader(mRequestQueue,
                new ImageLoader.ImageCache() {
                    private final LruCache<String, Bitmap>
                            cache = new LruCache<>(20);

                    @Override
                    public Bitmap getBitmap(String url) {
                        return cache.get(url);
                    }

                    @Override
                    public void putBitmap(String url, Bitmap bitmap) {
                        cache.put(url, bitmap);
                    }
                });
    }

    public static synchronized VolleySingleton getInstance(Context context) {
        if (mInstance == null) {
            mInstance = new VolleySingleton(context);
        }
        return mInstance;
    }

    private RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            // getApplicationContext() is key, it keeps you from leaking the
            // Activity or BroadcastReceiver if someone passes one in.
            mRequestQueue = Volley.newRequestQueue(mContext.getApplicationContext(), 10 * 1024 * 1024); // this for caching
        }
        return mRequestQueue;
    }

    public <T> void addToRequestQueue(Request<T> req) {
        getRequestQueue().add(req);
    }

    public ImageLoader getImageLoader() {
        return mImageLoader;
    }   
}

activity_main.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    tools:context=".MainActivity">       

        <com.android.volley.toolbox.NetworkImageView
            android:id="@+id/networkImageView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />       

</LinearLayout>

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.volleyapp" >

    <uses-permission android:name="android.permission.INTERNET" />

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>        
    </application>

</manifest>

Answer 2

你在错误的上下文中..初始化一个 class 成员

private final Context ctx = this;

然后在 onResponse 中使用 ctx

mImageLoader = MySingleton.getInstance(ctx).getImageLoader();

Answer 3

getInstance 方法的一个优化建议是使用双重检查锁定，因为它只在实例实际为空的情况下同步：

private volatile static VolleySingleton self;

public static VolleySingleton getInstance(Context context) {
    if (self == null) {
        //Using double checked locking
        synchronized (VolleySingleton.class) {
            if (self == null) {
                //Using app context prevents leaking of activity context
                self = new VolleySingleton(context.getApplicationContext());
            }
        }
    }
    return self;
}

可以找到有关单例和实例化它们的不同方法的更多信息here。

volatile 关键字确保所有线程立即看到该字段，并且不会在线程本地缓存。如果 getInstance 不需要 Context，link 中解释的其他方法比双重检查锁定更合适。

Android 添加 ImageLoader 时 getInstance(this) 中的 Volley 错误

Android Volley error in getInstance(this) when adding ImageLoader

singleton

android

android-volley