面向对象——单一职责原则

概念

• 一个对象应该只包含单一的职责，并且该职责被完整地封装在一个类中，即又定义有且仅有一个原因使类变更。简简单来说，一个类中应该是一组相关性很高的函数、数据的封装，尽力去做同一件事情。

优点

• 降低类的复杂性，类的职责清晰明确。比如数据职责和行为职责清晰明确。
• 提高类的可读性和维护性
• 变更引起的风险减低，变更是必不可少的，如果接口的单一职责做得好，一个接口修改只对相应的类有影响，对其他接口无影响，这对系统的扩展性、维护性都有非常大的帮助

参考示例

// 非单一写法
public class ImageLoaderNS {

    // lruCache图片缓存
    LruCache<String, Bitmap> bitmapLruCache;
    // 创建缓存池跟系统的cpu数量一致
    ExecutorService executorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

    private ImageLoaderNS() {
        initImageCache();
    }

    public static ImageLoaderNS newInstance() {
        return ImageLoader1Holder.IMAGE_LOADER_NS;
    }

    /**
     * 静态内部类的实现方式
     */
    private static class ImageLoader1Holder {
        private static final ImageLoaderNS IMAGE_LOADER_NS = new ImageLoaderNS();
    }

    /**
     * 初始化
     */
    private void initImageCache() {
        // 计算可使用的最大内存
        final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
        // 取四分之一可用的作为缓存
        final int cacheSize = maxMemory / 4;
        bitmapLruCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
            }
        };
    }

    public void displayImage(final String url, final ImageView imageView) {
        imageView.setTag(url);
        executorService.submit(new Runnable() {
            @Override
            public void run() {
                Bitmap bitmap = downLoadImage(url);

                if (bitmap == null) {
                    return;
                }
                if (imageView.getTag().equals(url)) {
                    imageView.setImageBitmap(bitmap);
                    bitmapLruCache.put(url, bitmap);
                }

            }
        });
    }

    private Bitmap downLoadImage(String imageUrl) {
        Bitmap bitmap = null;
        try {
            URL url = new URL(imageUrl);
            HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.connect();
            bitmap = BitmapFactory.decodeStream(urlConnection.getInputStream());
            urlConnection.disconnect();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return bitmap;
    }
}

//单一职责 图片缓存
public class ImageCache {
    // lruCache图片缓存
    LruCache<String, Bitmap> bitmapLruCache;

    public ImageCache() {
        initImageCache();
    }

    /**
     * 初始化
     */
    private void initImageCache() {
        // 计算可使用的最大内存
        final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
        // 取四分之一可用的作为缓存
        final int cacheSize = maxMemory / 4;
        bitmapLruCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
            }
        };
    }

    public void put(String url, Bitmap bitmap) {
        bitmapLruCache.put(url, bitmap);
    }

    public Bitmap get(String url) {
        return bitmapLruCache.get(url);
    }

}
//单一职责 图片加载
public class ImageLoaderSP {

    private ImageCache imageCache;
    // 创建缓存池跟系统的cpu数量一致
    ExecutorService executorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

    private ImageLoaderSP() {
        imageCache = new ImageCache();
    }

    public static ImageLoaderSP newInstance() {
        return ImageLoaderSP.ImageLoader1Holder.imageLoader1;
    }

    /**
     * 静态内部类的实现方式
     */
    private static class ImageLoader1Holder {
        private static final ImageLoaderSP imageLoader1 = new ImageLoaderSP();
    }


    public void displayImage(final String url, final ImageView imageView) {
        imageView.setTag(url);
        executorService.submit(new Runnable() {
            @Override
            public void run() {
                Bitmap bitmap = downLoadImage(url);

                if (bitmap == null) {
                    return;
                }
                if (imageView.getTag().equals(url)) {
                    imageView.setImageBitmap(bitmap);
                    imageCache.put(url, bitmap);
                }

            }
        });
    }

    private Bitmap downLoadImage(String imageUrl) {
        Bitmap bitmap = null;
        try {
            URL url = new URL(imageUrl);
            HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
            urlConnection.connect();
            bitmap = BitmapFactory.decodeStream(urlConnection.getInputStream());
            urlConnection.disconnect();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return bitmap;
    }
}

文章来参考Android源码设计模式，代码已经上传至github