论单一职责原则的使用场景

来源：互联网发布：怪物猎人ol数据库编辑：程序博客网时间：2024/06/05 17:05

单一职责原则的定义是：就一个类而言，应该仅有一个引起它变化的原因。简单来说，一个类中应该是一组相关性很高的函数，数据封装。就是对职责的定义，什么是类的职责，以及怎么划分类的职责。这是最重要的一环

举个例子做进一步的阐释，做出比较：

没有采用这种模式的例子：

import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.util.LruCache;import android.widget.ImageView;import java.io.IOException;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class ImageLoader {    LruCache<String, Bitmap> mImageCache;    // 线程池，线程数量为cpu的数量    ExecutorService mExecutorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());    public ImageLoader() {        initImageCache();    }    private void initImageCache() {        final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);        final int cacheSize = maxMemory / 4;        mImageCache = 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);        mExecutorService.submit(new Runnable() {            @Override            public void run() {                Bitmap bitmap = downloadImage(url);                if (bitmap == null) {                    return;                }                if (imageView.getTag().equals(url)) {                    imageView.setImageBitmap(bitmap);                }                mImageCache.put(url, bitmap);            }        });    }    public Bitmap downloadImage(String imageUrl) {        Bitmap bitmap = null;        try {            URL url = new URL(imageUrl);            try {                final HttpURLConnection conn = (HttpURLConnection) url.openConnection();                bitmap = BitmapFactory.decodeStream(conn.getInputStream());                conn.disconnect();            } catch (IOException e) {                e.printStackTrace();            }        } catch (MalformedURLException e) {            e.printStackTrace();        }        return bitmap;    }}

代码看起来有些杂乱，耦合度比较高，对后期的扩展性，灵活性而言差别较大，后期一旦改动，将会改动很多地方，不利于维护

优化之后，采用单一职责：在功能上一份为二：

import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.util.LruCache;import android.widget.ImageView;import java.io.IOException;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class ImageLoader2 {    // 图片缓存    LruCache<String, Bitmap> mImageCache;    // 线程池，线程数量为cpu的数量    ExecutorService mExecutorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());    public void displayImage(final String url, final ImageView imageView) {        imageView.setTag(url);        mExecutorService.submit(new Runnable() {            @Override            public void run() {                Bitmap bitmap = downloadImage(url);                if (bitmap == null) {                    return;                }                if (imageView.getTag().equals(url)) {                    imageView.setImageBitmap(bitmap);                }                mImageCache.put(url, bitmap);            }        });    }    public Bitmap downloadImage(String imageUrl) {        Bitmap bitmap = null;        try {            URL url = new URL(imageUrl);            try {                final HttpURLConnection conn = (HttpURLConnection) url.openConnection();                bitmap = BitmapFactory.decodeStream(conn.getInputStream());                conn.disconnect();            } catch (IOException e) {                e.printStackTrace();            }        } catch (MalformedURLException e) {            e.printStackTrace();        }        return bitmap;    }}

多了imageCache,负责具体的缓存

import android.graphics.Bitmap;import android.util.LruCache;public class ImageCache {    LruCache<String ,Bitmap> mImageCache;    public ImageCache(){        initImageCache();    }    private void initImageCache() {        final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);        final int cacheSize = maxMemory / 4;        mImageCache = 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){        mImageCache.put(url,bitmap);    }    public Bitmap get(String url){        return mImageCache.get(url);    }}

实现缓存和下载图片的功能分离，代码逻辑更加清晰。单一职责就是根据个人的经验，在代码上进行一些优化，让代码逻辑更加清晰，尽量的减少耦合，利于后期维护和扩展。

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