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
