项目开发中，往往会随着需求的改变而切换到其它图片加载框架上去。如果最初代码设计的耦合度太高，那么恭喜你，成功入坑了。至今无法忘却整个项目一行行去复制粘贴被支配的恐惧。:）
那么是否存在一种方式 能够一劳永逸地解决这个痛点呢？下面我们来分析一下图片加载框架面对的现状和解决思路。
问题现状
一个优秀的框架一般在代码设计的时候已经封装很不错了，对于开发者而言框架的使用也是很方便，但是为什么说我们往往还要去做这方面的框架封装呢？原因很简单，实际项目开发中，我们不得不面对着日新月异的需求变化，想要在这个变化中最大程度的实现代码的可扩展性和变通性（当然还可以偷懒），不能因为牵一发而动全身，同时要将框架适配到实际项目，框架的再封装设计显得尤为重要。
不多废话，我们可以开始今天的图片封装之路了。
设计思路
图片框架的封装主要需要满足以下三点：
低耦合，方便将来的代码扩展。至少要支持目前市场上使用率最高的图片框架Fresco、Glide、Picasso三者之间的切换
满足项目中各种需求
调用方便
谈到图片封装，最先想到的是把一些常用的功能点作为参数传入到方法内，然后调用图片加载框架实现我们图片的加载工作。比如说像下面这样
public interface ImageLoader {void loadImage(ImageView view, String path, int placeholderId, int errorId，boolean skipMemory);void loadImage(ImageView view, File file, int placeholderId, int errorId, boolean skipMemory);
}
然后分别写对应的ImageLoader实现类FrescoImageLoader、GlideImageLoader、PicassoImageLoader，最后采用策略的设计模式实现代码的切换。那么这种方式实际效果如何呢？实际开发中很明显的一个 问题就是，对于每一个需要的参数都需要进行对应的封装，就不止上面所提到的两个方法,我们需要封装大量的方法去满足实际的项目需要，而且每个框架的很多属性不一致，如果切换图片框架的话，还是需要大量的切换成本的。
于是我们想到了下面的这种思路
public interface ILoaderStrategy {
void loadImage(LoaderOptions options);/*** 清理内存缓存*/void clearMemoryCache();/*** 清理磁盘缓存*/void clearDiskCache();
}
提取各个框架通用的View,path/file文件路径,通过LoaderOptions解决大量不同参数传入的问题。这里需要说明的是，LoaderOptions中采用控件View，而不是ImageView，主要考虑到Fresco图片框架采用了DraweeView，这里保留了设计的扩展性。而图片参数类LoaderOptions采用了Builder设计模式：
public class LoaderOptions {public int placeholderResId;public int errorResId;public boolean isCenterCrop;public boolean isCenterInside;public boolean skipLocalCache; //是否缓存到本地public boolean skipNetCache;public Bitmap.Config config = Bitmap.Config.RGB_565;public int targetWidth;public int targetHeight;public float bitmapAngle; //圆角角度public float degrees; //旋转角度.注意:picasso针对三星等本地图片，默认旋转回0度，即正常位置。此时不需要自己rotatepublic Drawable placeholder;public View targetView;//targetView展示图片public BitmapCallBack callBack;public String url;public File file;public int drawableResId;public Uri uri;public LoaderOptions(String url) {this.url = url;}public LoaderOptions(File file) {this.file = file;}public LoaderOptions(int drawableResId) {this.drawableResId = drawableResId;}public LoaderOptions(Uri uri) {this.uri = uri;}public void into(View targetView) {this.targetView = targetView;ImageLoader.getInstance().loadOptions(this);}public void bitmap(BitmapCallBack callBack) {this.callBack = callBack;ImageLoader.getInstance().loadOptions(this);}public LoaderOptions placeholder(@DrawableRes int placeholderResId) {this.placeholderResId = placeholderResId;return this;}public LoaderOptions placeholder(Drawable placeholder) {this.placeholder = placeholder;return this;}public LoaderOptions error(@DrawableRes int errorResId) {this.errorResId = errorResId;return this;}public LoaderOptions centerCrop() {isCenterCrop = true;return this;}public LoaderOptions centerInside() {isCenterInside = true;return this;}public LoaderOptions config(Bitmap.Config config) {this.config = config;return this;}public LoaderOptions resize(int targetWidth, int targetHeight) {this.targetWidth = targetWidth;this.targetHeight = targetHeight;return this;}/*** 圆角* @param bitmapAngle 度数* @return*/public LoaderOptions angle(float bitmapAngle) {this.bitmapAngle = bitmapAngle;return this;}public LoaderOptions skipLocalCache(boolean skipLocalCache) {this.skipLocalCache = skipLocalCache;return this;}public LoaderOptions skipNetCache(boolean skipNetCache) {this.skipNetCache = skipNetCache;return this;}public LoaderOptions rotate(float degrees) {this.degrees = degrees;return this;}}
当然了，如果觉得有项目中需要可以以LoderOptions为基类继续扩展LoderOptions，不过现在这样在LoaderOptions上自行扩展基本上可以满足所有日常需要了。现在解决了代码设计的方向，那么接下来 我们要采取策略的方式实现图片框架的解耦。
import android.view.View;import com.squareup.picasso.Callback;import java.io.File;/*** 图片管理类，提供对外接口。* 静态代理模式，开发者只需要关心ImageLoader + LoaderOptions* Created by MhListener on /6/27.*/public class ImageLoader{private static ILoaderStrategy sLoader;private static volatile ImageLoader sInstance;private ImageLoader() {}//单例模式public static ImageLoader getInstance() {if (sInstance == null) {synchronized (ImageLoader.class) {if (sInstance == null) {//若切换其它图片加载框架，可以实现一键替换sInstance = new ImageLoader();}}}return sInstance;}//提供实时替换图片加载框架的接口public void setImageLoader(ILoaderStrategy loader) {if (loader != null) {sLoader = loader;}}public LoaderOptions load(String path) {return new LoaderOptions(path);}public LoaderOptions load(int drawable) {return new LoaderOptions(drawable);}public LoaderOptions load(File file) {return new LoaderOptions(file);}public LoaderOptions load(Uri uri) {return new LoaderOptions(uri);}public void loadOptions(LoaderOptions options) {sLoader.loadImage(options);}public void clearMemoryCache() {sLoader.clearMemoryCache();}public void clearDiskCache() {sLoader.clearDiskCache();}}
最后我们开始图片加载框架的具体实现方式，这里我实现了Picasso图片加载，开发者可以根据此例自行扩展GlideLoader或者FrescoLoader。
public class PicassoLoader implements ILoaderStrategy {private volatile static Picasso sPicassoSingleton;private final String PICASSO_CACHE = "picasso-cache";private static LruCache sLruCache = new LruCache(App.gApp);private static Picasso getPicasso() {if (sPicassoSingleton == null) {synchronized (PicassoLoader.class) {if (sPicassoSingleton == null) {sPicassoSingleton = new Picasso.Builder(App.gApp).memoryCache(sLruCache).build();}}}return sPicassoSingleton;}@Overridepublic void clearMemoryCache() {sLruCache.clear();}@Overridepublic void clearDiskCache() {File diskFile = new File(App.gApp.getCacheDir(), PICASSO_CACHE);if (diskFile.exists()) {//这边自行写删除代码//FileUtil.deleteFile(diskFile);}}@Overridepublic void loadImage(LoaderOptions options) {RequestCreator requestCreator = null;if (options.url != null) {requestCreator = getPicasso().load(options.url);} else if (options.file != null) {requestCreator = getPicasso().load(options.file);}else if (options.drawableResId != 0) {requestCreator = getPicasso().load(options.drawableResId);} else if (options.uri != null){requestCreator = getPicasso().load(options.uri);}if (requestCreator == null) {throw new NullPointerException("requestCreator must not be null");}if (options.targetHeight > 0 && options.targetWidth > 0) {requestCreator.resize(options.targetWidth, options.targetHeight);}if (options.isCenterInside) {requestCreator.centerInside();} else if (options.isCenterCrop) {requestCreator.centerCrop();}if (options.config != null) {requestCreator.config(options.config);}if (options.errorResId != 0) {requestCreator.error(options.errorResId);}if (options.placeholderResId != 0) {requestCreator.placeholder(options.placeholderResId);}if (options.bitmapAngle != 0) {requestCreator.transform(new PicassoTransformation(options.bitmapAngle));}if (options.skipLocalCache) {requestCreator.memoryPolicy(MemoryPolicy.NO_CACHE, MemoryPolicy.NO_STORE);}if (options.skipNetCache) {workPolicy(NetworkPolicy.NO_CACHE, NetworkPolicy.NO_STORE);}if (options.degrees != 0) {requestCreator.rotate(options.degrees);}if (options.targetView instanceof ImageView) {requestCreator.into(((ImageView)options.targetView));} else if (options.callBack != null){requestCreator.into(new PicassoTarget(options.callBack));}}class PicassoTarget implements Target {BitmapCallBack callBack;protected PicassoTarget(BitmapCallBack callBack) {this.callBack = callBack;}@Overridepublic void onBitmapLoaded(Bitmap bitmap, Picasso.LoadedFrom from) {if (this.callBack != null) {this.callBack.onBitmapLoaded(bitmap);}}@Overridepublic void onBitmapFailed(Exception e, Drawable errorDrawable) {if (this.callBack != null) {this.callBack.onBitmapFailed(e);}}@Overridepublic void onPrepareLoad(Drawable placeHolderDrawable) {}}class PicassoTransformation implements Transformation {private float bitmapAngle;protected PicassoTransformation(float corner){this.bitmapAngle = corner;}@Overridepublic Bitmap transform(Bitmap source) {float roundPx = bitmapAngle;//圆角的横向半径和纵向半径Bitmap output = Bitmap.createBitmap(source.getWidth(),source.getHeight(), Bitmap.Config.ARGB_8888);Canvas canvas = new Canvas(output);final int color = 0xff424242;final Paint paint = new Paint();final Rect rect = new Rect(0, 0, source.getWidth(),source.getHeight());final RectF rectF = new RectF(rect);paint.setAntiAlias(true);canvas.drawARGB(0, 0, 0, 0);paint.setColor(color);canvas.drawRoundRect(rectF, roundPx, roundPx, paint);paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_IN));canvas.drawBitmap(source, rect, rect, paint);source.recycle();return output;}@Overridepublic String key() {return "bitmapAngle()";}}}
好了，到了这里，关于图片框架的封装已经全部完成。而且该图片框架的封装已经成功应用到公司项目上，目前反馈良好。如有问题，欢迎交流指教！
如果感兴趣的话，欢迎在github给个star。代码已上传github链接
如果有考虑引用该封装的话，可以采用下面的方式:
//根目录下build.gradle配置
allprojects {
repositories {
...
maven { url 'https://jitpack.io' }
}
}
//项目build.gradle依赖
dependencies {
compile 'com.github.mhlistener:ImageLoader:1.0.5'
}
//使用方式
1.Application中全局设置
ImageLoader.getInstance().setGlobalImageLoader(new PicassoLoader());
2.界面中使用封装
ImageView imageView = findViewById(R.id.imageview);
String url = "/large/7a8aed7bgw1eutsd0pgiwj20go0p0djn.jpg";
ImageLoader.getInstance()
.load(url)
.angle(80)
.resize(400, 600)
.centerCrop()
.config(Bitmap.Config.RGB_565)
.placeholder(R.mipmap.test)
.error(R.mipmap.test)
.skipLocalCache(true)
.into(imageView);
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