android本地图片加载器,LruCache缓存机制

package com.dingmore.mobile.ui.picselect;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Point;
import android.os.Handler;
import android.os.Message;
import android.support.v4.util.LruCache;

/**
* 本地图片加载器,采用的是异步解析本地图片，单例模式利用getInstance()获取NativeImageLoader实例
* 调用loadNativeImage()方法加载本地图片，此类可作为一个加载本地图片的工具类
*
*
*/
public class NativeImageLoader {
private LruCache<String, Bitmap> mMemoryCache;
private static NativeImageLoader mInstance = new NativeImageLoader();
private ExecutorService mImageThreadPool = Executors.newFixedThreadPool(1);

private NativeImageLoader(){
//获取应用程序的最大内存
final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);

//用最大内存的1/4来存储图片
final int cacheSize = maxMemory / 4;
mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {

//获取每张图片的大小
@Override
protected int sizeOf(String key, Bitmap bitmap) {
return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
}
};
}

/**
* 通过此方法来获取NativeImageLoader的实例
* @return
*/
public static NativeImageLoader getInstance(){
return mInstance;
}

/**
* 加载本地图片，对图片不进行裁剪
* @param path
* @param mCallBack
* @return
*/
public Bitmap loadNativeImage(final String path, final NativeImageCallBack mCallBack){
return this.loadNativeImage(path, null, mCallBack);
}

/**
* 此方法来加载本地图片，这里的mPoint是用来封装ImageView的宽和高，我们会根据ImageView控件的大小来裁剪Bitmap
* 如果你不想裁剪图片，调用loadNativeImage(final String path, final NativeImageCallBack mCallBack)来加载
* @param path
* @param mPoint
* @param mCallBack
* @return
*/
public Bitmap loadNativeImage(final String path, final Point mPoint, final NativeImageCallBack mCallBack){
//先获取内存中的Bitmap
Bitmap bitmap = getBitmapFromMemCache(path);

final Handler mHander = new Handler(){

@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
mCallBack.onImageLoader((Bitmap)msg.obj, path);
}

};

//若该Bitmap不在内存缓存中，则启用线程去加载本地的图片，并将Bitmap加入到mMemoryCache中
if(bitmap == null){
mImageThreadPool.execute(new Runnable() {

@Override
public void run() {
//先获取图片的缩略图
Bitmap mBitmap = decodeThumbBitmapForFile(path, mPoint == null ? 0: mPoint.x, mPoint == null ? 0: mPoint.y);
Message msg = mHander.obtainMessage();
msg.obj = mBitmap;
mHander.sendMessage(msg);

//将图片加入到内存缓存
addBitmapToMemoryCache(path, mBitmap);
}
});
}
return bitmap;

}

/**
* 往内存缓存中添加Bitmap
*
* @param key
* @param bitmap
*/
private void addBitmapToMemoryCache(String key, Bitmap bitmap) {
if (getBitmapFromMemCache(key) == null && bitmap != null) {
mMemoryCache.put(key, bitmap);
}
}

/**
* 根据key来获取内存中的图片
* @param key
* @return
*/
private Bitmap getBitmapFromMemCache(String key) {
return mMemoryCache.get(key);
}

/**
* 根据View(主要是ImageView)的宽和高来获取图片的缩略图
* @param path
* @param viewWidth
* @param viewHeight
* @return
*/
private Bitmap decodeThumbBitmapForFile(String path, int viewWidth, int viewHeight){
BitmapFactory.Options options = new BitmapFactory.Options();
//设置为true,表示解析Bitmap对象，该对象不占内存
options.inJustDecodeBounds = true;
BitmapFactory.decodeFile(path, options);
//设置缩放比例
options.inSampleSize = computeScale(options, viewWidth, viewHeight);

//设置为false,解析Bitmap对象加入到内存中
options.inJustDecodeBounds = false;

return BitmapFactory.decodeFile(path, options);
}

/**
* 根据View(主要是ImageView)的宽和高来计算Bitmap缩放比例。默认不缩放
* @param options
* @param width
* @param height
*/
private int computeScale(BitmapFactory.Options options, int viewWidth, int viewHeight){
int inSampleSize = 1;
if(viewWidth == 0 || viewWidth == 0){
return inSampleSize;
}
int bitmapWidth = options.outWidth;
int bitmapHeight = options.outHeight;

//假如Bitmap的宽度或高度大于我们设定图片的View的宽高，则计算缩放比例
if(bitmapWidth > viewWidth || bitmapHeight > viewWidth){
int widthScale = Math.round((float) bitmapWidth / (float) viewWidth);
int heightScale = Math.round((float) bitmapHeight / (float) viewWidth);

//为了保证图片不缩放变形，我们取宽高比例最小的那个
inSampleSize = widthScale < heightScale ? widthScale : heightScale;
}
return inSampleSize;
}

/**
* 加载本地图片的回调接口
*
* @author xiaanming
*
*/
public interface NativeImageCallBack{
/**
* 当子线程加载完了本地的图片，将Bitmap和图片路径回调在此方法中
* @param bitmap
* @param path
*/
public void onImageLoader(Bitmap bitmap, String path);
}
}