Android list加载图片工具类

 总体思路

Handler + looper + message

核心类

package com.base.imagechoose.util;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.support.v4.util.LruCache;
import android.util.DisplayMetrics;
import android.view.ViewGroup;
import android.widget.ImageView;

import java.lang.reflect.Field;
import java.util.LinkedList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

/**
 * 图片加载类
 */
public class ImageLoader {


    private static ImageLoader mInstance;

    /**
     * 图片缓存
     */
    private LruCache<String, Bitmap> mLruCache;

    private ExecutorService mThreadPool;//线程池, 加载任务

    private static final int DAFULT_THREAD_COUNT = 1; //线程数量
    /**
     * 队列调度方式
     */
    private Type mType = Type.FIFO;

    /**
     * 任务队列
     */
    private LinkedList<Runnable> mTaskQueue;

    /**
     * 后台轮询线程
     */
    private Thread mPoolThread;
    private Handler mPoolTheadHandler;

    //信号量同步mPoolTheadHandler
    private Semaphore mSemaphorePoolTheadHandler = new Semaphore(0);
    /**
     * 用Semaphore来控制一个线程执行完才执行下一个
     */
    private Semaphore mSemaphorePool; //用信号量来达到线程Type规则

    /**
     * UI线程中的Handler
     */
    private Handler mUIHandler;

    //先进先出
    public enum Type {
        FIFO, FIFI;
    }


    private ImageLoader(int threadCount, Type type) {
        init(threadCount, type);
    }

    public static ImageLoader getInstance() {
        if (mInstance == null) {
            synchronized (ImageLoader.class) {
                if (mInstance == null) {
                    mInstance = new ImageLoader(DAFULT_THREAD_COUNT, Type.FIFO);
                }
            }
        }
        return mInstance;
    }

    public static ImageLoader getInstance(int threadCount, Type type) {
        if (mInstance == null) {
            synchronized (ImageLoader.class) {
                if (mInstance == null) {
                    mInstance = new ImageLoader(threadCount, type);
                }
            }
        }
        return mInstance;
    }

    private void init(int threadCount, Type type) {
        //后台轮询线程
        mPoolThread = new Thread(){

            @Override
            public void run() {
                Looper.prepare();
                mPoolTheadHandler = new Handler() {
                    @Override
                    public void handleMessage(Message msg) {
                        //线程池去取出一个任务进行执行
                        mThreadPool.execute(getTask());

                        try {
                            //如果信号量为3，到第4个就会阻塞住
                            mSemaphorePool.acquire();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                };
                //释放一个信号量
                mSemaphorePoolTheadHandler.release();
                Looper.loop();
            }
        };
        mPoolThread.start();

        //获取应用的最大可用内存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        //设置缓存空间
        int cacheMemory = maxMemory/8;

        //初始化缓存
        mLruCache = new LruCache<String, Bitmap>(cacheMemory) {
            @Override
            protected int sizeOf(String key, Bitmap value) {
                //返回内存，value每一行的大小*高度
                return value.getRowBytes()*value.getHeight();
            }
        };

        //创建线程池
        mThreadPool = Executors.newFixedThreadPool(threadCount);
        //初始化任务队列
        mTaskQueue = new LinkedList<>();

        mType = type;

        mSemaphorePool = new Semaphore(threadCount);
    }

    /**
     * 从任务队列取出一个方法
     * @return
     */
    private Runnable getTask() {

        if (mType == Type.FIFO) {
            return mTaskQueue.removeFirst();
        } else {
            return  mTaskQueue.removeLast();
        }

    }


    /**
     * 根据path为控件加载图片
     * @param path 路径
     * @param imageView 控件
     */
    public void  loadImage(final String path, final ImageView imageView) {

        imageView.setTag(path);

        if (mUIHandler == null) {
            mUIHandler = new Handler() {
                @Override
                public void handleMessage(Message msg) {
                    //获取得到的图片并在控件上显示
                    ImgBeanHolder holder = (ImgBeanHolder) msg.obj;

                    //当tag为传入path时才进行赋值
                    if (holder.imageView.getTag().equals(holder.path)) {
                        holder.imageView.setImageBitmap(holder.bm);
                    }
                }
            };
        }

        //根据path在缓存中获取图片
        Bitmap bm = getBitmapFromLruCache(path);

        if (bm != null) {
            refreashBitmap(path, imageView, bm);
        } else {
            //没有图片，添加一个任务
            addTasks(new Runnable(){
                @Override
                public void run() {
                        //加载图片
                        //图片压缩
                        //1.获得图片需要显示的大小
                       ImageSize imageSize = getImageViewSize(imageView);
                       //2.压缩图片
                      Bitmap bm = decodeSampleBitmapFromPath(path, imageSize.width, imageSize.height);

                      //3.把图片加入缓存
                    addBitmapToLruCache(path,bm);

                    refreashBitmap(path, imageView, bm);
                    //当前任务执行完后释放信号量
                    mSemaphorePool.release();
                }
            });
        }

    }

    /**
     * 发送handler显示图片
     * @param path
     * @param imageView
     * @param bm
     */
    private void refreashBitmap(String path, ImageView imageView, Bitmap bm) {
        Message message = new Message();
        ImgBeanHolder imgBeanHolder = new ImgBeanHolder();
        imgBeanHolder.bm = bm;
        imgBeanHolder.imageView = imageView;
        imgBeanHolder.path = path;
        message.obj = imgBeanHolder;
        mUIHandler.sendMessage(message);
    }

    /**
     * 将图片放入缓存
     * @param path
     * @param bm
     */
    private void addBitmapToLruCache(String path, Bitmap bm) {
        if(getBitmapFromLruCache(path) == null) {
            if (bm != null) {
                mLruCache.put(path, bm);
            }
        }

    }


    /**
     * 添加任务
     * @param runnable
     */
    private synchronized void addTasks(Runnable runnable) {
        //将任务放入任务队列中
        mTaskQueue.add(runnable);
        //发送一个通知去通知后台轮询执行任务
        try {
            if (mPoolTheadHandler == null)
            mSemaphorePoolTheadHandler.acquire();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        mPoolTheadHandler.sendEmptyMessage(0x110);
    }

    /**
     * 根据path在缓存中获取图片
     * @param key
     * @return
     */
    private Bitmap getBitmapFromLruCache(String key) {

        return mLruCache.get(key);
    }

    /**
     * 根据imageview获取图片需要压缩的宽和高
     * @param imageView
     */
    private ImageSize getImageViewSize(ImageView imageView) {
        ImageSize imageSize = new ImageSize();

        DisplayMetrics displayMetrics = imageView.getContext().getResources().getDisplayMetrics();
        ViewGroup.LayoutParams lp = imageView.getLayoutParams();

        int width = imageView.getWidth();

        if (width <= 0) {
            width = lp.width; //获取imageview在layout中声明的亮度
        }

        if (width <= 0) {
            width = getImageViewFileValue(imageView, "mMaxWidth"); //检查最大值
        }

        if (width <= 0) {
            width = displayMetrics.widthPixels;
        }

        int height = imageView.getHeight();

        if (height <= 0) {
            height = lp.height; //获取imageview在layout中声明的亮度
        }

        if (height <= 0) {
            height = getImageViewFileValue(imageView, "mMaxHeight"); //检查最大值 //检查最大值
        }

        if (height <= 0) {
            height = displayMetrics.heightPixels;
        }

        imageSize.width = width;
        imageSize.height = height;

        return imageSize;
    }

    private static int getImageViewFileValue(Object object,String fileName) {
        int value = 0;

        //得到当前ImageView的字段集合
        try {
            Field field = ImageView.class.getDeclaredField(fileName);
            field.setAccessible(true);

            int fieldValue =  field.getInt(object);

            if (fieldValue > 0 && fieldValue < Integer.MAX_VALUE) {
                value = fieldValue;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        return value;
    }


    /**
     * 根据图片需要的宽高对图片进行压缩
     * @param path
     * @param width
     * @param height
     * @return
     */
    private Bitmap decodeSampleBitmapFromPath(String path, int width, int height) {

        BitmapFactory.Options  options = new BitmapFactory.Options();
        //不加入内存
        options.inJustDecodeBounds = true;
        //获取实际的options
        BitmapFactory.decodeFile(path, options);

        //用实际的options和需要显示的宽高算出inSampleSize
        options.inSampleSize = caculateInSampleSize(options, width, height);

        //使用获取的inSampleSize再次解析图片
        //放入内存
        options.inJustDecodeBounds = false;

        Bitmap bitmap =  BitmapFactory.decodeFile(path, options);

        return bitmap;
    }

    /**
     * 用实际的options和需要显示的宽高算出inSampleSize
     * @param options
     * @param reqWidth
     * @param reqHeight
     * @return
     */
    private int caculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {

        int width = options.outWidth;
        int height = options.outHeight;

        int inSampleSize = 1;

        if (width > reqHeight || height > reqHeight) {
            int widthRadio = Math.round(width*1.0f/reqWidth);
            int heightRadio = Math.round(height*1.0f/reqHeight);

            inSampleSize = Math.max(widthRadio, heightRadio);
        }

        return inSampleSize;
    }

    private class ImageSize{
        int width;
        int height;
    }


    private class  ImgBeanHolder {
        Bitmap bm;
        String path;
        ImageView imageView;
    }

}