ImageLoader源码解析(五) 补充针对ListView或者RecycleView的优化

本文链接：https://blog.youkuaiyun.com/qq_33804553/article/details/77657894

本文详细解析了ImageLoader中PauseOnScrollListener类的工作原理，该类针对ListView、GridView及RecycleView进行了滑动时暂停加载图片的优化处理，并介绍了ImageLoader如何通过暂停和恢复加载任务来实现这一功能。

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ImageLoader源码解析(五) 补充针对ListView或者RecycleView的优化

1 前言

前几篇,我忽略了一个类,这个类在com.nostra13.universalimageloader.core.listener包下,它所做的事情是,针对ListView,GridView,RecycleView做优化,其实很简单,就是在滑动的时候暂停加载,在滑动结束后重新启动加载,我们来看下这个类

2 PauseOnScrollListener

public class PauseOnScrollListener implements OnScrollListener {

    private ImageLoader imageLoader;

    private final boolean pauseOnScroll;
    private final boolean pauseOnFling;
    private final OnScrollListener externalListener;

    /**
     * Constructor
     *
     * @param imageLoader   {@linkplain ImageLoader} instance for controlling
     * @param pauseOnScroll Whether {@linkplain ImageLoader#pause() pause ImageLoader} during touch scrolling
     * @param pauseOnFling  Whether {@linkplain ImageLoader#pause() pause ImageLoader} during fling
     */
    public PauseOnScrollListener(ImageLoader imageLoader, boolean pauseOnScroll, boolean pauseOnFling) {
        this(imageLoader, pauseOnScroll, pauseOnFling, null);
    }

    /**
     * Constructor
     *
     * @param imageLoader    {@linkplain ImageLoader} instance for controlling
     * @param pauseOnScroll  Whether {@linkplain ImageLoader#pause() pause ImageLoader} during touch scrolling
     * @param pauseOnFling   Whether {@linkplain ImageLoader#pause() pause ImageLoader} during fling
     * @param customListener Your custom {@link OnScrollListener} for {@linkplain AbsListView list view} which also
     *                       will be get scroll events
     */
    public PauseOnScrollListener(ImageLoader imageLoader, boolean pauseOnScroll, boolean pauseOnFling,
            OnScrollListener customListener) {
        this.imageLoader = imageLoader;
        this.pauseOnScroll = pauseOnScroll;
        this.pauseOnFling = pauseOnFling;
        externalListener = customListener;
    }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
        switch (scrollState) {
            case OnScrollListener.SCROLL_STATE_IDLE:
                imageLoader.resume();
                break;
            case OnScrollListener.SCROLL_STATE_TOUCH_SCROLL:
                if (pauseOnScroll) {
                    imageLoader.pause();
                }
                break;
            case OnScrollListener.SCROLL_STATE_FLING:
                if (pauseOnFling) {
                    imageLoader.pause();
                }
                break;
        }
        if (externalListener != null) {
            externalListener.onScrollStateChanged(view, scrollState);
        }
    }

    @Override
    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
        if (externalListener != null) {
            externalListener.onScroll(view, firstVisibleItem, visibleItemCount, totalItemCount);
        }
    }

这个类其实也没什么可讲的,就是根据状态变化,是否暂停加载,滑动结束后再启动加载,
在使用的时候.其实对于RecycleView是否可用,我没事试过,但是它提供了一个思路,一个实现滑动暂停加载的思路,RecycleView也有监听滑动的方法,再写一个类似的就ok了

其实最重要的是Imageloader是如何完成暂停和继续逻辑的

3 Imageloader.pause和resume

Imagelader

  public void pause() {
        engine.pause();
    }

    /**
     * Resumes waiting "load&display" tasks
     */
    public void resume() {
        engine.resume();
    }

可以看到,所有的操作又转到了ImageLoaderEngine

ImageLoaderEngine

    private final AtomicBoolean paused = new AtomicBoolean(false);
    private final AtomicBoolean networkDenied = new AtomicBoolean(false);
    private final Object pauseLock = new Object();
 void pause() {
        paused.set(true);
    }

    /**
     * Resumes engine work. Paused "load&display" tasks will continue its work.
     */
    void resume() {
        paused.set(false);
        synchronized (pauseLock) {
            pauseLock.notifyAll();
        }
    }

      AtomicBoolean getPause() {
        return paused;
    }

暂停的,只是设置了一下标志,唤醒,则是通过lock,唤醒所有在等待的线程

那么到底是在哪里暂停的呢

LoadAndDisplayImageTask


    @Override
    public void run() {
        if (waitIfPaused()) return;
        ....
    }
 /**
     * 任务是否暂停
     *
     * @return <b>true</b> - if task should be interrupted; <b>false</b> - otherwise
     */
    private boolean waitIfPaused() {
        AtomicBoolean pause = engine.getPause();
        if (pause.get()) {
            synchronized (engine.getPauseLock()) {
                if (pause.get()) {
                    L.d(LOG_WAITING_FOR_RESUME, memoryCacheKey);
                    try {
                    //核心代码,本线程等待
                        engine.getPauseLock().wait();
                    } catch (InterruptedException e) {
                        L.e(LOG_TASK_INTERRUPTED, memoryCacheKey);
                        return true;
                    }
                    L.d(LOG_RESUME_AFTER_PAUSE, memoryCacheKey);
                }
            }
        }
        //如果view已经被回收或者被重用,那么暂停
        return isTaskNotActual();
    }