专栏首页移动开发Glide 如何实现正确加载图片而没有错位

Glide 如何实现正确加载图片而没有错位

我们在平时的项目使用下面的代码

 GlideApp
                .with(context)
                .load(url)
                .into(imageView);

当我们在常见的列表界面中(如 recycleview 实现的列表),使用上面的代码,在我们快速滑动中,glide 是如何实现正确加载图片,而没有导致图片内容的错位或者是不正确呢?

要达到这样的效果,简而言之,就是要执行上面的代码后,glide 要把最新的图片加载到正确的对象上,而取消对象之前关联的图片加载请求。

我们首先从 into() 这个方法进行分析。

  /**
   * Sets the {@link ImageView} the resource will be loaded into, cancels any existing loads into
   * the view, and frees any resources Glide may have previously loaded into the view so they may    *  be reused.
   *
   * @see RequestManager#clear(Target)
   *
   * @param view The view to cancel previous loads for and load the new resource into.
   * @return The
   * {@link com.bumptech.glide.request.target.Target} used to wrap the given {@link ImageView}.
   */
  @NonNull
  public ViewTarget<ImageView, TranscodeType> into(@NonNull ImageView view) {
    Util.assertMainThread();
    Preconditions.checkNotNull(view);

    RequestOptions requestOptions = this.requestOptions;
    if (!requestOptions.isTransformationSet()
        && requestOptions.isTransformationAllowed()
        && view.getScaleType() != null) {
      // Clone in this method so that if we use this RequestBuilder to load into a View and then
      // into a different target, we don't retain the transformation applied based on the previous
      // View's scale type.
      switch (view.getScaleType()) {
        case CENTER_CROP:
          requestOptions = requestOptions.clone().optionalCenterCrop();
          break;
        case CENTER_INSIDE:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case FIT_CENTER:
        case FIT_START:
        case FIT_END:
          requestOptions = requestOptions.clone().optionalFitCenter();
          break;
        case FIT_XY:
          requestOptions = requestOptions.clone().optionalCenterInside();
          break;
        case CENTER:
        case MATRIX:
        default:
          // Do nothing.
      }
    }

    return into(
        glideContext.buildImageViewTarget(view, transcodeClass),
        /*targetListener=*/ null,
        requestOptions);
  }

其实从方法的注释上就已经证明了我上面的说法。注释大意如下:

给 ImagView 设置将要被加载的资源,取消任何已存在的与 ImageView x相关的加载,释放 Glide 之前可能给该 View 加载的资源,这样他们可以被复用。

可以看到 45 行的代码是关键,具体分析下代码。

private <Y extends Target<TranscodeType>> Y into(
      @NonNull Y target,
      @Nullable RequestListener<TranscodeType> targetListener,
      @NonNull RequestOptions options) {
    Util.assertMainThread();
    Preconditions.checkNotNull(target);
    if (!isModelSet) {
      throw new IllegalArgumentException("You must call #load() before calling #into()");
    }

    options = options.autoClone();
    Request request = buildRequest(target, targetListener, options);

    Request previous = target.getRequest();
    if (request.isEquivalentTo(previous)
        && !isSkipMemoryCacheWithCompletePreviousRequest(options, previous)) {
      request.recycle();
      // If the request is completed, beginning again will ensure the result is re-delivered,
      // triggering RequestListeners and Targets. If the request is failed, beginning again will
      // restart the request, giving it another chance to complete. If the request is already
      // running, we can let it continue running without interruption.
      if (!Preconditions.checkNotNull(previous).isRunning()) {
        // Use the previous request rather than the new one to allow for optimizations like skipping
        // setting placeholders, tracking and un-tracking Targets, and obtaining View dimensions
        // that are done in the individual Request.
        previous.begin();
      }
      return target;
    }

    requestManager.clear(target);
    target.setRequest(request);
    requestManager.track(target, request);

    return target;
  }

由于 Glide 的代码还是相当复杂的,这里我不会一行一行的分析具体实现,大家可以对感兴趣的地方自己去探索下,这里我们主要看下上面提到主要流程的实现。

可以看到 15 行代码,构建了一个 Request 并持有了 target 。这样便可以将结果通知给 target。

 Request request = buildRequest(target, targetListener, options);

上面的入参 target 是由

glideContext.buildImageViewTarget(view, transcodeClass)

构建,具体的实现这里不分析,我们只需知道 Target 是 Glide 对我们要加载目标的一个封装和抽象。

下面贴一下接口定义和实现帮助大家稍微理解下。实现其实有很多种,这里贴的是我们常用用法最容易使用到的内部实现。

/**
 * An interface that Glide can load a resource into and notify of relevant lifecycle events during a
 * load.
 *
 * <p> The lifecycle events in this class are as follows: <ul> <li>onLoadStarted</li>
 * <li>onResourceReady</li> <li>onLoadCleared</li> <li>onLoadFailed</li> </ul>
 *
 * The typical lifecycle is onLoadStarted -> onResourceReady or onLoadFailed -> onLoadCleared.
 * However, there are no guarantees. onLoadStarted may not be called if the resource is in memory or
 * if the load will fail because of a null model object. onLoadCleared similarly may never be called
 * if the target is never cleared. See the docs for the individual methods for details. </p>
 *
 * @param <R> The type of resource the target can display.
 */
public interface Target<R> extends LifecycleListener {
  /**
   * Indicates that we want the resource in its original unmodified width and/or height.
   */
  int SIZE_ORIGINAL = Integer.MIN_VALUE;

  /**
   * A lifecycle callback that is called when a load is started.
   *
   * <p> Note - This may not be called for every load, it is possible for example for loads to fail
   * before the load starts (when the model object is null).
   *
   * <p> Note - This method may be called multiple times before any other lifecycle method is
   * called. Loads can be paused and restarted due to lifecycle or connectivity events and each
   * restart may cause a call here.
   *
   * <p>You must ensure that any current Drawable received in {@link #onResourceReady(Object,
   * Transition)} is no longer displayed before redrawing the container (usually a View) or
   * changing its visibility.
   *
   * @param placeholder The placeholder drawable to optionally show, or null.
   */
  void onLoadStarted(@Nullable Drawable placeholder);

  /**
   * A lifecycle callback that is called when a load fails.
   *
   * <p> Note - This may be called before {@link #onLoadStarted(android.graphics.drawable.Drawable)
   * } if the model object is null.
   *
   * <p>You must ensure that any current Drawable received in {@link #onResourceReady(Object,
   * Transition)} is no longer displayed before redrawing the container (usually a View) or
   * changing its visibility.
   *
   * @param errorDrawable The error drawable to optionally show, or null.
   */
  void onLoadFailed(@Nullable Drawable errorDrawable);

  /**
   * The method that will be called when the resource load has finished.
   *
   * @param resource the loaded resource.
   */
  void onResourceReady(@NonNull R resource, @Nullable Transition<? super R> transition);

  /**
   * A lifecycle callback that is called when a load is cancelled and its resources are freed.
   *
   * <p>You must ensure that any current Drawable received in {@link #onResourceReady(Object,
   * Transition)} is no longer displayed before redrawing the container (usually a View) or
   * changing its visibility.
   *
   * @param placeholder The placeholder drawable to optionally show, or null.
   */
  void onLoadCleared(@Nullable Drawable placeholder);

  /**
   * A method to retrieve the size of this target.
   *
   * @param cb The callback that must be called when the size of the target has been determined
   */
  void getSize(@NonNull SizeReadyCallback cb);

  /**
   * Removes the given callback from the pending set if it's still retained.
   *
   * @param cb The callback to remove.
   */
  void removeCallback(@NonNull SizeReadyCallback cb);

  /**
   * Sets the current request for this target to retain, should not be called outside of Glide.
   */
  void setRequest(@Nullable Request request);

  /**
   * Retrieves the current request for this target, should not be called outside of Glide.
   */
  @Nullable
  Request getRequest();
}
/**
 * A base {@link com.bumptech.glide.request.target.Target} for displaying resources in {@link
 * android.widget.ImageView}s.
 *
 * @param <Z> The type of resource that this target will display in the wrapped {@link
 *            android.widget.ImageView}.
 */
// Public API.
@SuppressWarnings("WeakerAccess")
public abstract class ImageViewTarget<Z> extends ViewTarget<ImageView, Z>
    implements Transition.ViewAdapter {

  @Nullable
  private Animatable animatable;

  public ImageViewTarget(ImageView view) {
    super(view);
  }

  /**
   * @deprecated Use {@link #waitForLayout()} instead.
   */
  @SuppressWarnings({"deprecation"})
  @Deprecated
  public ImageViewTarget(ImageView view, boolean waitForLayout) {
    super(view, waitForLayout);
  }

  /**
   * Returns the current {@link android.graphics.drawable.Drawable} being displayed in the view
   * using {@link android.widget.ImageView#getDrawable()}.
   */
  @Override
  @Nullable
  public Drawable getCurrentDrawable() {
    return view.getDrawable();
  }

  /**
   * Sets the given {@link android.graphics.drawable.Drawable} on the view using {@link
   * android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
   *
   * @param drawable {@inheritDoc}
   */
  @Override
  public void setDrawable(Drawable drawable) {
    view.setImageDrawable(drawable);
  }

  /**
   * Sets the given {@link android.graphics.drawable.Drawable} on the view using {@link
   * android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
   *
   * @param placeholder {@inheritDoc}
   */
  @Override
  public void onLoadStarted(@Nullable Drawable placeholder) {
    super.onLoadStarted(placeholder);
    setResourceInternal(null);
    setDrawable(placeholder);
  }

  /**
   * Sets the given {@link android.graphics.drawable.Drawable} on the view using {@link
   * android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
   *
   * @param errorDrawable {@inheritDoc}
   */
  @Override
  public void onLoadFailed(@Nullable Drawable errorDrawable) {
    super.onLoadFailed(errorDrawable);
    setResourceInternal(null);
    setDrawable(errorDrawable);
  }

  /**
   * Sets the given {@link android.graphics.drawable.Drawable} on the view using {@link
   * android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)}.
   *
   * @param placeholder {@inheritDoc}
   */
  @Override
  public void onLoadCleared(@Nullable Drawable placeholder) {
    super.onLoadCleared(placeholder);
    if (animatable != null) {
      animatable.stop();
    }
    setResourceInternal(null);
    setDrawable(placeholder);
  }

  @Override
  public void onResourceReady(@NonNull Z resource, @Nullable Transition<? super Z> transition) {
    if (transition == null || !transition.transition(resource, this)) {
      setResourceInternal(resource);
    } else {
      maybeUpdateAnimatable(resource);
    }
  }

  @Override
  public void onStart() {
    if (animatable != null) {
      animatable.start();
    }
  }

  @Override
  public void onStop() {
    if (animatable != null) {
      animatable.stop();
    }
  }

  private void setResourceInternal(@Nullable Z resource) {
    // Order matters here. Set the resource first to make sure that the Drawable has a valid and
    // non-null Callback before starting it.
    setResource(resource);
    maybeUpdateAnimatable(resource);
  }

  private void maybeUpdateAnimatable(@Nullable Z resource) {
    if (resource instanceof Animatable) {
      animatable = (Animatable) resource;
      animatable.start();
    } else {
      animatable = null;
    }
  }

  protected abstract void setResource(@Nullable Z resource);
}

into(
      @NonNull Y target,
      @Nullable RequestListener<TranscodeType> targetListener,
      @NonNull RequestOptions options) 

方法中 第 15 至 19 行中,判断 Taget 中的之前的 Request 和最新构建的 Request 是否相同,如果相同回收最新的 Request ,让旧的 Request 继续运行。如果不同,就取消之前的 Request 和 target 的关联。

具体逻辑代码为 31 行

 requestManager.clear(target);

最终会触发下面的代码

  private void untrackOrDelegate(@NonNull Target<?> target) {
    boolean isOwnedByUs = untrack(target);
    // We'll end up here if the Target was cleared after the RequestManager that started the request
    // is destroyed. That can happen for at least two reasons:
    // 1. We call clear() on a background thread using something other than Application Context
    // RequestManager.
    // 2. The caller retains a reference to the RequestManager after the corresponding Activity or
    // Fragment is destroyed, starts a load with it, and then clears that load with a different
    // RequestManager. Callers seem especially likely to do this in retained Fragments (#2262).
    //
    // #1 is always an error. At best the caller is leaking memory briefly in something like an
    // AsyncTask. At worst the caller is leaking an Activity or Fragment for a sustained period of
    // time if they do something like reference the Activity RequestManager in a long lived
    // background thread or task.
    //
    // #2 is always an error. Callers shouldn't be starting new loads using RequestManagers after
    // the corresponding Activity or Fragment is destroyed because retaining any reference to the
    // RequestManager leaks memory. It's possible that there's some brief period of time during or
    // immediately after onDestroy where this is reasonable, but I can't think of why.
    if (!isOwnedByUs && !glide.removeFromManagers(target) && target.getRequest() != null) {
      Request request = target.getRequest();
      target.setRequest(null);
      request.clear();
    }
  }

可以看到, target 对应的 request 被置 null, 而旧的 request 被 “clear”。旧的 Request 被 clear 后,又是如何让资源没有去加载到关联的 Target 上的? 我们看其中 SingleRequest 的实现

  /**
   * Cancels the current load if it is in progress, clears any resources held onto by the request
   * and replaces the loaded resource if the load completed with the placeholder.
   *
   * <p> Cleared requests can be restarted with a subsequent call to {@link #begin()} </p>
   *
   * @see #cancel()
   */
  @Override
  public void clear() {
    Util.assertMainThread();
    assertNotCallingCallbacks();
    stateVerifier.throwIfRecycled();
    if (status == Status.CLEARED) {
      return;
    }
    cancel();
    // Resource must be released before canNotifyStatusChanged is called.
    if (resource != null) {
      releaseResource(resource);
    }
    if (canNotifyCleared()) {
      target.onLoadCleared(getPlaceholderDrawable());
    }
    // Must be after cancel().
    status = Status.CLEARED;
  }

可以看到 clear() 方法中先是 执行了 cancel(),该方法会取消加载资源请求与该 Request 的回调关联。

  /**
   * Cancels the current load but does not release any resources held by the request and continues
   * to display the loaded resource if the load completed before the call to cancel.
   *
   * <p> Cancelled requests can be restarted with a subsequent call to {@link #begin()}. </p>
   *
   * @see #clear()
   */
  void cancel() {
    assertNotCallingCallbacks();
    stateVerifier.throwIfRecycled();
    target.removeCallback(this);
    status = Status.CANCELLED;
    if (loadStatus != null) {
      loadStatus.cancel();
      loadStatus = null;
    }
  }

LoadStatus 实际上只是持有了回调和 EngineJob。

/**
 * A callback that listens for when a resource load completes successfully or fails due to an
 * exception.
 */
public interface ResourceCallback {

  /**
   * Called when a resource is successfully loaded.
   *
   * @param resource The loaded resource.
   */
  void onResourceReady(Resource<?> resource, DataSource dataSource);

  /**
   * Called when a resource fails to load successfully.
   *
   * @param e a non-null {@link GlideException}.
   */
  void onLoadFailed(GlideException e);
}
  /**
   * Allows a request to indicate it no longer is interested in a given load.
   */
  public static class LoadStatus {
    private final EngineJob<?> engineJob;
    private final ResourceCallback cb;

    LoadStatus(ResourceCallback cb, EngineJob<?> engineJob) {
      this.cb = cb;
      this.engineJob = engineJob;
    }

    public void cancel() {
      engineJob.removeCallback(cb);
    }
  }

EngineJob 是负责加载资源,并加载成功后回调回去,这里 SingleRequest 实现了回调,所以它便可得知资源加载完成并获取到。这里不再分析 EngineJob 实现,以免偏离主流程太远。

所以 cancel() 调用后,即使旧的加载请求完成也不会回调到 Tareget 上。

    target.setRequest(request);
    requestManager.track(target, request);

方法中,Target 持有了最新的 request , requestManager.track() 方法测触发了 request 的加载请求,实际是由内部 Engine 和 EngineJob 负责。当顺利加载成功后便回调到 Target 对象上,触发 target.onResourceReady(result, animation) 方法,图片便被正确显示出来了。

实际上,还是有很多细节流程。这里只是大概介绍了主流程,希望对大家有所帮助。

以上代码基于 glide v4.7.1 版本

本文参与腾讯云自媒体分享计划,欢迎正在阅读的你也加入,一起分享。

我来说两句

0 条评论
登录 后参与评论

相关文章

  • 用path画一个抽象的树叶

    源码地址:https://github.com/X-FAN/LeafView 只是个简单的demo,大家可以参考下

    夏洛克的猫
  • 关于通知的一些小问题

    最近做下载,有一个通知显示进度的需求。这个过程中碰到一些问题,这里总结下,方便大家排查。

    夏洛克的猫
  • java中Map转换List

    夏洛克的猫
  • IIS 安装 pydio

    Introduction In this how-to, we’ll see the installation of Pydio (Put Your Data ...

    庞小明
  • 新的Feren OS做得更好

    Feren OS现在是围绕UbuntuLinux18.04和KDE Plasma桌面而不是LinuxMint构建的。肉桂桌面版可能在今年晚些时候退役。

    柴艺
  • From High Ceph Latency to Kernel Patch with eBPF/BCC

    There are a lot of tools for debugging kernel and userspace programs in Linux. M...

    Linux阅码场
  • 12个Kubernetes配置的最佳实践

    到目前为止,我们大多数人都听说过人为错误,在导致数据泄露中所扮演的角色。自7月以来,Capital One发生的黑客入侵事件只是一连串安全事件中的最新一起,这些...

    CNCF
  • splinter 学习笔记

    In order to install Splinter, make sure Python is installed. Note: only Python 2...

    Criss@陈磊
  • 内核开发知识第一讲.内核中的数据类型.重要数据结构.常用内核API函数.

      在内核中.程序的编写不能简单的用基本数据类型了. 因为操作系统不同.很有可能造成数据类型的长度不一.而产生重大问题.所以在内核中.

    IBinary
  • ReentrantLock 实现原理笔记(一)

    singleThreadSum: 20000100000 multiThreadSumNoLock:19496951532 multiThreadSumUs...

    一个会写诗的程序员

扫码关注云+社区

领取腾讯云代金券