Glide的自问自答

发表于2023-03-10更新于2023-04-26

1. Glide的缓存有哪些

内存缓存和磁盘缓存

2. 缓存对应的实现类是什么

内存缓存:

  1. ActiveResource
  2. LruResourceCache
  • ActiveResource是使用HaspMap进行的数据存储.键值是用弱引用进行的包装.为什么要用弱引用,因为他缓存的是正在使用的图片,当页面被销毁了,你还拿着对象的引用,会造成内存泄漏,所以要用弱引用进行包装.
  • LruResourceCache他缓存的是所有曾经加载过的图片信息,他和ActiveResource的区别是,他底层使用了Lru算法,然后Lru算法的实现是通过LinkedHashmap.为什么LruResourceCache要用到Lru算法,然后还不用弱引用.因为他缓存的不是正在使用的图片,不会存在内存泄漏的问题.然后如果他也使用了弱引用,就会导致缓存的资源被频繁的清空,也就失去了缓存的意义了.当然,缓存的大小总是有个边界的,所以使用了Lru算法,在超过缓存大小后删除那些最近最少使用的资源.这个实现就是用到了LinkedHashMap.LinkedHashMap继承HashMap,他重写了newNode方法,键值封装到了LinkedHashMapEntry里面,维护了一个双向链表.这样我们就能得到我们的插入顺序了.新插入的值是插在链表的尾部的.默认情况下LinkedHashMap的插入顺序和遍历顺序是一样的,但是我们通过改变accessOrder值为true

3. 如何全局设置Glide的加载配置,并且他的原理是什么,为什么可以设置一个全局的加载配置

在Glide里,配置信息都是写入BaseRequesOptions里面.我们每次发起请求通过Glide.with(context)先获取当前页面的RequestManager,然后在调用load创建RequestBuilder对象,这个对象是每次调用load都会创建一个新的.我们可以看到RequestBuilder是继承BaseRequestOptions的.看下对象创建的代码:

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class RequestManager{
    public RequestBuilder<Drawable> load(@Nullable String string) {
      return asDrawable().load(string);
    }
    public RequestBuilder<Drawable> asDrawable() {
      return as(Drawable.class);
    }
    public <ResourceType> RequestBuilder<ResourceType> as(
        @NonNull Class<ResourceType> resourceClass) {
      return new RequestBuilder<>(glide, this, resourceClass, context);
    }
}
class RquestBuilder{
    protected RequestBuilder(
        @NonNull Glide glide,
        RequestManager requestManager,
        Class<TranscodeType> transcodeClass,
        Context context) {
      ----
      apply(requestManager.getDefaultRequestOptions());
    }
}

其他都是浮云.重点是RequestBuilder的构造函数里面的apply方法.

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public T apply(@NonNull BaseRequestOptions<?> o) {
  BaseRequestOptions<?> other = o;

  if (isSet(other.fields, SIZE_MULTIPLIER)) {
    sizeMultiplier = other.sizeMultiplier;
  }
  if (isSet(other.fields, USE_UNLIMITED_SOURCE_GENERATORS_POOL)) {
    useUnlimitedSourceGeneratorsPool = other.useUnlimitedSourceGeneratorsPool;
  }
  if (isSet(other.fields, USE_ANIMATION_POOL)) {
    useAnimationPool = other.useAnimationPool;
  }
  if (isSet(other.fields, DISK_CACHE_STRATEGY)) {
    diskCacheStrategy = other.diskCacheStrategy;
  }
  if (isSet(other.fields, PRIORITY)) {
    priority = other.priority;
  }
  if (isSet(other.fields, ERROR_PLACEHOLDER)) {
    errorPlaceholder = other.errorPlaceholder;
    errorId = 0;
    fields &= ~ERROR_ID;
  }
  if (isSet(other.fields, ERROR_ID)) {
    errorId = other.errorId;
    errorPlaceholder = null;
    fields &= ~ERROR_PLACEHOLDER;
  }
  if (isSet(other.fields, PLACEHOLDER)) {
    placeholderDrawable = other.placeholderDrawable;
    placeholderId = 0;
    fields &= ~PLACEHOLDER_ID;
  }
  if (isSet(other.fields, PLACEHOLDER_ID)) {
    placeholderId = other.placeholderId;
    placeholderDrawable = null;
    fields &= ~PLACEHOLDER;
  }
  if (isSet(other.fields, IS_CACHEABLE)) {
    isCacheable = other.isCacheable;
  }
  if (isSet(other.fields, OVERRIDE)) {
    overrideWidth = other.overrideWidth;
    overrideHeight = other.overrideHeight;
  }
  if (isSet(other.fields, SIGNATURE)) {
    signature = other.signature;
  }
  if (isSet(other.fields, RESOURCE_CLASS)) {
    resourceClass = other.resourceClass;
  }
  if (isSet(other.fields, FALLBACK)) {
    fallbackDrawable = other.fallbackDrawable;
    fallbackId = 0;
    fields &= ~FALLBACK_ID;
  }
  if (isSet(other.fields, FALLBACK_ID)) {
    fallbackId = other.fallbackId;
    fallbackDrawable = null;
    fields &= ~FALLBACK;
  }
  if (isSet(other.fields, THEME)) {
    theme = other.theme;
  }
  if (isSet(other.fields, TRANSFORMATION_ALLOWED)) {
    isTransformationAllowed = other.isTransformationAllowed;
  }
  if (isSet(other.fields, TRANSFORMATION_REQUIRED)) {
    isTransformationRequired = other.isTransformationRequired;
  }
  if (isSet(other.fields, TRANSFORMATION)) {
    transformations.putAll(other.transformations);
    isScaleOnlyOrNoTransform = other.isScaleOnlyOrNoTransform;
  }
  if (isSet(other.fields, ONLY_RETRIEVE_FROM_CACHE)) {
    onlyRetrieveFromCache = other.onlyRetrieveFromCache;
  }

  // Applying options with dontTransform() is expected to clear our transformations.
  if (!isTransformationAllowed) {
    transformations.clear();
    fields &= ~TRANSFORMATION;
    isTransformationRequired = false;
    fields &= ~TRANSFORMATION_REQUIRED;
    isScaleOnlyOrNoTransform = true;
  }

  fields |= other.fields;
  options.putAll(other.options);

  return selfOrThrowIfLocked();
}

可以看到apply方法就是一个拷贝的操作,他将requestManager.getDefaultRequestOptions()默认配置赋值给了新的RequestBuilder.所以我们可以操作的点出来了.
我们通过修改getDefultRequestoptions就可以实现修改全局的GLide加载属性了. 这里我拿修改Glide的编码格式为DecodeFormat.PREFER_RGB_565来举例.(应用场景:在手机内存太小的手机上,用来减少图片的内存占用).

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@GlideModule
class MyGlideModule : AppGlideModule(){
    override fun applyOptions(context: Context, builder: GlideBuilder) {
        super.applyOptions(context, builder)
        val activityManager:ActivityManager = context.getSystemService(Context.ALARM_SERVICE) as ActivityManager
        if(activityManager!=null){
            var memoryInfo = ActivityManager.MemoryInfo()
            activityManager.getMemoryInfo(memoryInfo)
            builder.setDefaultRequestOptions(RequestOptions().format(DecodeFormat.PREFER_RGB_565))
        }
    }
}
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<meta-data
    android:name="com.adventure.live.MyGlideModule"
    android:value="GlideModule" />

实现方法就是继承AppGlideModule重写applyOptions方法.