Android四大图片缓存框架之-Fresco之initialize（二）

我们今天来了解下Fresco的初始化过程。
以系统默认的初始化参数为例。

Fresco.initialize(getApplicationContext());

进入到 com.facebook.drawee.backends.pipeline.Fresco中

public static void initialize(Context context) {
        ImagePipelineFactory.initialize(context);
        initializeDrawee(context);
    }

我们先看ImagePipelineFactory.initialize(context);方法

public static void initialize(Context context) {
        initialize(ImagePipelineConfig.newBuilder(context).build());
    }

我们继续看ImagePipelineConfig.newBuilder(context)方法，

public static ImagePipelineConfig.Builder newBuilder(Context context) {
        return new ImagePipelineConfig.Builder(context, null);
    }

这里调用了ImagePipelineCongif的一个内部类Builder。

private Builder(Context context) {
            this.mDownsampleEnabled = false;
            this.mResizeAndRotateEnabledForNetwork = true;
            this.mContext = (Context)Preconditions.checkNotNull(context);
        }

这里只初始化了3个参数。其他的一些相关的参数并没有在这里进行初始化，那么，其他的一些设置是怎么实现的呢。别着急，我们接下来看
ImagePipelineConfig.newBuilder(context).build()的build()方法。

public ImagePipelineConfig build() {
            return new ImagePipelineConfig(this, null);
        }

这里返回了ImagePipelineConfig的一个实例。我们看下他的构造函数吧。

private ImagePipelineConfig(ImagePipelineConfig.Builder builder) {
        this.mAnimatedImageFactory = builder.mAnimatedImageFactory;
        this.mBitmapMemoryCacheParamsSupplier = (Supplier)(builder.mBitmapMemoryCacheParamsSupplier == null?new DefaultBitmapMemoryCacheParamsSupplier((ActivityManager)builder.mContext.getSystemService("activity")):builder.mBitmapMemoryCacheParamsSupplier);
        this.mCacheKeyFactory = (CacheKeyFactory)(builder.mCacheKeyFactory == null?DefaultCacheKeyFactory.getInstance():builder.mCacheKeyFactory);
        this.mContext = (Context)Preconditions.checkNotNull(builder.mContext);
        this.mDownsampleEnabled = builder.mDownsampleEnabled;
        this.mEncodedMemoryCacheParamsSupplier = (Supplier)(builder.mEncodedMemoryCacheParamsSupplier == null?new DefaultEncodedMemoryCacheParamsSupplier():builder.mEncodedMemoryCacheParamsSupplier);
        this.mImageCacheStatsTracker = (ImageCacheStatsTracker)(builder.mImageCacheStatsTracker == null?NoOpImageCacheStatsTracker.getInstance():builder.mImageCacheStatsTracker);
        this.mImageDecoder = builder.mImageDecoder;
        this.mIsPrefetchEnabledSupplier = builder.mIsPrefetchEnabledSupplier == null?new Supplier() {
            public Boolean get() {
                return Boolean.valueOf(true);
            }
        }:builder.mIsPrefetchEnabledSupplier;
        this.mMainDiskCacheConfig = builder.mMainDiskCacheConfig == null?getDefaultMainDiskCacheConfig(builder.mContext):builder.mMainDiskCacheConfig;
        this.mMemoryTrimmableRegistry = (MemoryTrimmableRegistry)(builder.mMemoryTrimmableRegistry == null?NoOpMemoryTrimmableRegistry.getInstance():builder.mMemoryTrimmableRegistry);
        this.mNetworkFetcher = (NetworkFetcher)(builder.mNetworkFetcher == null?new HttpUrlConnectionNetworkFetcher():builder.mNetworkFetcher);
        this.mPlatformBitmapFactory = builder.mPlatformBitmapFactory;
        this.mPoolFactory = builder.mPoolFactory == null?new PoolFactory(PoolConfig.newBuilder().build()):builder.mPoolFactory;
        this.mProgressiveJpegConfig = (ProgressiveJpegConfig)(builder.mProgressiveJpegConfig == null?new SimpleProgressiveJpegConfig():builder.mProgressiveJpegConfig);
        this.mRequestListeners = (Set)(builder.mRequestListeners == null?new HashSet():builder.mRequestListeners);
        this.mResizeAndRotateEnabledForNetwork = builder.mResizeAndRotateEnabledForNetwork;
        this.mSmallImageDiskCacheConfig = builder.mSmallImageDiskCacheConfig == null?this.mMainDiskCacheConfig:builder.mSmallImageDiskCacheConfig;
        int decodeThreads = this.mPoolFactory.getFlexByteArrayPoolMaxNumThreads();
        this.mExecutorSupplier = (ExecutorSupplier)(builder.mExecutorSupplier == null?new DefaultExecutorSupplier():builder.mExecutorSupplier);
    }

看到这里，就明白了，构造函数中根据判断ImagePipelineConfig.Builder对象的成员变量是否为空来初始化，不为空，则用我们设置好的，为空，那么就用系统的。我们以mBitmapMemoryCacheParamsSupplier为例。

public class DefaultBitmapMemoryCacheParamsSupplier implements Supplier<MemoryCacheParams> {
    private static final int MAX_CACHE_ENTRIES = 256;
    private static final int MAX_EVICTION_QUEUE_SIZE = 2147483647;
    private static final int MAX_EVICTION_QUEUE_ENTRIES = 2147483647;
    private static final int MAX_CACHE_ENTRY_SIZE = 2147483647;
    private final ActivityManager mActivityManager;

    public DefaultBitmapMemoryCacheParamsSupplier(ActivityManager activityManager) {
        this.mActivityManager = activityManager;
    }

    public MemoryCacheParams get() {
        return new MemoryCacheParams(this.getMaxCacheSize(), 256, 2147483647, 2147483647, 2147483647);
    }

    private int getMaxCacheSize() {
        int maxMemory = Math.min(this.mActivityManager.getMemoryClass() * 1048576, 2147483647);
        return maxMemory < 33554432?4194304:(maxMemory < 67108864?6291456:(VERSION.SDK_INT <= 9?8388608:maxMemory / 4));
    }
}

看到这些敏感的数字就知道，这里就是配置LruCache大小的地方了。(猜测)。我想，这里会在某个地方调用get方法来获取系统设置的参数。接着看下这些参数的意思。

public MemoryCacheParams(int maxCacheSize, int maxCacheEntries, int maxEvictionQueueSize, int maxEvictionQueueEntries, int maxCacheEntrySize) {
        this.maxCacheSize = maxCacheSize;
        this.maxCacheEntries = maxCacheEntries;
        this.maxEvictionQueueSize = maxEvictionQueueSize;
        this.maxEvictionQueueEntries = maxEvictionQueueEntries;
        this.maxCacheEntrySize = maxCacheEntrySize;
    }

• 内存缓存的最大Size
• 缓存的最大条目，应该就是缓存图片的最大数目
• 驱逐队列的Size，(以下是我猜测的内容，有待验证)，驱逐队列指的是重Lrucache中淘汰下来的图片，但是近期可能会用到的，暂时存放在这里。
• 驱逐队列的数目
• 单个缓存条目的最大大小

以上纯属个人意见，如有错误，请及时更正。