Glide之旅 —— DecodeJob

前言

glide是谷歌推荐的Android图片加载框架，其优秀的缓存策略、Activity的生命周期的继承、GIF图片的支持等都是为人所称道的地方。下面是用glide加载一张图片的调用。

private void loadImage() {
    Glide.with(this)
         .load("http://pic2.orsoon.com/2017/0118/20170118011032176.jpg")
         .into(ivTest);
}

那么，该框架是如何实际运作的呢，我会通过“Glide之旅”系列博客尽可能详细地将我的心得记录下来。“Glide之旅”系列文章汇总：

• Glide之旅 —— Registry

• Glide之旅 —— DecodeJob

概述

EngineJob(com.bumptech.glide.load.engine.DecodeJob)是整个glide开始进行图片编解码、缓存等一系列的动作的发动机。

源码分析

首先明确一点，在DecodeJob中初始化变量的值是来源于 com.bumptech.glide.request.SingleRequest：

package com.bumptech.glide.request;

...

public final class SingleRequest<R> implements Request,
        SizeReadyCallback,
        ResourceCallback,
        FactoryPools.Poolable {

    ...

    @Override
    public void onSizeReady(int width, int height) {

        ...

        loadStatus = engine.load(
                glideContext,
                model,
                requestOptions.getSignature(),
                this.width,
                this.height,
                requestOptions.getResourceClass(),
                transcodeClass,
                priority,
                requestOptions.getDiskCacheStrategy(),
                requestOptions.getTransformations(),
                requestOptions.isTransformationRequired(),
                requestOptions.getOptions(),
                requestOptions.isMemoryCacheable(),
                requestOptions.getUseUnlimitedSourceGeneratorsPool(),
                requestOptions.getOnlyRetrieveFromCache(),
                this);

        ...

    }

    ...

}

先来看看执行解码操作的入口函数：

package com.bumptech.glide.load.engine;

...

class DecodeJob implements DataFetcherGenerator.FetcherReadyCallback,
        Runnable, Comparable>, Poolable {

    ...

    @Override
    public void run() {
        try {
            if (isCancelled) {
                notifyFailed();
                return;
            }
            runWrapped();
        } catch (RuntimeException e) {
            if (Log.isLoggable(TAG, Log.DEBUG)) {
                Log.d(TAG, "DecodeJob threw unexpectedly"
                        + ", isCancelled: " + isCancelled
                        + ", stage: " + stage, e);
            }
            if (stage != Stage.ENCODE) {
                notifyFailed();
            }
            if (!isCancelled) {
                throw e;
            }
        }
    }

    ...

}

也就是说，一般情况下，直接跳转到runWrapped()中去：

package com.bumptech.glide.load.engine;

...

class DecodeJob implements DataFetcherGenerator.FetcherReadyCallback,
        Runnable, Comparable>, Poolable {

    ...

    private void runWrapped() {
        switch (runReason) {
            case INITIALIZE:
                stage = getNextStage(Stage.INITIALIZE);
                currentGenerator = getNextGenerator();
                runGenerators();
                break;
            case SWITCH_TO_SOURCE_SERVICE:
                runGenerators();
                break;
            case DECODE_DATA:
                decodeFromRetrievedData();
                break;
            default:
                throw new IllegalStateException("Unrecognized run reason: " + runReason);
        }
    }

    ...

}

runReason在初始状态下设定为INITIALIZE，这个时候再看一下getNextGenerator(Stage.INITIALIZE)和getNextGenerator()：

package com.bumptech.glide.load.engine;

...

class DecodeJob implements DataFetcherGenerator.FetcherReadyCallback,
        Runnable, Comparable>, Poolable {

    ...

    private DataFetcherGenerator getNextGenerator() {
        switch (stage) {
            case RESOURCE_CACHE:
                return new ResourceCacheGenerator(decodeHelper, this);
            case DATA_CACHE:
                return new DataCacheGenerator(decodeHelper, this);
            case SOURCE:
                return new SourceGenerator(decodeHelper, this);
            case FINISHED:
                return null;
            default:
                throw new IllegalStateException("Unrecognized stage: " + stage);
        }
    }

    ...

    private Stage getNextStage(Stage current) {
        switch (current) {
            case INITIALIZE:
                return diskCacheStrategy.decodeCachedResource()
                        ? Stage.RESOURCE_CACHE : getNextStage(Stage.RESOURCE_CACHE);
            case RESOURCE_CACHE:
                return diskCacheStrategy.decodeCachedData()
                        ? Stage.DATA_CACHE : getNextStage(Stage.DATA_CACHE);
            case DATA_CACHE:
                return onlyRetrieveFromCache ? Stage.FINISHED : Stage.SOURCE;
            case SOURCE:
            case FINISHED:
                return Stage.FINISHED;
            default:
                throw new IllegalArgumentException("Unrecognized stage: " + current);
        }
    }

    ...

}

由于默认情况下，diskCacheStrategy的值为DiskCacheStrategy.AUTOMATIC：

package com.bumptech.glide.load.engine;

...

public abstract class DiskCacheStrategy {

    ...

    public static final DiskCacheStrategy AUTOMATIC = new DiskCacheStrategy() {
        @Override
        public boolean isDataCacheable(DataSource dataSource) {
            return dataSource == DataSource.REMOTE;
        }

        @Override
        public boolean isResourceCacheable(boolean isFromAlternateCacheKey, DataSource dataSource,
                                        EncodeStrategy encodeStrategy) {
            return ((isFromAlternateCacheKey && dataSource == DataSource.DATA_DISK_CACHE)
                    || dataSource == DataSource.LOCAL)
                    && encodeStrategy == EncodeStrategy.TRANSFORMED;
        }

        @Override
        public boolean decodeCachedResource() {
            return true;
        }

        @Override
        public boolean decodeCachedData() {
            return true;
        }
    };

    ...

}

由上面可以分析出，在runReason为INITIALIZE的情况下，stage此时为RESOURCE_CACHE，currentGenerator为ResourceCacheGenerator的实例，再回到runWrapped()中去，接下来调用runGenerators()：

package com.bumptech.glide.load.engine;

...

class DecodeJob implements DataFetcherGenerator.FetcherReadyCallback,
        Runnable, Comparable>, Poolable {

    ...

    private void runGenerators() {
        currentThread = Thread.currentThread();
        startFetchTime = LogTime.getLogTime();
        boolean isStarted = false;
        while (!isCancelled && currentGenerator != null
                && !(isStarted = currentGenerator.startNext())) {
            stage = getNextStage(stage);
            currentGenerator = getNextGenerator();

            if (stage == Stage.SOURCE) {
                reschedule();
                return;
            }
        }
        if ((stage == Stage.FINISHED || isCancelled) && !isStarted) {
            notifyFailed();
        }
    }

    ...

}

从这个函数源码看出，完整执行的情况下，会依次调用ResourceCacheGenerator、DataCacheGenerator和SourceGenerator中的startNext()，接下来，分别来看看三个DataFetcherGenerator的实现类是怎么进行的：

package com.bumptech.glide.load.engine;

...

class ResourceCacheGenerator implements DataFetcherGenerator, DataFetcher.DataCallback