Glide模块工作原理

每个Glide模块包含一个Decoder，和一个Transcoder

Decoder负责将DataSource转换为对应的Resource

Transcoder负责将对应的Resource，转换为Drawable

GlideModule示例

这里以SVG为例，展示下GlideModule是如何定义的

class SvgDecoder : ResourceDecoder<InputStream, SVG> {

    override fun handles(source: InputStream, options: Options): Boolean {
        return true
    }

    override fun decode(
        source: InputStream, width: Int, height: Int, options: Options
    ): Resource<SVG>? {
        try {
            val svg = SVG.getFromInputStream(source)
            if (width != Target.SIZE_ORIGINAL)
                svg.documentWidth = width.toFloat()
            if (height != Target.SIZE_ORIGINAL)
                svg.documentHeight = height.toFloat()
            return SimpleResource(svg)
        } catch (e: Throwable) {
            return null
        }
    }
}

class SvgTranscoder : ResourceTranscoder<SVG, PictureDrawable> {

    override fun transcode(
        toTranscode: Resource<SVG>, options: Options
    ): Resource<PictureDrawable> {
        val svg = toTranscode.get()
        val picture = svg.renderToPicture()
        val drawable = PictureDrawable(picture)
        return SimpleResource(drawable)
    }
}

@GlideModule
class SvgLibraryGlideModule : LibraryGlideModule() {

    override fun registerComponents(context: Context, glide: Glide, registry: Registry) {
        registry.register(SVG::class.java, PictureDrawable::class.java, SvgTranscoder())
        registry.append(InputStream::class.java, SVG::class.java, SvgDecoder())
    }
}

Decode流程

Glide的加载原理，是采用HitTest的方法

轮询所有注册的Decoder，挨个尝试解码DataSource

如果解码成功，则视为该Decoder对应的Resource

再找到Decoder对应的Transcoder，将对应的Resource转为Drawable

这样，便可以将对应资源加载到ImageView当中了

class DecodePath {

    private Resource<ResourceType> decodeResourceWithList(
            DataRewinder<DataType> rewinder,
            int width,
            int height,
            Options options,
            List<Throwable> exceptions
    ) {
        Resource<ResourceType> result = null;
        // hit test decoder
        for (int i = 0, size = decoders.size(); i < size; i++) {
            ResourceDecoder<DataType, ResourceType> decoder = decoders.get(i);
            try {
                DataType data = rewinder.rewindAndGet();
                if (decoder.handles(data, options)) {
                    data = rewinder.rewindAndGet();
                    result = decoder.decode(data, width, height, options);
                }
            } catch (Throwable e) {
                Log.v(TAG, "Failed to decode data for " + decoder, e);
                exceptions.add(e);
            }
            if (result != null) {
                break;
            }
        }
        // hit nothing
        if (result == null) {
            throw new GlideException(failureMessage, new ArrayList<>(exceptions));
        }
        return result;
    }
}

Glide日志分析

以下报错日志可以看出来，Glide逐个尝试了每个Decoder

以及每个Decoder的解码过程，如果成功，最终都将解出Drawable

Cause (1 of 4): class com.bumptech.glide.load.engine.GlideException: Failed DecodePath{FileInputStream->GifDrawable->Drawable}
Cause (2 of 4): class com.bumptech.glide.load.engine.GlideException: Failed DecodePath{FileInputStream->Bitmap->Drawable}
Cause (3 of 4): class com.bumptech.glide.load.engine.GlideException: Failed DecodePath{FileInputStream->BitmapDrawable->Drawable}
Cause (4 of 4): class com.bumptech.glide.load.engine.GlideException: Failed DecodePath{FileInputStream->SVG->Drawable}