Android中图片占用内存的深入分析

    override fun initData() {
        val options = BitmapFactory.Options()
        val bitmap = BitmapFactory.decodeResource(resources, R.drawable.test_xxhdpi, options)
        ShowLogUtil.info("width: ${options.outWidth},height: ${options.outHeight},config: ${options.inPreferredConfig},占用内存: ${bitmap.allocationByteCount}")
        options.inSampleSize = 2
        val bitmap2 = BitmapFactory.decodeResource(resources, R.drawable.test_xxhdpi, options)
        ShowLogUtil.info("width: ${options.outWidth},height: ${options.outHeight},config: ${options.inPreferredConfig},占用内存: ${bitmap2.allocationByteCount}")
        options.inPreferredConfig = Bitmap.Config.RGB_565
        val bitmap3 = BitmapFactory.decodeResource(resources, R.drawable.test_xxhdpi, options)
        ShowLogUtil.info("width: ${options.outWidth},height: ${options.outHeight},config: ${options.inPreferredConfig},占用内存: ${bitmap3.allocationByteCount}")
    }

    override fun initData() {
        val options1 = BitmapFactory.Options()
        val bitmap = BitmapFactory.decodeResource(resources, R.drawable.test_xxhdpi, options1)
        ShowLogUtil.info(
            "width: ${options1.outWidth},height: ${options1.outHeight},config: ${options1.inPreferredConfig},占用内存: ${bitmap.allocationByteCount},inDensity: ${options1.inDensity},inTargetDensity: ${options1.inTargetDensity}"
        )
        val options2 = BitmapFactory.Options()
        val bitmap2 = BitmapFactory.decodeResource(resources, R.drawable.test_xhdpi, options2)
        ShowLogUtil.info(
            "width: ${options2.outWidth},height: ${options2.outHeight},config: ${options2.inPreferredConfig},占用内存: ${bitmap2.allocationByteCount},inDensity: ${options2.inDensity},inTargetDensity: ${options2.inTargetDensity}"
        )
        val options3 = BitmapFactory.Options()
        val bitmap3 = BitmapFactory.decodeResource(resources, R.drawable.test_hdpi, options3)
        ShowLogUtil.info(
            "width: ${options3.outWidth},height: ${options3.outHeight},config: ${options3.inPreferredConfig},占用内存: ${bitmap3.allocationByteCount},inDensity: ${options3.inDensity},inTargetDensity: ${options3.inTargetDensity}"
        )
    }

    public static Bitmap decodeResource(Resources res, int id, Options opts) {
        validate(opts);
        Bitmap bm = null;
        InputStream is = null; 
        
        try {
            final TypedValue value = new TypedValue();
            // 打开资源流，并初始化一些属性。
            is = res.openRawResource(id, value);
 
            // 解析图片资源。
            bm = decodeResourceStream(res, value, is, null, opts);
        } catch (Exception e) {
            /*  do nothing.
                If the exception happened on open, bm will be null.
                If it happened on close, bm is still valid.
            */
        } finally {
            try {
                if (is != null) is.close();
            } catch (IOException e) {
                // Ignore
            }
        }
 
        if (bm == null && opts != null && opts.inBitmap != null) {
            throw new IllegalArgumentException("Problem decoding into existing bitmap");
        }
 
        return bm;
    }

    @Nullable
    public static Bitmap decodeResourceStream(@Nullable Resources res, @Nullable TypedValue value,
            @Nullable InputStream is, @Nullable Rect pad, @Nullable Options opts) {
        validate(opts);
        // 检查 Options 对象是否为空，为空则创建一个默认对象。
        if (opts == null) {
            opts = new Options();
        }
        // 查看 Options 对象是否设置了 inDensity，这里是默认的，所以没有设置，TypedValue 对象在上面的 decodeResource() 方法中也是创建的一个默认对象，所以不为 null，必然进这个 if 代码块。
        if (opts.inDensity == 0 && value != null) {
            final int density = value.density;
            if (density == TypedValue.DENSITY_DEFAULT) {
                opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
            } else if (density != TypedValue.DENSITY_NONE) {
                // TypedValue 对象在上面的 decodeResource() 方法中调用 Resources.openRawResource() 方法的时候 density 会赋值成对应的资源文件所在目录的 density 值，所以会走到这里，给 Options 的 inDensity 属性赋值。
                opts.inDensity = density;
            }
        }
        
        // Options 的 inTargetDensity 默认没有赋值，所以会进 if 代码块，赋值为手机屏幕的 densityDpi。
        if (opts.inTargetDensity == 0 && res != null) {
            opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
        }
        
        return decodeStream(is, pad, opts);
    }

    @Nullable
    public static Bitmap decodeStream(@Nullable InputStream is, @Nullable Rect outPadding,
            @Nullable Options opts) {
        ...
        try {
            if (is instanceof AssetManager.AssetInputStream) {
                final long asset = ((AssetManager.AssetInputStream) is).getNativeAsset();
                bm = nativeDecodeAsset(asset, outPadding, opts, Options.nativeInBitmap(opts),
                    Options.nativeColorSpace(opts));
            } else {
                // 这里的资源并非从 assets 目录中加载，所以进入 else 代码块。
                bm = decodeStreamInternal(is, outPadding, opts);
            }
        ...
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_GRAPHICS);
        }
        return bm;
    }

    private static Bitmap decodeStreamInternal(@NonNull InputStream is,
            @Nullable Rect outPadding, @Nullable Options opts) {
        // ASSERT(is != null);
        byte [] tempStorage = null;
        if (opts != null) tempStorage = opts.inTempStorage;
        if (tempStorage == null) tempStorage = new byte[DECODE_BUFFER_SIZE];
        return nativeDecodeStream(is, tempStorage, outPadding, opts,
                Options.nativeInBitmap(opts),
                Options.nativeColorSpace(opts));
    }

static jobject nativeDecodeStream(JNIEnv* env, jobject clazz, jobject is, jbyteArray storage,
        jobject padding, jobject options) {
 
    jobject bitmap = NULL;
    std::unique_ptr<SkStream> stream(CreateJavaInputStreamAdaptor(env, is, storage));
 
    if (stream.get()) {
        std::unique_ptr<SkStreamRewindable> bufferedStream(
                SkFrontBufferedStream::Make(std::move(stream), SkCodec::MinBufferedBytesNeeded()));
        SkASSERT(bufferedStream.get() != NULL);
        bitmap = doDecode(env, std::move(bufferedStream), padding, options);
    }
    return bitmap;
}

static jobject doDecode(JNIEnv* env, std::unique_ptr<SkStreamRewindable> stream,
                        jobject padding, jobject options) {
    // Set default values for the options parameters.
    ...
    // 初始化缩放比为 1.0
    float scale = 1.0f;
    ...
    if (options != NULL) {
        ...
        // 获取 java 中的 Options 对象中的 density，targetDensity，计算出缩放比，两者都是在 java 代码中的 decodeResourceStream() 方法赋值的。
        if (env->GetBooleanField(options, gOptions_scaledFieldID)) {
            const int density = env->GetIntField(options, gOptions_densityFieldID);
            const int targetDensity = env->GetIntField(options, gOptions_targetDensityFieldID);
            const int screenDensity = env->GetIntField(options, gOptions_screenDensityFieldID);
            // 如加载的是 xhdpi 中的图片则 inDensity 为 320，使用的测试机分辨率为 1920*1080，则 targetDensity 为 480，所以 scale 为 480/320=1.5
            if (density != 0 && targetDensity != 0 && density != screenDensity) {
                scale = (float) targetDensity / density;
            }
        }
    }
    ...
    int scaledWidth = size.width();
    int scaledHeight = size.height();
    ...
    // Scale is necessary due to density differences.
    if (scale != 1.0f) {
		// 需要缩放的话，计算缩放后的宽高。宽高分别乘以缩放比 scale。
        willScale = true;
        scaledWidth = static_cast<int>(scaledWidth * scale + 0.5f);
        scaledHeight = static_cast<int>(scaledHeight * scale + 0.5f);
    }
    ...
	if (willScale) {
        ...
        outputBitmap.setInfo(
                bitmapInfo.makeWH(scaledWidth, scaledHeight).makeColorType(scaledColorType));
		...
    } else {
        outputBitmap.swap(decodingBitmap);
    }
	...
    // now create the java bitmap
    return bitmap::createBitmap(env, defaultAllocator.getStorageObjAndReset(),
            bitmapCreateFlags, ninePatchChunk, ninePatchInsets, -1);
}