android --Resource

大家都知道android 里面引用资源，我们用R.drawable.xx就可以引用到我们想要的资源，其实我们都不知道，在这简单的，有系统的努力在支持着。其实大家可以想到我们想要获取一个drawable图片资源的时候也可以这样：Resource res=getResource();

res.getDrawable(R.drawble.icon);

下面我们来学习一下Resource类。继续上面所说的，查找资源我们想看一下getDrawable的函数原型

public Drawable getDrawable(int id) throws NotFoundException {
        synchronized (mTmpValue) {
            TypedValue value = mTmpValue;
            getValue(id, value, true);
            return loadDrawable(value, id);
        }
    }

就是通过一个资源的id返回一个drawable。我们继续向下看。

Drawable.ConstantState cs = sPreloadedDrawables.get(key);
        if (cs != null) {
            dr = cs.newDrawable(this);
        } else {
            if (value.type >= TypedValue.TYPE_FIRST_COLOR_INT &&
                    value.type <= TypedValue.TYPE_LAST_COLOR_INT) {
                dr = new ColorDrawable(value.data);
            }

            if (dr == null) {
                if (value.string == null) {
                    throw new NotFoundException(
                            "Resource is not a Drawable (color or path): " + value);
                }

                String file = value.string.toString();

                if (DEBUG_LOAD) Log.v(TAG, "Loading drawable for cookie "
                        + value.assetCookie + ": " + file);

                if (file.endsWith(".xml")) {
                    try {
                        XmlResourceParser rp = loadXmlResourceParser(
                                file, id, value.assetCookie, "drawable");
                        dr = Drawable.createFromXml(this, rp);
                        rp.close();
                    } catch (Exception e) {
                        NotFoundException rnf = new NotFoundException(
                            "File " + file + " from drawable resource ID #0x"
                            + Integer.toHexString(id));
                        rnf.initCause(e);
                        throw rnf;
                    }

                } else {
                    try {
                        InputStream is = mAssets.openNonAsset(
                                value.assetCookie, file, AssetManager.ACCESS_STREAMING);
        //                System.out.println("Opened file " + file + ": " + is);
                        dr = Drawable.createFromResourceStream(this, value, is,
                                file, null);
                        is.close();
        //                System.out.println("Created stream: " + dr);
                    } catch (Exception e) {
                        NotFoundException rnf = new NotFoundException(
                            "File " + file + " from drawable resource ID #0x"
                            + Integer.toHexString(id));
                        rnf.initCause(e);
                        throw rnf;
                    }
                }
            }
        }

这里会去查找value的值，最终通过AssetManager读取我们安装文件里面的文件。

到这里大家都应该明白了吧！在我们最简单的引用背后，系统是如何帮我们查找资源的，其实还是通过流的形式，大家可能会问那这么R.drawable.xx有什么关系，其实到Drawable还有一个转换的过程，大家自己去查找看吧。

下一次讲解关于横竖屏切换实现原理，也是与Resource类有关的。