有些时候需要将res中的资源文件加载进来然后做一些修改，然后再赋值给控件使用，比如这样：

Drawable drawable = getResources().getDrawable(int id);

现在我需要把资源文件id所对应的Drawable对象加载进来，然后做向下转型处理，如果我不调试查看本身的类型的话，该怎么知道它到底是哪种类型呢？我们来通过源代码查看究竟：

首先我们找到getResources().getDrawable()方法：

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

这个方法并没有做什么，进入到loadDrawable方法中：

/*package*/ Drawable loadDrawable(TypedValue value, int id)            throws NotFoundException {        if (TRACE_FOR_PRELOAD) {            // Log only framework resources            if ((id >>> 24) == 0x1) {                final String name = getResourceName(id);                if (name != null) android.util.Log.d("PreloadDrawable", name);            }        }        final long key = (((long) value.assetCookie) << 32) | value.data;        Drawable dr = getCachedDrawable(key);        if (dr != null) {            return dr;        }        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;                    }                }            }        }        if (dr != null) {            dr.setChangingConfigurations(value.changingConfigurations);            cs = dr.getConstantState();            if (cs != null) {                if (mPreloading) {                    sPreloadedDrawables.put(key, cs);                } else {                    synchronized (mTmpValue) {                        //Log.i(TAG, "Saving cached drawable @ #" +                        //        Integer.toHexString(key.intValue())                        //        + " in " + this + ": " + cs);                        mDrawableCache.put(key, new WeakReference
    
     (cs));                    }                }            }        }        return dr;    }
    

这一大段我们看主要的：

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;                    }                }

这里以selector说明，selector是定义在xml中的，我们就找到这个地方，它原来是使用XmlResourceParser来对xml资源文件进行解析的，咱们进去createFromXml方法内部一探究竟：

/**     * Create a drawable from an XML document. For more information on how to     * create resources in XML, see     * Drawable Resources.     */    public static Drawable createFromXml(Resources r, XmlPullParser parser)            throws XmlPullParserException, IOException {        AttributeSet attrs = Xml.asAttributeSet(parser);        int type;        while ((type=parser.next()) != XmlPullParser.START_TAG &&                type != XmlPullParser.END_DOCUMENT) {            // Empty loop        }        if (type != XmlPullParser.START_TAG) {            throw new XmlPullParserException("No start tag found");        }        Drawable drawable = createFromXmlInner(r, parser, attrs);        if (drawable == null) {            throw new RuntimeException("Unknown initial tag: " + parser.getName());        }        return drawable;    }

从这段代码中我们可以发现，最主要的工作是由方法createFromXmlInner来完成的，我们进去看看：

/**     * Create from inside an XML document.  Called on a parser positioned at     * a tag in an XML document, tries to create a Drawable from that tag.     * Returns null if the tag is not a valid drawable.     */    public static Drawable createFromXmlInner(Resources r, XmlPullParser parser, AttributeSet attrs)    throws XmlPullParserException, IOException {        Drawable drawable;        final String name = parser.getName();        if (name.equals("selector")) {            drawable = new StateListDrawable();        } else if (name.equals("level-list")) {            drawable = new LevelListDrawable();        } else if (name.equals("layer-list")) {            drawable = new LayerDrawable();        } else if (name.equals("transition")) {            drawable = new TransitionDrawable();        } else if (name.equals("color")) {            drawable = new ColorDrawable();        } else if (name.equals("shape")) {            drawable = new GradientDrawable();        } else if (name.equals("scale")) {            drawable = new ScaleDrawable();        } else if (name.equals("clip")) {            drawable = new ClipDrawable();        } else if (name.equals("rotate")) {            drawable = new RotateDrawable();        } else if (name.equals("animated-rotate")) {            drawable = new AnimatedRotateDrawable();                    } else if (name.equals("animation-list")) {            drawable = new AnimationDrawable();        } else if (name.equals("inset")) {            drawable = new InsetDrawable();        } else if (name.equals("bitmap")) {            drawable = new BitmapDrawable();            if (r != null) {               ((BitmapDrawable) drawable).setTargetDensity(r.getDisplayMetrics());            }        } else if (name.equals("nine-patch")) {            drawable = new NinePatchDrawable();            if (r != null) {                ((NinePatchDrawable) drawable).setTargetDensity(r.getDisplayMetrics());             }        } else {            throw new XmlPullParserException(parser.getPositionDescription() +                    ": invalid drawable tag " + name);        }        drawable.inflate(r, parser, attrs);        return drawable;    }

噢噢，原来是这样啊，我定义什么样类型的图片，最后就有什么样的Java类型与之对应，原来我的selector对应StateListDrawable，恍然大悟，接下来就容易处理了。