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<Drawable.ConstantState>(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
* <a href="{@docRoot}guide/topics/resources/drawable-resource.html">Drawable Resources</a>.
*/
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。