原文出处：http://www.ccbu.cc/android/android-drawable
Android系统中将可绘制对象被抽象为Drawable,不同的绘制资源对应着不同的Drawable类型。Android FrameWork提供了常用的Drawable，Android控件的绘制资源基本都是通过Drawable形式实现的。一般情况下，开发者是不会直接接触Drawable的具体实现的，Drawable资源一般都放在res/drawable目录下，用户通过图片，xml格式的Drawable资源来使用。 Android内置的比较常用的Drawable类型包括：ColorDrawable、GradientDrawable、ShapeDrawable、BitmapDrawable、 NinePatchDrawable、InsetDrawable、ClipDrawable、ScaleDrawable、RotateDrawable、AnimationDrawable、LayerDrawable、LevelListDrawable、StateListDrawable、TransitionDrawable。 一般情况下，除了图片资源是直接放在res/drawable下（android studio中图片放在res/minmap下），其他的Drawable都是以xml格式实现的，开发者通过在xml中使用shape,selector,level-list等标签来实现对应的Drawable，从而实现相应的可绘制资源的定义，最终view通过绘制这些Drawable来实现我们想要的显示效果。
Drawable中xml标签与Drawable对象的对应关系如以下表格所示:
| xml标签 | Drawable对象 |
|---|---|
| StateListDrawable | |
| LevelListDrawable | |
| LayerDrawable | |
| TransitionDrawable | |
| ColorDrawable | |
| GradientDrawable | |
| ScaleDrawable | |
| ScaleDrawable | |
| ClipDrawable | |
| RotateDrawable | |
| AnimationDrawable | |
| InsetDrawable | |
| BitmapDrawable | |
| NinePatchDrawable |
下面我们通过Framework的源代码来探索一下Drawable的加载过程，一般情况下，我们通过getResources().getDrawable(int id);的方式去加载。顺着这条线往下看，getDrawable->loadDrawable->loadDrawableForCookie->Drawable.createFromXml->createFromXmlInner,来看android\graphics\drawable\Drawable.java的createFromXmlInner函数的实现代码。
public static Drawable createFromXmlInner(Resources r, XmlPullParser parser, AttributeSet attrs,
Theme theme) throws XmlPullParserException, IOException {
final Drawable drawable;
final String name = parser.getName();
if (name.equals("selector")) {
drawable = new StateListDrawable();
} else if (name.equals("animated-selector")) {
drawable = new AnimatedStateListDrawable();
} 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("ripple")) {
drawable = new RippleDrawable();
} else if (name.equals("color")) {
drawable = new ColorDrawable();
} else if (name.equals("shape")) {
drawable = new GradientDrawable();
} else if (name.equals("vector")) {
drawable = new VectorDrawable();
} else if (name.equals("animated-vector")) {
drawable = new AnimatedVectorDrawable();
} 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")) {
//noinspection deprecation
drawable = new BitmapDrawable(r);
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, theme);
return drawable;
}
上面的对应关系就从这里来的，现在是不是一下子明朗了呢。在函数结尾，Drawable通过条用inflate函数解析AttributeSet中的属性信息，并设置Drawable相应的属性。看一下BitmapDrawable吧，
public void inflate(Resources r, XmlPullParser parser, AttributeSet attrs, Theme theme)
throws XmlPullParserException, IOException {
super.inflate(r, parser, attrs, theme);
final TypedArray a = obtainAttributes(r, theme, attrs, R.styleable.BitmapDrawable);
updateStateFromTypedArray(a);
verifyState(a);
a.recycle();
}
继续看updateStateFromTypedArray
private void updateStateFromTypedArray(TypedArray a) throws XmlPullParserException {
final Resources r = a.getResources();
final BitmapState state = mBitmapState;
// Account for any configuration changes.
state.mChangingConfigurations |= a.getChangingConfigurations();
// Extract the theme attributes, if any.
state.mThemeAttrs = a.extractThemeAttrs();
final int srcResId = a.getResourceId(R.styleable.BitmapDrawable_src, 0);
if (srcResId != 0) {
final Bitmap bitmap = BitmapFactory.decodeResource(r, srcResId);
if (bitmap == null) {
throw new XmlPullParserException(a.getPositionDescription() +
": requires a valid src attribute");
}
state.mBitmap = bitmap;
}
state.mTargetDensity = r.getDisplayMetrics().densityDpi;
final boolean defMipMap = state.mBitmap != null ? state.mBitmap.hasMipMap() : false;
setMipMap(a.getBoolean(R.styleable.BitmapDrawable_mipMap, defMipMap));
state.mAutoMirrored = a.getBoolean(
R.styleable.BitmapDrawable_autoMirrored, state.mAutoMirrored);
state.mBaseAlpha = a.getFloat(R.styleable.BitmapDrawable_alpha, state.mBaseAlpha);
final int tintMode = a.getInt(R.styleable.BitmapDrawable_tintMode, -1);
if (tintMode != -1) {
state.mTintMode = Drawable.parseTintMode(tintMode, Mode.SRC_IN);
}
final ColorStateList tint = a.getColorStateList(R.styleable.BitmapDrawable_tint);
if (tint != null) {
state.mTint = tint;
}
final Paint paint = mBitmapState.mPaint;
paint.setAntiAlias(a.getBoolean(
R.styleable.BitmapDrawable_antialias, paint.isAntiAlias()));
paint.setFilterBitmap(a.getBoolean(
R.styleable.BitmapDrawable_filter, paint.isFilterBitmap()));
paint.setDither(a.getBoolean(R.styleable.BitmapDrawable_dither, paint.isDither()));
setGravity(a.getInt(R.styleable.BitmapDrawable_gravity, state.mGravity));
final int tileMode = a.getInt(R.styleable.BitmapDrawable_tileMode, TILE_MODE_UNDEFINED);
if (tileMode != TILE_MODE_UNDEFINED) {
final Shader.TileMode mode = parseTileMode(tileMode);
setTileModeXY(mode, mode);
}
final int tileModeX = a.getInt(R.styleable.BitmapDrawable_tileModeX, TILE_MODE_UNDEFINED);
if (tileModeX != TILE_MODE_UNDEFINED) {
setTileModeX(parseTileMode(tileModeX));
}
final int tileModeY = a.getInt(R.styleable.BitmapDrawable_tileModeY, TILE_MODE_UNDEFINED);
if (tileModeY != TILE_MODE_UNDEFINED) {
setTileModeY(parseTileMode(tileModeY));
}
// Update local properties.
initializeWithState(state, r);
}
updateStateFromTypedArray函数中，BitmapDrawable获取到src属性后，通过BitmapFactory加载图像文件，并读取用户配置的属性，设置BitmapDrawable的其他属性。其他的Drawable的实现方式类似。