Drawable

android.graphics.drawable. Drawable是个虚类。
它的直接子类有
BitmapDrawable, ClipDrawable, ColorDrawable, DrawableContainer, GradientDrawable, 
InsetDrawable, LayerDrawable, NinePatchDrawable, PictureDrawable, RotateDrawable, 
ScaleDrawable, ShapeDrawable
间接子类有
AnimationDrawable, LevelListDrawable, PaintDrawable, StateListDrawable, TransitionDrawable

Class Overview

A Drawable is a general abstraction for "something that can be drawn." Most often you will deal with Drawable as the type of resource retrieved for drawing things to the screen; the Drawable class provides a generic API for dealing with an underlying visual resource that may take a variety of forms. Unlike a View, a Drawable does not have any facility to receive events or otherwise interact with the user.

In addition to simple drawing, Drawable provides a number of generic mechanisms for its client to interact with what is being drawn:

• The setBounds(Rect) method must be called to tell the Drawable where it is drawn and how large it should be. All Drawables should respect the requested size, often simply by scaling their imagery. A client can find the preferred size for some Drawables with the getIntrinsicHeight() and getIntrinsicWidth()methods.这个用来设置Drawable要画好大
• The getPadding(Rect) method can return from some Drawables information about how to frame content that is placed inside of them. For example, a Drawable that is intended to be the frame for a button widget would need to return padding that correctly places the label inside of itself.这个接口用于返回padding，这个padding其实表示的是控件的content的padding
• The setState(int[]) method allows the client to tell the Drawable in which state it is to be drawn, such as "focused", "selected", etc. Some drawables may modify their imagery based on the selected state.
• The setLevel(int) method allows the client to supply a single continuous controller that can modify the Drawable is displayed, such as a battery level or progress level. Some drawables may modify their imagery based on the current level.
• A Drawable can perform animations by calling back to its client through the Drawable.Callback interface. All clients should support this interface (viasetCallback(Drawable.Callback)) so that animations will work. A simple way to do this is through the system facilities such assetBackgroundDrawable(Drawable) and ImageView.

Drawable定义了一个可画的对象一些接口。
它是抽象类。我们只能实例化它的实现类。Drawable就是一个可画的对象，其可能是一张位图（BitmapDrawable），也可能是一个图形（ShapeDrawable），还有可能是一个图层（LayerDrawable），我们根据画图的需求，创建相应的可画对象。 

注意1：如果要手动调用 draw(Canvas canvas)来画Drawable，
一定要在之前通过
void        setBounds(Rect bounds)
来设置其需要画到的区域。
注意2: 可通过setCallback(Drawable.Callback)来实现动画。

构造函数

Public Constructors
	Drawable()

主要函数

Public Methods
void	clearColorFilter()
final  Rect	copyBounds() Return a copy of the drawable's bounds in a new Rect.
final void	copyBounds( Rect bounds) Return a copy of the drawable's bounds in the specified Rect (allocated by the caller).
static  Drawable	createFromPath( String pathName) Create a drawable from file path name.
static  Drawable	createFromResourceStream( Resources res,  TypedValue value,  InputStream is,  String srcName,  BitmapFactory.Options opts) Create a drawable from an inputstream, using the given resources and value to determine density information.
static  Drawable	createFromResourceStream( Resources res,  TypedValue value,  InputStream is,  String srcName) Create a drawable from an inputstream, using the given resources and value to determine density information.
static  Drawable	createFromStream( InputStream is,  String srcName) Create a drawable from an inputstream
static  Drawable	createFromXml( Resources r,  XmlPullParser parser) Create a drawable from an XML document.
static  Drawable	createFromXmlInner( Resources r,  XmlPullParser parser,  AttributeSet attrs) Create from inside an XML document.
abstract void	draw( Canvas canvas) Draw in its bounds (set via setBounds) respecting optional effects such as alpha (set via setAlpha) and color filter (set via setColorFilter).
final  Rect	getBounds() Return the drawable's bounds Rect.
Drawable.Callback	getCallback() Return the current  Drawable.Callback implementation attached to this Drawable.
int	getChangingConfigurations() Return a mask of the configuration parameters for which this drawable may change, requiring that it be re-created.
Drawable.ConstantState	getConstantState() Return a  Drawable.ConstantState instance that holds the shared state of this Drawable.
Drawable	getCurrent()
int	getIntrinsicHeight() Return the intrinsic height of the underlying drawable object.
int	getIntrinsicWidth() Return the intrinsic width of the underlying drawable object.
final int	getLevel() Retrieve the current level.
int	getMinimumHeight() Returns the minimum height suggested by this Drawable.
int	getMinimumWidth() Returns the minimum width suggested by this Drawable.
abstract int	getOpacity() Return the opacity/transparency of this Drawable.
boolean	getPadding( Rect padding) Return in padding the insets suggested by this Drawable for placing content inside the drawable's bounds.
int[]	getState() Describes the current state, as a union of primitve states, such as  state_focused,  state_selected, etc.
Region	getTransparentRegion() Returns a Region representing the part of the Drawable that is completely transparent.
void	inflate( Resources r,  XmlPullParser parser,  AttributeSet attrs) Inflate this Drawable from an XML resource.
void	invalidateSelf() Use the current  Drawable.Callback implementation to have this Drawable redrawn.
boolean	isStateful() Indicates whether this view will change its appearance based on state.
final boolean	isVisible()
void	jumpToCurrentState() If this Drawable does transition animations between states, ask that it immediately jump to the current state and skip any active animations.
Drawable	mutate() Make this drawable mutable.
static int	resolveOpacity(int op1, int op2) Return the appropriate opacity value for two source opacities.
void	scheduleSelf( Runnable what, long when) Use the current  Drawable.Callback implementation to have this Drawable scheduled.
abstract void	setAlpha(int alpha) Specify an alpha value for the drawable.
void	setBounds(int left, int top, int right, int bottom) Specify a bounding rectangle for the Drawable.
void	setBounds( Rect bounds) Specify a bounding rectangle for the Drawable.
final void	setCallback( Drawable.Callback cb) Bind a  Drawable.Callback object to this Drawable.
void	setChangingConfigurations(int configs) Set a mask of the configuration parameters for which this drawable may change, requiring that it be re-created.
abstract void	setColorFilter( ColorFilter cf) Specify an optional colorFilter for the drawable.
void	setColorFilter(int color,  PorterDuff.Mode mode) Specify a color and porterduff mode to be the colorfilter for this drawable.
void	setDither(boolean dither) Set to true to have the drawable dither its colors when drawn to a device with fewer than 8-bits per color component.
void	setFilterBitmap(boolean filter) Set to true to have the drawable filter its bitmap when scaled or rotated (for drawables that use bitmaps).
final boolean	setLevel(int level) Specify the level for the drawable.
boolean	setState(int[] stateSet) Specify a set of states for the drawable.
boolean	setVisible(boolean visible, boolean restart) Set whether this Drawable is visible.
void	unscheduleSelf( Runnable what) Use the current  Drawable.Callback implementation to have this Drawable unscheduled.

Protected Methods
void	onBoundsChange( Rect bounds) Override this in your subclass to change appearance if you recognize the specified state.
boolean	onLevelChange(int level) Override this in your subclass to change appearance if you vary based on level.
boolean	onStateChange(int[] state) Override this in your subclass to change appearance if you recognize the specified state.

直接子类
BitmapDrawable     A Drawable that wraps a bitmap. You can create a BitmapDrawable from a file path, 
                                an input stream, through XML inflation, or from a Bitmap object. 
                            它的本质是个bitmap,它提供了针对bitmap的实现。
注意：Android supports bitmap files in a three formats:  .png (preferred),  .jpg (acceptable),  .gif (discouraged).
也是说Android把.png,.jpg,.gif都是看为BitmapDrawable,Android应该是把这三种图片转为bitmap来处理。
其实任何图片绘画时最后都是转为bitmap来处理
        BitmapDrawable bitmapDrawable=(BitmapDrawable)context.getResources().getDrawable(R.drawable.icon);
        Bitmap bitmap=bitmapDrawable.getBitmap();
可以通过上面的形式把BitmapDrawable转化为Bitmap
ClipDrawable     A Drawable that clips another Drawable based on this Drawable's current level value. 
具体参照《 ClipDrawable》
ColorDrawable     A specialized Drawable that fills the Canvas with a specified color, with respect to the clip region. 
DrawableContainer
    它应该是专门用来存放Drawable的。
    如何直接使用它还不知道。它不能在XML文件中定义。
    它的子类AnimationDrawable, LevelListDrawable, StateListDrawable        
GradientDrawable     A Drawable with a color gradient for buttons, backgrounds, etc.
    It can be defined in an XML file with the <shape> element. For more information。 
InsetDrawable     A Drawable that insets another Drawable by a specified distance. 
LayerDrawable     A Drawable that manages an array of other Drawables. 
NinePatchDrawable     A resizeable bitmap, with stretchable areas that you define. 
它对应的是.9.png文件。关于此请参考《 个性缩放图片NinePatchDrawable》

PictureDrawable     Drawable subclass that wraps a Picture, allowing the picture to be used whereever a Drawable is supported. 
RotateDrawable     
A Drawable that can rotate another Drawable based on the current level value. 
ScaleDrawable     A Drawable that changes the size of another Drawable based on its current level value. 
ShapeDrawable     A Drawable object that draws primitive shapes. 关于此请参考《 ShapeDrawable》

ColorDrawable
    A specialized Drawable that fills the Canvas with a specified color, 
    with respect to the clip region. Note that a ColorDrawable ignores the ColorFilter. 
    It also ignores the Bounds, meaning it will draw everywhere in the current clip, 
    even if setBounds(...) was called with a smaller area.
    It can be defined in an XML file with the <color> element.
注意1:它会忽略掉ColorFilter和Bounds，它会把整个clip区域都填充成指定的颜色。
在XML中定义和引用ColorDrawable
A color resource can also be used as a drawable in XML. For example,
when creating a state list drawable, you can reference a color resource for the android:drawable attribute 
(android:drawable="@color/green
在values下的 Colors.xml文件：
<?xml version="1.0" encoding="UTF-8"?>
<resources>
< color  name="red">#FFFF0000</color>
</resources>
像引用Drawable一样引用它
<TextView  
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="@string/hello"
    android:background=" @color/red "
    />