android 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

主要函数

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"
     />