Android graphic, what's drawable, it's practice and principle

 

 What is Drawable

 

     Drawable.java中，对drawable有一段注释。drawable是something can be drawn的一个抽象。 

 

    我们可以把一个resource对象，变成drawable, 然后显示到屏幕。

   

   drawable正是提供了这样的api 。 drawable和view不一样，它没有任何和user进行交互的能力。

 

 Drawable -

 

1)作为Android平下通用的图形对象，它可以装载常用格式的图像，比如GIF、PNG、JPG，当然也支持BMP，当然还提供一些高级的可视化对象，比如　　渐变、图形等

 2) 它还可以往view的canvas上面，直接作图。例如画出各种shape.

  

Practice　

 1)的例子

 //定义UI组件
final Button b1 = (Button) findViewById(R.id.Button01);
b1.setBackgroundResource(R.drawable.a);
 
final Button b2 = (Button) findViewById(R.id.Button02);
b2.setBackgroundColor(Color.parseColor("#FF00FF"));
 
final Drawable dra =  this.getResources().getDrawable(R.drawable.a);
final Drawable drb =  this.getResources().getDrawable(R.drawable.b);
 
//定义按钮点击监听器
OnClickListener ocl = new OnClickListener() {
 
@SuppressWarnings("deprecation")
@Override
public void onClick(View v) {
 
switch (v.getId()) {
case R.id.Button01:
 
if(switch1 == true) {
b1.setBackgroundDrawable(dra);
switch1 = false;
} else {
b1.setBackgroundDrawable(drb);
switch1 = true;
}
 
break;
}
 
}
 
};
 
//给按钮们绑定点击监听器
b1.setOnClickListener(ocl);
 

　　

 

principle: How drawable element is shown?

 

在View::draw()的过程中，首先会看看view的mBackground的成员变量，调用该成员变量的setBounds()和draw函数。

绘图的结果，会保存在canvas当中。

 

 1. Draw the background
13675         *      2. If necessary, save the canvas' layers to prepare for fading
13676         *      3. Draw view's content
13677         *      4. Draw children
13678         *      5. If necessary, draw the fading edges and restore layers
13679         *      6. Draw decorations (scrollbars for instance)
13680         */

 final Drawable background = mBackground;
13687            if (background != null) {
13688                final int scrollX = mScrollX;
13689                final int scrollY = mScrollY;
13690
13691                if (mBackgroundSizeChanged) {
13692                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
13693                    mBackgroundSizeChanged = false;
13694                }
13695
13696                if ((scrollX | scrollY) == 0) {
13697                    background.draw(canvas);
13698                } else {
13699                    canvas.translate(scrollX, scrollY);
13700                    background.draw(canvas);
13701                    canvas.translate(-scrollX, -scrollY);
13702                }
13703            }

2）的例子

  public class CustomDrawableView extends View {
      private ShapeDrawable mDrawable;

      public CustomDrawableView(Context context) {
      super(context);

      int x = 10;
      int y = 10;
      int width = 300;
      int height = 50;

      mDrawable = new ShapeDrawable(new OvalShape());
      mDrawable.getPaint().setColor(0xff74AC23);
      mDrawable.setBounds(x, y, x + width, y + height);
      }

      protected void onDraw(Canvas canvas) {
      mDrawable.draw(canvas);
      }
      }

 

principle of drawing:

类似于1），作图过程中， drawable的draw函数会画图到canvas. 在上面的例子中，作图过程，最终调用的是：

public class OvalShape extends RectShape {

    /**
     * OvalShape constructor.
     */
    public OvalShape() {}

    @Override
    public void draw(Canvas canvas, Paint paint) {
        canvas.drawOval(rect(), paint);
    }
}

　所以drawable不过是对canvas drawing primitive的一个封装，使得用起来更容易。

 

 

 

 

reference:

http://developer.android.com/guide/topics/graphics/2d-graphics.html
http://www.cnblogs.com/feisky/archive/2010/01/08/1642567.html
http://developer.android.com/guide/topics/graphics/2d-graphics.html#shape-drawable