Android: Bitmap/Canvas/Drawable

接触到自定义View之后,经常会遇到Canvas与Paint, 从使用上不难理解Paint, 但是对于Canvas和Bitmap以及Drawable之间的关系不是很清楚. 今天看了下代码, 尝试去区分一下.

Canvas

The Canvas class holds the "draw" calls. To draw something, you need
4 basic components: A Bitmap to hold the pixels, a Canvas to host
the draw calls (writing into the bitmap), a drawing primitive (e.g. Rect,
Path, text, Bitmap), and a paint (to describe the colors and styles for the
drawing).
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这个是Canvas类中写的, 如果我们要绘制什么的话, 需要:

1. 一个Bitmap去持有像素
2. 一个Canvas来做主绘制调用, 并且绘制在Bitmap上
3. 一个绘制原型, 比如矩形, path, text, Bitmap
4. 一个画笔, 用来描述绘制的颜色和样式

从这里我们就可以知道, 绘制调用是传到Canvas里, 但是绘制的位置是绘制在一个Bitmap上.

那么Bitmap是啥呢?

Bitmap

位图, 其实可以理解为 int[] buffer , 也就是说这里有个缓存, 用来保存每个像素的信息

而Canvas类中有个Bitmap对象:

public class Canvas extends BaseCanvas {
  ...
  // may be null
  private Bitmap mBitmap;
  
  public Canvas(@NonNull Bitmap bitmap) {
    ...
    mBitmap = bitmap;
    ...
  }
  
  public void setBitmap(@Nullable Bitmap bitmap) {
    ...
    mBitmap = bitmap;
  }
}
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因此实际绘制是绘制在Canvas所持有的Bitmap上

Drawable

Drawable是一个抽象, 描述所有可绘制的对象, 平时很少直接使用Drawable, 通常是使用drawable资源的方式获取Drawable对象.

与View的关系

public class View implements Drawable.Callback, KeyEvent.Callback, AccessibilityEventSource {
  ...
  private Drawable mBackground; // 背景
  ...

  public void setBackground(Drawable background) {
    setBackgroundDrawable(background);
  }

  @Deprecated
  public void setBackgroundDrawable(Drawable background) {
    ...
    mBackground = background;
    ...
  }

  public void draw(Canvas canvas) {
    ...
    // Step 1, draw the background, if needed
    if (!dirtyOpaque) {
      drawBackground(canvas);
    }
    ...
  }

  private void drawBackground(Canvas canvas) {
    final Drawable background = mBackground;
    ...
    background.draw(canvas);
    ...
  }

  @Override
  protected void onDraw(Canvas canvas) {
  }

}
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