Android: Bitmap/Canvas/Drawable

摘要： 接觸到自定義View之後,經常會遇到Canvas與Paint, 從使用上不難理解Paint, 但是對於Canvas和Bitmap以及Drawable之間的關係不是很清楚. 今天看了下程式碼, 嘗試去區分一下. Canvas The Canvas class holds the "...

接觸到自定義View之後,經常會遇到Canvas與Paint, 從使用上不難理解Paint, 但是對於Canvas和Bitmap以及Drawable之間的關係不是很清楚. 今天看了下程式碼, 嘗試去區分一下.

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).
複製程式碼

這個是Canvas類中寫的, 如果我們要繪製什麼的話, 需要:

1. 一個Bitmap去持有畫素
2. 一個Canvas來做主繪製呼叫, 並且繪製在Bitmap上
3. 一個繪製原型, 比如矩形, path, text, Bitmap
4. 一個畫筆, 用來描述繪製的顏色和樣式

從這裡我們就可以知道, 繪製呼叫是傳到Canvas裡, 但是繪製的位置是繪製在一個Bitmap上.

那麼Bitmap是啥呢?

Bitmap

點陣圖, 其實可以理解為int[] buffer , 也就是說這裡有個快取, 用來儲存每個畫素的資訊

而Canvas類中有個Bitmap物件:

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;
}
}
複製程式碼

因此實際繪製是繪製在Canvas所持有的Bitmap上

Drawable

Drawable是一個抽象, 描述所有可繪製的物件, 平時很少直接使用Drawable, 通常是使用drawable資源的方式獲取Drawable物件.

資源型別	檔案字尾	Drawable型別
Bitmap File	.png .jpg .gif	BitmapDrawable
Nine-Patch File	.9.png	NinePatchDrawable
Shape Drawable	.xml	ShapeDrawable
State List	.xml	StateListDrawable

與View的關係

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) {
}

}
複製程式碼