blocks|key|184214|text|来自Android：
http://developer.android.com/reference/android/graphics/Canvas.html类保存“绘制”调用。要绘制一些东西，您需要4个基本组件:保存像素的http://developer.android.com/reference/android/graphics/Bitmap.html、承载绘图调用的画布(写入位图)、绘图原语(例如Rect、Path、text、bitmap)和油漆(用于描述绘图的颜色和样式)。|type|ordered-list-item|depth|inlineStyleRanges|entityRanges|offset|length|data|184215|我猜你指的是这个片段：
@重写受保护的无效onDraw(画布){+if+(mBitmap+!=+null)+{+canvas.drawBitmap(mBitmap，0，0，null)；}
它看起来像一个继承的onDraw方法的重写，默认情况下它很可能“什么都不做”，因此实际上给它一些行为的重写，在这种情况下，给出一个非空位图实例，让画布绘制它。|style|CODE|184216|unstyled|entityMap|0|LINK|mutability|MUTABLE|url|http://developer.android.com/reference/android/graphics/Canvas.html|1|http://developer.android.com/reference/android/graphics/Bitmap.html^0|B|1V|0|33|1V|1|0|2W|6|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]|$A|W|B|X|1|Y]]|C|$]]|$1|D|3|E|5|6|7|Z|8|@$A|10|B|11|F|G]]|9|@]|C|$]]|$1|H|3|-4|5|I|7|12|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]|Q|$5|L|M|N|C|$O|R]]]]

<ol>
<li>From the Android SDK:

<blockquote>
 The <a href="http://developer.android.com/reference/android/graphics/Canvas.html" rel="nofollow">Canvas</a> class holds the "draw" calls. To draw something, you need 4 basic components: A <a href="http://developer.android.com/reference/android/graphics/Bitmap.html" rel="nofollow">Bitmap</a> 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).
</blockquote></li>
<li>I'm assuming you're referring to this snippet:

<pre><code>@Override protected void onDraw(Canvas canvas) {
 if (mBitmap != null) {
 canvas.drawBitmap(mBitmap, 0, 0, null);
 }
}
</code></pre>

Well it looks like an override of an inherited <code>onDraw</code> method which by default probably 'does nothing', hence the override to actually give it some behaviour, in this case given a non-null Bitmap instance, make the canvas draw it.</li>
</ol>

blocks|key|79965|text|如果你去看开发人员参考资料：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|79966|drawBitmap(位图位图，浮动左，浮动顶部，油漆)
绘制指定的位图，其上/左角位于(x，y)，使用指定的油漆，由当前矩阵.转换。|blockquote|offset|length|style|BOLD|79967|如果您看到类中不存在mBitmap，那是因为var来自于来自另一个活动的扩展。|79968|画布还具有一个setBitmap(位图位图)函数。那么解决方案就是在画布上画，如果你已经设置了一个位图对象。|79969|entityMap^0|0|T|Z|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@$E|Q|F|R|G|H]]|9|@]|A|$]]|$1|I|3|J|5|6|7|S|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|T|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|U|8|@]|9|@]|A|$]]]|N|$]]

if you go to the developper refference:
<blockquote>
drawBitmap(Bitmap bitmap, float left, float top, Paint paint)
Draw the
specified bitmap, with its top/left corner at (x,y), using the
specified paint, transformed by the current matrix.
</blockquote>
Then if you see that mBitmap doesn't exist in the class , thats cause that var comes from the extend from another activity .
Canvas also has a setBitmap(Bitmap bitmap) function . Then the solution is that that paint in canvas if you have set into it a bitmap object.

blocks|key|2008254|text|画布与位图|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|2008255|Bitmap是其名称所表示的:作为位图的普通图像。Canvas类是位图的编辑器。如果使用它来更改位图数据，它将保存所有绘图方法。此原则的行为类似于共享首选项(如果您已经使用过它们)，您有一个包含首选项的SharedPreferences类，以及一个用于更改内容的Editor类。|CODE|2008256|绘制圆圈|2008257|此代码执行类似于双缓冲的操作。drawPoint()基本上是在mBitmap对象中画一个圆圈。但是这个位图对象还没有可见。它存在于记忆中。当调用onDraw()时，它有一个表示视图绘图面的Canvas参数。drawBitmap()在这里所做的就是使用内存中准备好的位图，并将其绘制到视图中，以使其可见。|2008258|使用过的画布mCanvas绑定到mBitmap内部的onSizeChanged()|2008259|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Double_buffering#Double_buffering^0|0|5|0|0|6|P|6|2T|H|3N|6|0|0|4|0|F|B|V|7|20|8|2M|6|2V|C|8|3|0|0|6|7|G|7|Q|F|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Z|8|@$9|10|A|11|B|H]|$9|12|A|13|B|H]|$9|14|A|15|B|H]|$9|16|A|17|B|H]]|D|@]|E|$]]|$1|I|3|J|5|6|7|18|8|@$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|K|3|L|5|6|7|1B|8|@$9|1C|A|1D|B|H]|$9|1E|A|1F|B|H]|$9|1G|A|1H|B|H]|$9|1I|A|1J|B|H]|$9|1K|A|1L|B|H]]|D|@$9|1M|A|1N|1|1O]]|E|$]]|$1|M|3|N|5|6|7|1P|8|@$9|1Q|A|1R|B|H]|$9|1S|A|1T|B|H]|$9|1U|A|1V|B|H]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|1W|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

Canvas vs Bitmap
A <code>Bitmap</code> is what the name suggests: A normal image as a bitmap. The <code>Canvas</code> class is an editor for bitmaps. You use it to change the bitmap data, it holds all drawing methods. This principle behaves similar to the shared preferences (if you already worked with them), you have a <code>SharedPreferences</code> class that holds the preferences, and an <code>Editor</code> class to change things.
Drawing the circles
This code does something similar to <a href="http://en.wikipedia.org/wiki/Double_buffering#Double_buffering" rel="nofollow noreferrer">double buffering</a>. <code>drawPoint()</code> basically draws a circle into the <code>mBitmap</code> object¹. But this bitmap object is not yet visible. It exists in the memory. When <code>onDraw()</code> is called, it has a <code>Canvas</code> argument that represents the drawing surface of the view. All that <code>drawBitmap()</code> does here is use the prepared bitmap from the memory and draw it inside the views graphical representation to make it visible.
¹ The used canvas <code>mCanvas</code> is tied to <code>mBitmap</code> inside <code>onSizeChanged()</code>

I am looking at one of the sample applications from Google, which deals with touch drawing using canvas: 

<a href="http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/graphics/TouchPaint.html" rel="nofollow">http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/graphics/TouchPaint.html</a>

I have a few doubts:

<ol>
<li>I am not able to understand what's the role of Canvas versus the role
of the bitmap.</li>
<li>In the <code>drawPoint</code> function, I am not able to
understand this code snippet:</li>
</ol>

<blockquote>
<pre><code>mCanvas.drawCircle(x, y, radius, mPaint);
mRect.set((int) (x - radius - 2), (int) (y - radius - 2),
 (int) (x + radius + 2), (int) (y + radius + 2));
invalidate(mRect);
</code></pre>
</blockquote>

If the circle is already drawn into the canvas above, then what happens in the <code>onDraw</code> function where the following code is given:

<code>canvas.drawBitmap(mBitmap, 0, 0, null);</code>

Android - canvas drawing doubts

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我正在查看Google的一个示例应用程序，它使用画布处理触摸绘图：我有几个疑问：我无法理解画布的角色和位图的角色。在drawPoint函数中，我无法理解这个代码片段：mCanvas.drawCircle (x，y，radius，mPaint)；mRect.set((int) (x - radius - 2)，(int)...

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