blink渲染知识5 - absulte/fixed and relative positioning



Posted by DaveHyatt onTuesday, August 14th, 2007 at 1:32 pm

The position property in CSS can be used to position anobject relative to a specific containing block. It has fourvalues: ‘static’, ‘absolute’, ‘fixed’ and ‘relative’.Static positioning is the default and means that the object is justpositioned using the normal rules of block and line layout.

RelativePositioning

Relative positioning is exactly like static positioning exceptthat the CSS left, top, right and bottomproperties can be used to apply a translation to the object. TheisRelPositioned method can be used to ask if a rendereris using relative positioning.

bool isRelPositioned() const

The translation offset that will be applied can be obtained usingthe following methods:

int relativePositionOffsetX() const;
int relativePositionOffsetY()
const;

Relative positioning is literally nothing more than a paint-timetranslation. As far as layout is concerned, the object is at itsoriginal position. Below is an example that uses relative positioningto shift part of a line up by a few pixels. As you can see, the linelays out as though the object was at the original position.

<div style="border:5px solid black; padding:20px;
width:300px">
Here is a line of text.
<span style="position:relative;top:-10px;
background-color: #eeeeee">
This part is shifted<br> up a bit,
</span>
but the rest of the line is in its original position.
</div>

Here is aline of text. This part isshifted
up a bit
, but the rest of the line is in itsoriginal position.

Absoluteand Fixed Positioning

Fixed positioned objects are positioned relative to the viewport,i.e., the visible page area of your browser window. Absolutepositioned objects are positioned relative to the containingblock, which is the nearest enclosing ancestor block with aposition other than ‘static’. If no such block exists, then theinitial containing block (the RenderView) is used. For more detailson containing blocks, see the previous article.

The isPositioned method can be used to find out if arenderer is absolute or fixed positioned.

bool isPositioned() const

When an object is absolute/fixed positioned, it becomesblock-level. Even if the CSS display type is set to inline (orinline-block/table), the effective display type becomes block-levelonce an object is positioned. The isInline method willreturn false for positioned elements.

The RenderStyle can give both display values. Thereare times where the original display type is relevant and needed bylayout, and the following methods can be used to obtain both displaytypes.

EDisplay display() const;
EDisplay originalDisplay() const;
ThePositioned Objects List

Every block has a positioned objects list that contains all of theabsolute/fixed positioned renderers for which it is the containingblock. It is the block’s responsibility to place these positionedchildren. The following methods can be used to manipulate thepositioned objects list:

void insertPositionedObject(RenderObject*);
void
removePositionedObject(RenderObject*);

The layoutOnlyPositionedObjects method is used to layout only the positioned objects for a block. If only positionedobjects changed, then this method returns true. Thisindicates that the layout method doesn’t have to lay out any of itsnormal children and can just return early.

bool layoutOnlyPositionedObjects

The layoutPositionedObjects method takes care of theplacement of positioned objects. It takes a boolean argument thatindicates whether relayout should be forced on all of the objects.Relayout can be necessary under a variety of circumstances that willbe covered in future articles.

bool layoutPositionedObjects(bool relayoutChildren)

Coordinatesof Positioned Objects

The coordinates of positioned objects in CSS are relative to thepadding edge of the containing block. For example specifying a leftand top position of (0, 0) for an absolute positioned object willresult in the object being placed just inside the containing block’stop left border. Here is an example:

<div style="position:relative;border:5px solid
black;width:300px;height:300px;">
<div
style="position:absolute;width:200px;height:200px;background-color:purple"></div>
</div>



In WebCore, coordinate positions are always relative to theborder edge, so in the above example, the object is actually at (5,5).

When the desired coordinates are omitted from CSS, WebCore has todetermine an appropriate place for the positioned object. CSS has aset of extremely complex rules for this, which we will delve into inmore detail in future articles.

InlineRelative Positioned Containers

It is possible for a relative positioned inline to act as a“containing block” for an absolutely positioned descendant. Thisis another extremely complex edge case that warrants its own article.For now it is enough to know that such a construct is possible, andthat the code does have to deal with it.



关于定位,这里还有一篇文章讲的不错。

http://www.jb51.net/web/77495.html

### 解决方案 在使用 ESP-IDF 工具链通过 CMake 构建项目时,如果 `bootloader-configure` 步骤失败,通常可能是由于以下几个原因引起的: #### 1. **工具链版本不兼容** ESP-IDF 的不同版本可能需要特定的工具链支持。例如,从 V4.x 升级到 V5.x 后,某些 API 或数据类型定义可能会发生变化,这可能导致编译错误。具体来说,`xTaskHandle` 是 FreeRTOS 中的一个常见类型名,但在新版本中其定义方式可能发生改变[^1]。 解决方案: - 确认当前使用的工具链版本是否匹配所选的 ESP-IDF 版本。 - 如果未安装最新版工具链,请按照官方文档重新配置开发环境并更新工具链。 ```bash ./install.sh esp32c3 source ./export.sh ``` #### 2. **CMake 配置文件问题** 有时项目的 CMakeLists.txt 文件可能存在语法错误或者路径设置不当,从而导致构建过程中的某个阶段(如 `bootloader-configure`)无法正常完成。 检查方法: - 打开项目的根目录下的 `CMakeLists.txt` 文件,确认是否存在拼写错误或其他异常情况。 - 使用以下命令手动测试 CMake 是否能够成功解析项目结构: ```bash cmake -G "Unix Makefiles" -DIDF_PATH=/path/to/esp-idf .. ``` 其中 `/path/to/esp-idf` 应替换为实际的 ESP-IDF 安装路径。 #### 3. **依赖库缺失或损坏** 当系统缺少必要的依赖项时,也可能引发类似的错误消息。对于 Linux 用户而言,可以通过修复包管理器的状态来尝试解决问题[^2]。 处理步骤如下: - 清理已知的问题软件包状态记录; - 更新可用资源列表以及升级现有程序至最新稳定发行版。 ```bash sudo dpkg --configure -a sudo apt-get update && sudo apt-get dist-upgrade ``` 以上操作完成后再次启动完整的构建流程看看是否有改善效果。 --- ### 提供一段示例代码用于验证基础功能是否正常工作 下面是一个简单的 Blink LED 示例程序,可以用来初步判断开发板及其配套驱动是否能正常使用硬件外设接口。 ```cpp #include <Arduino.h> void setup() { pinMode(2, OUTPUT); // 设置 GPIO2 作为输出端口 } void loop() { digitalWrite(2, HIGH); delay(500); digitalWrite(2, LOW); delay(500); } ``` 将其上传至目标设备后观察指示灯闪烁频率即可得知连接状况如何。 ---
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