iphone开发 Quratz 2D 学习 ---什么是 graphics context（图形上下文）

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1.graphics context（图形上下文）是一个不透明的数据类型（CGContextRef）。我们可以把graphics context想象成为一个绘图目标。有几种graphics context：bitmap graphics context，PDF graphics context，window graphics context，layer context。

2.IOS中获得graphics context是在方法drawRect：中，调用方法UIGraphicsGetCurrentContext得到上下文。drawRect：方法不需要调用super

CGContextRefc =UIGraphicsGetCurrentContext();

3.调用方法CGContextMoveToPoint开始一个新的路径，

调用方法CGContextAddLineToPoint添加一个单一的线条。

4.When you want to construct a path in a graphics context, you signal Quartz by calling the function<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); "><a href="http://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CGContext/Reference/reference.html#//apple_ref/c/func/CGContextBeginPath" target="_self" style="text-decoration: none; color: rgb(51, 102, 204); ">CGContextBeginPath</a></code>. Next, you set the starting point for the first shape, or subpath, in the path by calling the function<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); "><a href="http://developer.apple.com/library/ios/documentation/GraphicsImaging/Reference/CGContext/Reference/reference.html#//apple_ref/c/func/CGContextMoveToPoint" target="_self" style="text-decoration: none; color: rgb(51, 102, 204); ">CGContextMoveToPoint</a></code>. After you establish the first point, you can add lines, arcs, and curves to the path, keeping in mind the following:

Before you begin a new path, call the function<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); ">CGContextBeginPath</code>.
Lines, arcs, and curves are drawn starting at the current point. An empty path has no current point; you must call<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); ">CGContextMoveToPoint</code>to set the starting point for the first subpath or call a convenience function that implicitly does this for you.
When you want to close the current subpath within a path, call the function<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); ">CGContextClosePath</code>to connect a segment to the starting point of the subpath. Subsequent path calls begin a new subpath, even if you do not explicitly set a new starting point.
When you draw arcs, Quartz draws a line between the current point and the starting point of the arc.
Quartz routines that add ellipses and rectangles add a new closed subpath to the path.
You must call a painting function to fill or stroke the path because creating a path does not draw the path. See<span style="word-wrap: normal; word-break: normal; line-height: normal; margin-top: 0px; font: normal normal normal 100%/normal 'Lucida Grande', Geneva, Helvetica, Arial, sans-serif; margin-bottom: 0.833em; padding-bottom: 0.833em; vertical-align: top; "><a href="http://developer.apple.com/library/ios/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_paths/dq_paths.html#//apple_ref/doc/uid/TP30001066-CH211-CJBEIHHJ" style="text-decoration: none; color: rgb(51, 102, 204); ">“Painting a Path”</a>for detailed information.

是一个不透明的数据类型（CGContextRef）。我们可以把graphics context想象成为一个绘图目标。有几种graphics context：bitmap graphics context，PDF graphics context，window graphics context，layer context。

2.IOS中获得graphics context是在方法drawRect：中，调用方法UIGraphicsGetCurrentContext得到上下文。drawRect：方法不需要调用super

CGContextRefc =UIGraphicsGetCurrentContext();

3.调用方法CGContextMoveToPoint开始一个新的路径，

调用方法CGContextAddLineToPoint添加一个单一的线条。

Before you begin a new path, call the function<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); ">CGContextBeginPath</code>.
Lines, arcs, and curves are drawn starting at the current point. An empty path has no current point; you must call<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); ">CGContextMoveToPoint</code>to set the starting point for the first subpath or call a convenience function that implicitly does this for you.
When you want to close the current subpath within a path, call the function<code style="font-size: 13px; font-family: Courier, Consolas, monospace; color: rgb(102, 102, 102); ">CGContextClosePath</code>to connect a segment to the starting point of the subpath. Subsequent path calls begin a new subpath, even if you do not explicitly set a new starting point.
When you draw arcs, Quartz draws a line between the current point and the starting point of the arc.
Quartz routines that add ellipses and rectangles add a new closed subpath to the path.
You must call a painting function to fill or stroke the path because creating a path does not draw the path. See<span style="word-wrap: normal; word-break: normal; line-height: normal; margin-top: 0px; font: normal normal normal 100%/normal 'Lucida Grande', Geneva, Helvetica, Arial, sans-serif; margin-bottom: 0.833em; padding-bottom: 0.833em; vertical-align: top; "><a href="http://developer.apple.com/library/ios/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_paths/dq_paths.html#//apple_ref/doc/uid/TP30001066-CH211-CJBEIHHJ" style="text-decoration: none; color: rgb(51, 102, 204); ">“Painting a Path”</a>for detailed information.

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