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<HTML><HEAD><TITLE>Xlib Programming Manual: Window Attributes: Backing Store</TITLE></HEAD><BODY><H1 ALIGN=center>3.2.4 Backing Store Attribute</H1>Some implementations of the X server may choose to maintain the contents of <B><A HREF="../create.html#InputClass">InputOutput</A></B>windows.If the X server maintains the contents of a window, the off-screen saved pixelsare known as backing store.The backing store advises the X server on what to do with the contents of a window.The backing-store attribute can be set to<B>NotUseful</B>(default),<B>WhenMapped</B>,or<B>Always</B>.<P>A backing-store attribute of <B>NotUseful</B>advises the X server that maintaining contents is unnecessary, although some X implementations maystill choose to maintain contents and, therefore, not generate <B><A HREF="../../events/exposure/expose.html">Expose</A></B>events.A backing-store attribute of <B>WhenMapped</B>advises the X server that maintaining contents of obscured regions when the window is mapped would be beneficial.In this case,the server may generate an <B><A HREF="../../events/exposure/expose.html">Expose</A></B> event when the window is created.A backing-store attribute of <B>Always</B>advises the X server that maintaining contents even when the window is unmapped would be beneficial. Even if the window is larger than its parent, this is a request to the X server to maintain complete contents, not just the region within the parent window boundaries. While the X server maintains the window's contents, <B><A HREF="../../events/exposure/expose.html">Expose</A></B> events normally are not generated, but the X server may stop maintaining contents at any time. <P>When the contents of obscured regions of a window are being maintained,regions obscured by noninferior windows are included in the destinationof graphics requests (and source, when the window is the source).However, regions obscured by inferior windows are not included.<H5 ALIGN=right><I>Next: <A HREF="save-under.html">Save Under Flag</A></I></H5><HR><ADDRESS><A HREF="http://tronche.com/">Christophe Tronche</A>, <A HREF="mailto:ch.tronche@computer.org">ch.tronche@computer.org</A></ADDRESS></BODY></HTML>⌨️ 快捷键说明
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