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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"><title>Streams and File I/O</title><meta name="generator" content="DocBook XSL Stylesheets V1.68.1"><link rel="start" href="index.html" title="Cairo: A Vector Graphics Library"><link rel="up" href="language-bindings.html" title="Appendix A. Creating a language binding for cairo"><link rel="prev" href="bindings-overloading.html" title="Overloading and optional arguments"><link rel="next" href="bindings-errors.html" title="Error handling"><meta name="generator" content="GTK-Doc V1.6 (XML mode)"><link rel="stylesheet" href="style.css" type="text/css"><link rel="part" href="pt01.html" title="Part I. Tutorial"><link rel="part" href="pt02.html" title="Part II. Reference"><link rel="chapter" href="Drawing.html" title="Drawing"><link rel="chapter" href="Fonts.html" title="Fonts"><link rel="chapter" href="Surfaces.html" title="Surfaces"><link rel="chapter" href="Support.html" title="Utilities"><link rel="index" href="ix01.html" title="Index"><link rel="appendix" href="language-bindings.html" title="Appendix A. Creating a language binding for cairo"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="2"><tr valign="middle"><td><a accesskey="p" href="bindings-overloading.html"><img src="left.png" width="24" height="24" border="0" alt="Prev"></a></td><td><a accesskey="u" href="language-bindings.html"><img src="up.png" width="24" height="24" border="0" alt="Up"></a></td><td><a accesskey="h" href="index.html"><img src="home.png" width="24" height="24" border="0" alt="Home"></a></td><th width="100%" align="center">Cairo: A Vector Graphics Library</th><td><a accesskey="n" href="bindings-errors.html"><img src="right.png" width="24" height="24" border="0" alt="Next"></a></td></tr></table><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="bindings-streams"></a>Streams and File I/O</h2></div></div></div><p> Various places in the cairo API deal with reading and writing data, whether from and to files, or to other sources and destinations. In these cases, what is typically provided in the C API is a simple version that just takes a filename, and a complex version that takes a callback function. An example is the PNG handling functions: </p><pre class="programlisting">cairo_surface_t *cairo_image_surface_create_from_png (const char *filename);cairo_surface_t *cairo_image_surface_create_from_png_stream (cairo_read_func_t read_func, void *closure);cairo_status_tcairo_surface_write_to_png (cairo_surface_t *surface, const char *filename);cairo_status_tcairo_surface_write_to_png_stream (cairo_surface_t *surface, cairo_write_func_t write_func, void *closure);</pre><p> The expectation is that the filename version will be mapped literally in the language binding, but the callback version will be mapped to a version that takes a language stream object. For example, in Java, the four functions above might be mapped to: </p><pre class="programlisting">static public ImageSurface createFromPNG (String filename) throws IOException;static public ImageSurface createFromPNG (InputStream stream) throws IOException;public void writeToPNG (String filename) throws IOException;public void writeToPNG (OutputStream stream) throws IOException;</pre><p> In many cases, it will be better to implement the filename version internally using the stream version, rather than building it on top of the filename version in C. The reason for this is that will naturally give a more standard handling of file errors for the language, as seen in the above Java example, where <code class="methodname">createFromPNG()</code> is marked as raising an exception. Propagating exceptions from inside the callback function to the caller will pose a challenge to the language binding implementor, since an exception must not propagate through the Cairo code. A technique that will be useful in some cases is to catch the exception in the callback, store the exception object inside a structure pointed to by <em class="parameter"><code>closure</code></em>, and then rethrow it once the function returns. </p><p class="remark"><i><span class="remark"> I'm not sure how to handle this for <code class="function">cairo_pdf_surface_create_for_callback()</code>. Other than keep a “exception to rethrow” thread-specific variable that is checked after <span class="emphasis"><em>every</em></span> call to a Cairo function. </span></i></p></div></body></html>⌨️ 快捷键说明
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