Trace解析

今天在读关于图片解析的源码，看到这样的代码：

/** * Decode an input stream into a bitmap. If the input stream is null, or * cannot be used to decode a bitmap, the function returns null. * The stream's position will be where ever it was after the encoded data * was read. * * @param is The input stream that holds the raw data to be decoded into a * bitmap. * @param outPadding If not null, return the padding rect for the bitmap if * it exists, otherwise set padding to [-1,-1,-1,-1]. If * no bitmap is returned (null) then padding is * unchanged. * @param opts null-ok; Options that control downsampling and whether the * image should be completely decoded, or just is size returned. * @return The decoded bitmap, or null if the image data could not be * decoded, or, if opts is non-null, if opts requested only the * size be returned (in opts.outWidth and opts.outHeight) * * <p class="note">Prior to {@link android.os.Build.VERSION_CODES#KITKAT}, * if {@link InputStream#markSupported is.markSupported()} returns true, * <code>is.mark(1024)</code> would be called. As of * {@link android.os.Build.VERSION_CODES#KITKAT}, this is no longer the case.</p> */ public static Bitmap decodeStream(InputStream is, Rect outPadding, Options opts) { // we don't throw in this case, thus allowing the caller to only check // the cache, and not force the image to be decoded. if (is == null) { return null; } Bitmap bm = null; Trace.traceBegin(Trace.TRACE_TAG_GRAPHICS, "decodeBitmap"); try { if (is instanceof AssetManager.AssetInputStream) { final long asset = ((AssetManager.AssetInputStream) is).getNativeAsset(); bm = nativeDecodeAsset(asset, outPadding, opts); } else { bm = decodeStreamInternal(is, outPadding, opts); } if (bm == null && opts != null && opts.inBitmap != null) { throw new IllegalArgumentException("Problem decoding into existing bitmap"); } setDensityFromOptions(bm, opts); } finally { Trace.traceEnd(Trace.TRACE_TAG_GRAPHICS); } return bm; }

其中有两句代码：

Trace.traceBegin(Trace.TRACE_TAG_GRAPHICS, "decodeBitmap"); Trace.traceEnd(Trace.TRACE_TAG_GRAPHICS);

很好奇，这个Trace是干嘛的？之前接触到有一个TraceView！TraceView 是 Android 平台配备一个很好的性能分析的工具。它可以通过图形化的方式让我们了解我们要跟踪的程序的性能，并且能具体到 method！

跟进源码瞧个究竟，看到这个代码的文件注释：

/** * Writes trace events to the system trace buffer. These trace events can be * collected and visualized using the Systrace tool. * * <p>This tracing mechanism is independent of the method tracing mechanism * offered by {@link Debug#startMethodTracing}. In particular, it enables * tracing of events that occur across multiple processes. * <p>For information about using the Systrace tool, read <a * href="{@docRoot}tools/debugging/systrace.html">Analyzing Display and Performance * with Systrace</a>. */

翻译过来就是：

将跟踪事件写入系统跟踪缓冲区。 可以使用Systrace工具收集和可视化这些跟踪事件。

此跟踪机制独立于startMethodTracing（）提供的方法跟踪机制。 特别地，它使得能够跟踪跨多个进程发生的事件。

有关使用Systrace工具的信息，请参阅使用Systrace分析显示和性能。

哈哈，出来一个东西—— Systrace。等我完了研究下这个Systrace.

接下来看着两个方法：

Trace.traceBegin

/** * Writes a trace message to indicate that a given section of code has * begun. Must be followed by a call to {@link #traceEnd} using the same * tag. * * @param traceTag The trace tag. * @param methodName The method name to appear in the trace. * * @hide */ public static void traceBegin(long traceTag, String methodName) { if (isTagEnabled(traceTag)) { nativeTraceBegin(traceTag, methodName); } }

写入跟踪信息这标志着要追踪的代码已经开始运行。必须在后面调用traceEnd，并且使用相同的tag.

参数一：根据的标记tag; 参数二：在追踪路径中显示的方法名。

Trace.traceEnd

/** * Writes a trace message to indicate that the current method has ended. * Must be called exactly once for each call to {@link #traceBegin} using the same tag. * * @param traceTag The trace tag. * * @hide */ public static void traceEnd(long traceTag) { if (isTagEnabled(traceTag)) { nativeTraceEnd(traceTag); } }

写入追踪信息标志着当前方法已经结束。必须和traceBegin成对的调用并且使用同一个tag.

参数：追踪tag和traceBegin传入的要一致。

至此为止，明白了图片解析里的这两句代码是干嘛的：

/*
 * Writes trace events to the kernel trace buffer. These trace events can be
 * collected using the “atrace” program for offline analysis.
 */

将这些追踪时间写入内部追踪缓存中。这些追踪事件可以被“atrace”程序收集起来，进行离线分析。

但是，再往里面走的话，发现了这东西竟然也走到了Native层：

private static native void nativeTraceBegin(long tag, String name); private static native void nativeTraceEnd(long tag);

这两个方法是在哪儿呢？仔细看看了文件发现了有在各种tag上有两句注释：

// These tags must be kept in sync with system/core/include/cutils/trace.h. // They should also be added to frameworks/native/cmds/atrace/atrace.cpp.

好吧，原来这东西到最后也给C++去做了！

那么问题了，我有什么理由不去好好学下C++,NDK 呢？