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？
