关于如何优化activity的启动速度， view 的绘制速度， 可参考这个sdk里的文档。 android-sdk-windows-1.5_r1/docs/resources/articles/window-bg-speed.html。

看完后你就知道 android:windowBackground 太重要了，影响到绘制效率。

这里要说的是另外一点， 不是这个windowBackground 。

android 为了提高滚动等各方面的绘制速度，可以为每一个view建立一个缓存，使用 View.buildDrawingCache为自己的view 建立相应的缓存，

这个所谓的缓存，实际上就是一个Bitmap对象。只是 这个 bitmap 对象可以有多种格式而已，如

Bitmap.Config.ARGB_8888;

Bitmap.Config.ARGB_4444;

Bitmap.Config.ARGB_8888;

Bitmap.Config.ARGB_8888;

Bitmap.Config.RGB_565；

默认的格式是Bitmap.Config.ARGB_8888.，但大多数嵌入式设备使用的显示格式都是Bitmap.Config.RGB_565. 对于后者， 并没有

alpha 值，所以绘制的时候不需要计算alpha合成，速递当让快些。其次，RGB_565可以直接使用优化了的memcopy函数，效率相对高出许多。

所以， 在用buildDrawingCache建立缓存时， 可以使用RGB_565格式。但是如何制定这个格式呢 ？buildDrawingCache有两个版本，     buildDrawingCache(boolean) 和 buildDrawingCache()。并没有任何参数可以设置rgb格式，看看源码先：

public void buildDrawingCache(boolean autoScale) {

if ((mPrivateFlags & DRAWING_CACHE_VALID) == 0 || (autoScale ?

(mDrawingCache == null || mDrawingCache.get() == null) :

(mUnscaledDrawingCache == null || mUnscaledDrawingCache.get() == null))) {

if (ViewDebug.TRACE_HIERARCHY) {

ViewDebug.trace(this, ViewDebug.HierarchyTraceType.BUILD_CACHE);

}

if (Config.DEBUG && ViewDebug.profileDrawing) {

EventLog.writeEvent(60002, hashCode());

}

int width = mRight - mLeft;

int height = mBottom - mTop;

final AttachInfo attachInfo = mAttachInfo;

final boolean scalingRequired = attachInfo != null && attachInfo.mScalingRequired;

if (autoScale && scalingRequired) {

width = (int) ((width * attachInfo.mApplicationScale) + 0.5f);

height = (int) ((height * attachInfo.mApplicationScale) + 0.5f);

}

final int drawingCacheBackgroundColor = mDrawingCacheBackgroundColor;

final boolean opaque = drawingCacheBackgroundColor != 0 ||

(mBGDrawable != null && mBGDrawable.getOpacity() == PixelFormat.OPAQUE);

if (width <= 0 || height <= 0 ||

(width * height * (opaque ? 2 : 4) > // Projected bitmap size in bytes

ViewConfiguration.get(mContext).getScaledMaximumDrawingCacheSize())) {

destroyDrawingCache();

return;

}

boolean clear = true;

Bitmap bitmap = autoScale ? (mDrawingCache == null ? null : mDrawingCache.get()) :

(mUnscaledDrawingCache == null ? null : mUnscaledDrawingCache.get());

if (bitmap == null || bitmap.getWidth() != width || bitmap.getHeight() != height) {

Bitmap.Config quality;

if (!opaque) {

switch (mViewFlags & DRAWING_CACHE_QUALITY_MASK) {

case DRAWING_CACHE_QUALITY_AUTO:

quality = Bitmap.Config.ARGB_8888;

break;

case DRAWING_CACHE_QUALITY_LOW:

quality = Bitmap.Config.ARGB_4444;

break;

case DRAWING_CACHE_QUALITY_HIGH:

quality = Bitmap.Config.ARGB_8888;

break;

default:

quality = Bitmap.Config.ARGB_8888;

break;

}

} else {

quality = Bitmap.Config.RGB_565;

}

// Try to cleanup memory

if (bitmap != null) bitmap.recycle();

try {

bitmap = Bitmap.createBitmap(width, height, quality);

bitmap.setDensity(getResources().getDisplayMetrics().densityDpi);

if (autoScale) {

mDrawingCache = new SoftReference(bitmap);

} else {

mUnscaledDrawingCache = new SoftReference(bitmap);

}

} catch (OutOfMemoryError e) {

// If there is not enough memory to create the bitmap cache, just

// ignore the issue as bitmap caches are not required to draw the

// view hierarchy

if (autoScale) {

mDrawingCache = null;

} else {

mUnscaledDrawingCache = null;

}

return;

}

clear = drawingCacheBackgroundColor != 0;

}

Canvas canvas;

if (attachInfo != null) {

canvas = attachInfo.mCanvas;

if (canvas == null) {

canvas = new Canvas();

}

canvas.setBitmap(bitmap);

// Temporarily clobber the cached Canvas in case one of our children

// is also using a drawing cache. Without this, the children would

// steal the canvas by attaching their own bitmap to it and bad, bad

// thing would happen (invisible views, corrupted drawings, etc.)

attachInfo.mCanvas = null;

} else {

// This case should hopefully never or seldom happen

canvas = new Canvas(bitmap);

}

if (clear) {

bitmap.eraseColor(drawingCacheBackgroundColor);

}

computeScroll();

final int restoreCount = canvas.save();

if (autoScale && scalingRequired) {

final float scale = attachInfo.mApplicationScale;

canvas.scale(scale, scale);

}

canvas.translate(-mScrollX, -mScrollY);

mPrivateFlags |= DRAWN;

// Fast path for layouts with no backgrounds

if ((mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) {

if (ViewDebug.TRACE_HIERARCHY) {

ViewDebug.trace(this, ViewDebug.HierarchyTraceType.DRAW);

}

mPrivateFlags &= ~DIRTY_MASK;

dispatchDraw(canvas);

} else {

draw(canvas);

}

canvas.restoreToCount(restoreCount);

if (attachInfo != null) {

// Restore the cached Canvas for our siblings

attachInfo.mCanvas = canvas;

}

mPrivateFlags |= DRAWING_CACHE_VALID;

}

}

看完后明白了，至少跟两个因素有关 drawingCacheBackgroundColor 和 mBGDrawable.

用setDrawingCacheBackgroundColor(0xffff0000)设置为 非默认颜色后，建立的缓存就是rgb565了，可以用下列方法验证一下:

final Bitmap cache = mContent.getDrawingCache();

if (cache != null) {

Config cfg = cache.getConfig();

Log.d(TAG, "----------------------- cache.getConfig() = " + cfg);