优化安卓应用内存的神秘方法以及背后的原理，一般人我不告诉他

WindowManagerGlobal.getInstance().startTrimMemory(TRIM_MEMORY_COMPLETE);

@Override
public void drawBitmap(Bitmap bitmap, float left, float top, Paint paint) {
throwIfCannotDraw(bitmap);
// Shaders are ignored when drawing bitmaps
int modifiers = paint != null ? setupModifiers(bitmap, paint) : MODIFIER_NONE;
try {
final int nativePaint = paint == null ? 0 : paint.mNativePaint;
nDrawBitmap(mRenderer, bitmap.mNativeBitmap, bitmap.mBuffer, left, top, nativePaint);
} finally {
if (modifiers != MODIFIER_NONE) nResetModifiers(mRenderer, modifiers);
}
}

static void android_view_GLES20Canvas_drawBitmap(JNIEnv* env, jobject clazz,
OpenGLRenderer* renderer, SkBitmap* bitmap, jbyteArray buffer, float left,
float top, SkPaint* paint) {
// This object allows the renderer to allocate a global JNI ref to the buffer object.
JavaHeapBitmapRef bitmapRef(env, bitmap, buffer);
renderer->drawBitmap(bitmap, left, top, paint);
}

status_t OpenGLRenderer::drawBitmap(SkBitmap* bitmap, float left, float top, SkPaint* paint) {
const float right = left + bitmap->width();
const float bottom = top + bitmap->height();
if (quickReject(left, top, right, bottom)) {
return DrawGlInfo::kStatusDone;
}
mCaches.activeTexture(0);
Texture* texture = getTexture(bitmap);
if (!texture) return DrawGlInfo::kStatusDone;
const AutoTexture autoCleanup(texture);
if (CC_UNLIKELY(bitmap->getConfig() == SkBitmap::kA8_Config)) {
drawAlphaBitmap(texture, left, top, paint);
} else {
drawTextureRect(left, top, right, bottom, texture, paint);
}

Texture* TextureCache::get(SkBitmap* bitmap) {
Texture* texture = mCache.get(bitmap);
if (!texture) {
if (bitmap->width() > mMaxTextureSize || bitmap->height() > mMaxTextureSize) {
ALOGW("Bitmap too large to be uploaded into a texture (%dx%d, max=%dx%d)",
bitmap->width(), bitmap->height(), mMaxTextureSize, mMaxTextureSize);
return NULL;
}
const uint32_t size = bitmap->rowBytes() * bitmap->height();
// Don't even try to cache a bitmap that's bigger than the cache
if (size < mMaxSize) {
while (mSize + size > mMaxSize) {
mCache.removeOldest();
}
}
texture = new Texture();
texture->bitmapSize = size;
generateTexture(bitmap, texture, false);
if (size < mMaxSize) {
mSize += size;
TEXTURE_LOGD("TextureCache::get: create texture(%p): name, size, mSize = %d, %d, %d",
bitmap, texture->id, size, mSize);
if (mDebugEnabled) {
ALOGD("Texture created, size = %d", size);
}
mCache.put(bitmap, texture);
} else {
texture->cleanup = true;
}
} else if (bitmap->getGenerationID() != texture->generation) {
generateTexture(bitmap, texture, true);
}
return texture;
}

static void startTrimMemory(int level) {
if (sEgl == null || sEglConfig == null) return;
Gl20RendererEglContext managedContext =
(Gl20RendererEglContext) sEglContextStorage.get();
// We do not have OpenGL objects
if (managedContext == null) {
return;
} else {
usePbufferSurface(managedContext.getContext());
}
if (level >= ComponentCallbacks2.TRIM_MEMORY_COMPLETE) {
GLES20Canvas.flushCaches(GLES20Canvas.FLUSH_CACHES_FULL);
} else if (level >= ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN) {
GLES20Canvas.flushCaches(GLES20Canvas.FLUSH_CACHES_MODERATE);
}
}

void Caches::flush(FlushMode mode) {
FLUSH_LOGD("Flushing caches (mode %d)", mode);
// We must stop tasks before clearing caches
if (mode > kFlushMode_Layers) {
tasks.stop();
}
switch (mode) {
case kFlushMode_Full:
textureCache.clear();
patchCache.clear();
dropShadowCache.clear();
gradientCache.clear();
fontRenderer->clear();
fboCache.clear();
dither.clear();
// fall through
case kFlushMode_Moderate:
fontRenderer->flush();
textureCache.flush();
pathCache.clear();
// fall through
case kFlushMode_Layers:
layerCache.clear();
renderBufferCache.clear();
break;
}
clearGarbage();
}

GLES20Canvas.flushCaches(GLES20Canvas.FLUSH_CACHES_MODERATE)对应的是kFlushMode_Moderate
GLES20Canvas.flushCaches(GLES20Canvas.FLUSH_CACHES_LAYERS)对应的是kFlushMode_Layers