Android获取Drawable的方式一般是Resources.getDrawable(int),Framework会返回给你一个顶层抽象的Drawable对象。而在Framework中,系统使用了享元的方式来节省内存。为了证明这一点,我们来写一个小demo:
我们在我们的Android项目中引入一个简单的图片test.png。由于我们只是为了享元的结论,我们定义一个简单的Activity,并复写它的onCreate方法:
List<Bitmap> list = new ArrayList<Bitmap>();
Bitmap bitmap = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
for (int i = 0; i < 10; i ++) {
bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.test);
list.add(bitmap);
}
ImageView iv = new ImageView(this);
iv.setImageBitmap(bitmap);
this.setContentView(iv);
}
可能你这里有疑惑为何要需要一个list把Bitmap存储起来,这重要是为了避免GC引起的内存释放。好了我们将我们的内存打印出来会发现我们加入了10个Bitmap占用的实际内存是:26364K。我们在转化成为Drawable的方式:
List<Drawable> list = new ArrayList<Drawable>();
Drawable bitmap = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
for (int i = 0; i < 10; i ++) {
bitmap = this.getResources().getDrawable(R.drawable.test);
list.add(bitmap);
}
ImageView iv = new ImageView(this);
iv.setImageDrawable(bitmap);
this.setContentView(iv);
}
我们再打印内存,发现内存已经降到了:7844K,这部分数据基本就证明了我们的结论。那么有没有可能是Resources缓存了相同的drawable。当然不是,你可以写一个简单代码测试一下:
Drawable d1 = this.getResources().getDrawable(R.drawable.test);
Drawable d2 = this.getResources().getDrawable(R.drawable.test);
System.out.println(">>>d1 == d2 ? = "+(d1 == d2));
你会发现输出的是false。实际上,享元这点我们基本达成了共识,关键Framwork来包装Drawable的时候还引入了组合模式,Framework本身缓存的是你这个Drawable的核心元数据。
Resources.java
Drawable loadDrawable(TypedValue value, int id)
throws NotFoundException {
...
Drawable dr = getCachedDrawable(isColorDrawable ? mColorDrawableCache : mDrawableCache, key);
...
}
从代码可以看出,系统对Drawable主要分成两大类,实际上还有一类熟悉预加载类的Drawable,不过不作为我们讨论的重点,由于我们load的并不属于color类型的Drawable,因此我们对应的享元池由mDrawableCache对象实现。
Resources.java
private Drawable getCachedDrawable(
LongSparseArray<WeakReference<ConstantState>> drawableCache,
long key) {
synchronized (mAccessLock) {
WeakReference<Drawable.ConstantState> wr = drawableCache.get(key);
if (wr != null) { // we have the key
Drawable.ConstantState entry = wr.get();
if (entry != null) {
//Log.i(TAG, "Returning cached drawable @ #" +
// Integer.toHexString(((Integer)key).intValue())
// + " in " + this + ": " + entry);
return entry.newDrawable(this);
}
else { // our entry has been purged
drawableCache.delete(key);
}
}
}
return null;
}
我们通过调用代码,会发现我们存储在数据池中的根本不是我们的Drawable对象,而是一个叫做Drawable.ConstantState类型的对象,而且用了弱引用包装起来。ConstantState是一个抽象类,有多个子类的实现
public static abstract class ConstantState {
/**
* Create a new drawable without supplying resources the caller
* is running in. Note that using this means the density-dependent
* drawables (like bitmaps) will not be able to update their target
* density correctly. One should use {@link #newDrawable(Resources)}
* instead to provide a resource.
*/
public abstract Drawable newDrawable();
/**
* Create a new Drawable instance from its constant state. This
* must be implemented for drawables that change based on the target
* density of their caller (that is depending on whether it is
* in compatibility mode).
*/
public Drawable newDrawable(Resources res) {
return newDrawable();
}
/**
* Return a bit mask of configuration changes that will impact
* this drawable (and thus require completely reloading it).
*/
public abstract int getChangingConfigurations();
/**
* @hide
*/
public Bitmap getBitmap() {
return null;
}
}
由于我们使用的是BitmapDrawable,而BitmapDrawable对应的ConstantState是BitmapState
final static class BitmapState extends ConstantState {
Bitmap mBitmap;
int mChangingConfigurations;
int mGravity = Gravity.FILL;
Paint mPaint = new Paint(DEFAULT_PAINT_FLAGS);
Shader.TileMode mTileModeX = null;
Shader.TileMode mTileModeY = null;
int mTargetDensity = DisplayMetrics.DENSITY_DEFAULT;
boolean mRebuildShader;
boolean mAutoMirrored;
BitmapState(Bitmap bitmap) {
mBitmap = bitmap;
}
BitmapState(BitmapState bitmapState) {
this(bitmapState.mBitmap);
mChangingConfigurations = bitmapState.mChangingConfigurations;
mGravity = bitmapState.mGravity;
mTileModeX = bitmapState.mTileModeX;
mTileModeY = bitmapState.mTileModeY;
mTargetDensity = bitmapState.mTargetDensity;
mPaint = new Paint(bitmapState.mPaint);
mRebuildShader = bitmapState.mRebuildShader;
mAutoMirrored = bitmapState.mAutoMirrored;
}
@Override
public Bitmap getBitmap() {
return mBitmap;
}
@Override
public Drawable newDrawable() {
return new BitmapDrawable(this, null);
}
@Override
public Drawable newDrawable(Resources res) {
return new BitmapDrawable(this, res);
}
@Override
public int getChangingConfigurations() {
return mChangingConfigurations;
}
}
我们可以看到BitmapState对应的newDrawable方法,它将自己作为参数传递给BitmapDrawable对象,也就是说BitmapDrawble组合了同一个的BitmapState。这样就实现了同一个Bitmap资源的复用。
跟到这,相信大家都跟我一样了解了Bitmap是如何从cache中取出,我们接下来看一下ConstantState是如何存入的。
Resources.loadDrawable()
{
...
InputStream is = mAssets.openNonAsset(
value.assetCookie, file, AssetManager.ACCESS_STREAMING);
// System.out.println("Opened file " + file + ": " + is);
// MIUI MOD:
// dr = Drawable.createFromResourceStream(this, value, is, file, null);
dr = createFromResourceStream(this, value, is, file, id);
is.close();
...
if (dr != null) {
dr.setChangingConfigurations(value.changingConfigurations);
cs = dr.getConstantState();
if (cs != null) {
if (mPreloading) {
final int changingConfigs = cs.getChangingConfigurations();
if (isColorDrawable) {
if (verifyPreloadConfig(changingConfigs, 0, value.resourceId,
"drawable")) {
sPreloadedColorDrawables.put(key, cs);
}
} else {
if (verifyPreloadConfig(changingConfigs,
LAYOUT_DIR_CONFIG, value.resourceId, "drawable")) {
if ((changingConfigs&LAYOUT_DIR_CONFIG) == 0) {
// If this resource does not vary based on layout direction,
// we can put it in all of the preload maps.
sPreloadedDrawables[0].put(key, cs);
sPreloadedDrawables[1].put(key, cs);
} else {
// Otherwise, only in the layout dir we loaded it for.
final LongSparseArray<Drawable.ConstantState> preloads
= sPreloadedDrawables[mConfiguration.getLayoutDirection()];
preloads.put(key, cs);
}
}
}
} else {
synchronized (mAccessLock) {
//Log.i(TAG, "Saving cached drawable @ #" +
// Integer.toHexString(key.intValue())
// + " in " + this + ": " + cs);
if (isColorDrawable) {
mColorDrawableCache.put(key, new WeakReference<Drawable.ConstantState>(cs));
} else {
mDrawableCache.put(key, new WeakReference<Drawable.ConstantState>(cs));
}
}
}
}
... }
可以看出,当你新生成一个Drawable的时候,就会将Drawable的ConstantState从Drawable中取出,然后放入你Cache池中。