Question

Does Android maintain a memory cache of application drawable resources and reuse them, or it is a good practice to preload all drawables that may be assigned dynamically to different widgets?

For instance:

public static final int[] SETS = {
        R.drawable.set0, R.drawable.set1, R.drawable.set2,
        R.drawable.set3, R.drawable.set4, R.drawable.set5, R.drawable.set6,
        R.drawable.set7, R.drawable.set8, R.drawable.set9, R.drawable.set10};
public Drawable[] sets;

void init() {
    load(sets, SETS);
}

public void load(Drawable[] d, int[] ids) {
    for (int i = 0; i < ids.length; i++) {
        if (ids[i] == 0)
            d[i] = null;
        else
            d[i] = context.getResources().getDrawable(ids[i]);
    }
}

Answer 1

This smells like an unnecessary pre-optimization. However, android does cache drawables so you don’t have to pre-load them. Relevant code from ApplicationContext

  /*package*/ Drawable loadDrawable(TypedValue value, int id)
            throws NotFoundException {
        .
        .
        .

        final long key = (((long) value.assetCookie) << 32) | value.data;
        Drawable dr = getCachedDrawable(key);

        if (dr != null) {
            return dr;
        }

        .
        .
        .

        if (dr != null) {
            dr.setChangingConfigurations(value.changingConfigurations);
            cs = dr.getConstantState();
            if (cs != null) {
                if (mPreloading) {
                    sPreloadedDrawables.put(key, cs);
                } else {
                    synchronized (mTmpValue) {
                        //Log.i(TAG, "Saving cached drawable @ #" +
                        //        Integer.toHexString(key.intValue())
                        //        + " in " + this + ": " + cs);
                        mDrawableCache.put(key, new WeakReference<Drawable.ConstantState>(cs));
                    }
                }
            }
        }

        return dr;
    }

    private Drawable getCachedDrawable(long key) {
        synchronized (mTmpValue) {
            WeakReference<Drawable.ConstantState> wr = mDrawableCache.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
                    mDrawableCache.delete(key);
                }
            }
        }
        return null;
    }