点滴：ImageView setImageResource方法对资源的复用

从源代码分析，一步一步贴代码。

1，setImageResource 设置mResource

public void setImageResource(int resId) {        if (mUri != null || mResource != resId) {            updateDrawable(null);            mResource = resId;            mUri = null;            final int oldWidth = mDrawableWidth;            final int oldHeight = mDrawableHeight;            resolveUri();            if (oldWidth != mDrawableWidth || oldHeight != mDrawableHeight) {                requestLayout();            }            invalidate();        }    }

2，根据mResource加载Drawable

private void resolveUri() {        if (mDrawable != null) {            return;        }        Resources rsrc = getResources();        if (rsrc == null) {            return;        }        Drawable d = null;        if (mResource != 0) {            try {                d = rsrc.getDrawable(mResource);            } catch (Exception e) {                Log.w("ImageView", "Unable to find resource: " + mResource, e);                // Don't try again.                mUri = null;            }        }

3，Resource.java getDrawable调用loadDrawable

public Drawable getDrawable(int id) throws NotFoundException {        TypedValue value;        synchronized (mAccessLock) {            value = mTmpValue;            if (value == null) {                value = new TypedValue();            } else {                mTmpValue = null;            }            getValue(id, value, true);        }        Drawable res = loadDrawable(value, id);        synchronized (mAccessLock) {            if (mTmpValue == null) {                mTmpValue = value;            }        }        return res;    }

4，Resource.java  关键的在getCachedDrawable

Drawable loadDrawable(TypedValue value, int id)            throws NotFoundException {...        Drawable dr = getCachedDrawable(isColorDrawable ? mColorDrawableCache : mDrawableCache, key);        if (dr != null) {            return dr;        }        Drawable.ConstantState cs;        if (isColorDrawable) {            cs = sPreloadedColorDrawables.get(key);        } else {            cs = sPreloadedDrawables[mConfiguration.getLayoutDirection()].get(key);        }        if (cs != null) {            dr = cs.newDrawable(this);        } else {.......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;    }

5，BitmapDrawable

final static class BitmapState extends ConstantState {        Bitmap mBitmap;        int mChangingConfigurations;        int mGravity = Gravity.FILL;        Paint mPaint = new Paint(DEFAULT_PAINT_FLAGS);        Shader.TileMode mTileModeX;        Shader.TileMode mTileModeY;        int mTargetDensity = DisplayMetrics.DENSITY_DEFAULT;        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);        }        @Override        public Drawable newDrawable() {            return new BitmapDrawable(this);        }                @Override        public int getChangingConfigurations() {            return mChangingConfigurations;        }    }    private BitmapDrawable(BitmapState state) {        mBitmapState = state;        mTargetDensity = state.mTargetDensity;        setBitmap(state.mBitmap);    }

                                             
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