Bitmap處理之創建可自動回收資源的ImageView

1.創建可自動回收資源的BitmapDrawable

繼承BitmapDrawable，設置標誌位 mCacheRefCount cache計數，mDisplayRefCount 顯示計數，mHasBeenDisplayed判斷是否已經顯示過了具體的code：

    private int mCacheRefCount = 0;  //cache計數
    private int mDisplayRefCount = 0; //顯示計數

    private boolean mHasBeenDisplayed;//是否已經顯示過

判斷bitmap是否還存在並且可用

    private synchronized boolean hasValidBitmap() {
        Bitmap bitmap = getBitmap();
        return bitmap != null && !bitmap.isRecycled();
    }

判斷bitmap的狀態

    private synchronized void checkState() {
        // If the drawable cache and display ref counts = 0, and this drawable
        // has been displayed, then recycle
        if (mCacheRefCount <= 0 && mDisplayRefCount <= 0 && mHasBeenDisplayed
                && hasValidBitmap()) {

            getBitmap().recycle();
        }
    }

通過cache計數，顯示計數，是否已經顯示，以及bitmap是否還存在，如果四個條件都滿足，就顯式的回收bitmap資源

判斷bitmap是否已經cache

    public void setIsCached(boolean isCached) {
        synchronized (this) {
            if (isCached) {
                mCacheRefCount++;
            } else {
                mCacheRefCount--;
            }
        }

        // Check to see if recycle() can be called
        checkState();
    }

調用的地方是，如果已經將某個bitmap加入了cache(memory cache,disk cache)setIsCached(true)這時cache計數就會增加1，如果某個bitmap從cache中刪除，就setIsCached(false)這時cache計數就減去1，然後調用checkState()方法，來判斷bitmap是否需要回收

判斷bitmap是否已經顯示

    public void setIsDisplayed(boolean isDisplayed) {
        synchronized (this) {
            if (isDisplayed) {
                mDisplayRefCount++;
                mHasBeenDisplayed = true;
            } else {
                mDisplayRefCount--;
            }
        }

        // Check to see if recycle() can be called
        checkState();
    }

完整的代碼

public class RecyclingBitmapDrawable extends BitmapDrawable {

    static final String LOG_TAG = "CountingBitmapDrawable";

    private int mCacheRefCount = 0;
    private int mDisplayRefCount = 0;

    private boolean mHasBeenDisplayed;

    public RecyclingBitmapDrawable(Resources res, Bitmap bitmap) {
        super(res, bitmap);
    }

    /**
     * Notify the drawable that the displayed state has changed. Internally a
     * count is kept so that the drawable knows when it is no longer being
     * displayed.
     *
     * @param isDisplayed - Whether the drawable is being displayed or not
     */
    public void setIsDisplayed(boolean isDisplayed) {
        synchronized (this) {
            if (isDisplayed) {
                mDisplayRefCount++;
                mHasBeenDisplayed = true;
            } else {
                mDisplayRefCount--;
            }
        }

        // Check to see if recycle() can be called
        checkState();
    }

    /**
     * Notify the drawable that the cache state has changed. Internally a count
     * is kept so that the drawable knows when it is no longer being cached.
     *
     * @param isCached - Whether the drawable is being cached or not
     */
    public void setIsCached(boolean isCached) {
        synchronized (this) {
            if (isCached) {
                mCacheRefCount++;
            } else {
                mCacheRefCount--;
            }
        }

        // Check to see if recycle() can be called
        checkState();
    }

    private synchronized void checkState() {
        // If the drawable cache and display ref counts = 0, and this drawable
        // has been displayed, then recycle
        if (mCacheRefCount <= 0 && mDisplayRefCount <= 0 && mHasBeenDisplayed
                && hasValidBitmap()) {
            if (BuildConfig.DEBUG) {
                Log.d(LOG_TAG, "No longer being used or cached so recycling. "
                        + toString());
            }

            getBitmap().recycle();
        }
    }

    private synchronized boolean hasValidBitmap() {
        Bitmap bitmap = getBitmap();
        return bitmap != null && !bitmap.isRecycled();
    }

}

2.創建可自動回收資源的ImageView

基於RecyclingBitmapDrawable來實現 RecyclingImageView

具體實現

重載Imageiew的setImageDrawable()方法

    public void setImageDrawable(Drawable drawable) {
        // Keep hold of previous Drawable
        final Drawable previousDrawable = getDrawable();

        // Call super to set new Drawable
        super.setImageDrawable(drawable);

        // Notify new Drawable that it is being displayed
        notifyDrawable(drawable, true);

        // Notify old Drawable so it is no longer being displayed
        notifyDrawable(previousDrawable, false);
    }

在設置時，顯獲得前一個drawable資源，然後發出通知，通知的是實現

    private static void notifyDrawable(Drawable drawable, final boolean isDisplayed) {
        if (drawable instanceof RecyclingBitmapDrawable) {
            // The drawable is a CountingBitmapDrawable, so notify it
            ((RecyclingBitmapDrawable) drawable).setIsDisplayed(isDisplayed);
        } else if (drawable instanceof LayerDrawable) {
            // The drawable is a LayerDrawable, so recurse on each layer
            LayerDrawable layerDrawable = (LayerDrawable) drawable;
            for (int i = 0, z = layerDrawable.getNumberOfLayers(); i < z; i++) {
                notifyDrawable(layerDrawable.getDrawable(i), isDisplayed);
            }
        }
    }

主要是調用RecyclingBitmapDrawable的setIsDisplayed方法，在通過ImageView設置時，當前的Drawable的顯示計數加1，而前一個Drawable資源的顯示計數減1，然後檢查狀態，這樣前一個Drawable資源就有可能被回收

http://developer.android.com/training/displaying-bitmaps/process-bitmap.html#concurrency