本頁面由 Cloud Translation API 翻譯而成。

管理點陣圖記憶體

注意：在大多數情況下，建議您使用 Glide 程式庫在應用程式中擷取、解碼，並顯示點陣圖。Glide 在處理上述作業，以及其他在 Android 中使用點陣圖和其他圖片的相關任務時，大多能將複雜之處化繁為簡。如要瞭解如何使用和下載 Glide，請造訪 GitHub 的 Glide 存放區。

除了「快取點陣圖」中所述的步驟之外，您還可以採行一些措施進行垃圾收集和點陣圖重複使用。我們會依照您指定的 Android 版本，提供相應的建議策略。此類別包含的 BitmapFun 範例應用程式會說明如何設計應用程式，以在不同的 Android 版本中有效運作。

為輔助說明本課程，以下介紹 Android 強化點陣圖記憶體管理的過程：

在 Android 2.2 (API 級別 8) 以下版本中，發生垃圾收集時，應用程式的執行緒就會停止。這會引發延遲現象，進一步也可能造成效能下降。Android 2.3 於是新增了並行垃圾收集；這表示系統不再參照點陣圖後，會立即取回記憶體。
在 Android 2.3.3 (API 級別 10) 以下版本中，點陣圖的備份像素資料是儲存在原生記憶體中。這點與點陣圖不同，因為點陣圖是儲存在 Dalvik 堆積中。原生記憶體中的像素資料不會以可預測的方式釋出，而這可能會導致應用程式短暫超出記憶體限制和當機情況。從 Android 3.0 (API 級別 11) 到 Android 7.1 (API 級別 25)，像素資料都是與相關點陣圖一起儲存在 Dalvik 堆積中。在 Android 8.0 (API 級別 26) 以上版本中，點陣圖像素資料會儲存在原生堆積中。

以下章節說明如何針對不同的 Android 版本，讓點陣圖記憶體獲得最佳化管理。

管理 Android 2.3.3 以下版本的記憶體

在 Android 2.3.3 (API 級別 10) 以下版本中，建議使用 recycle()。如果要在應用程式中顯示大量的點陣圖資料，就可能會發生 OutOfMemoryError 錯誤。recycle() 方法可允許應用程式盡快收回記憶體。

注意：只有在確定不再使用點陣圖時，才應使用 recycle()。如果呼叫 recycle() 之後又嘗試繪製點陣圖，系統就會顯示錯誤訊息："Canvas: trying to use a recycled bitmap"。

下列程式碼片段提供呼叫 recycle() 的範例。這會使用參照計數 (在變數 mDisplayRefCount 和 mCacheRefCount 中) 追蹤點陣圖目前正在顯示或是快取中。如果符合以下條件，程式碼就會回收點陣圖：

mDisplayRefCount 和 mCacheRefCount 的參考計數都是 0。
點陣圖不是 null，且尚未回收。

Kotlin

private var cacheRefCount: Int = 0
private var displayRefCount: Int = 0
...
// Notify the drawable that the displayed state has changed.
// Keep a count to determine when the drawable is no longer displayed.
fun setIsDisplayed(isDisplayed: Boolean) {
    synchronized(this) {
        if (isDisplayed) {
            displayRefCount++
            hasBeenDisplayed = true
        } else {
            displayRefCount--
        }
    }
    // Check to see if recycle() can be called.
    checkState()
}

// Notify the drawable that the cache state has changed.
// Keep a count to determine when the drawable is no longer being cached.
fun setIsCached(isCached: Boolean) {
    synchronized(this) {
        if (isCached) {
            cacheRefCount++
        } else {
            cacheRefCount--
        }
    }
    // Check to see if recycle() can be called.
    checkState()
}

@Synchronized
private fun checkState() {
    // If the drawable cache and display ref counts = 0, and this drawable
    // has been displayed, then recycle.
    if (cacheRefCount <= 0
            && displayRefCount <= 0
            && hasBeenDisplayed
            && hasValidBitmap()
    ) {
        getBitmap()?.recycle()
    }
}

@Synchronized
private fun hasValidBitmap(): Boolean =
        getBitmap()?.run {
            !isRecycled
        } ?: false

Java

private int cacheRefCount = 0;
private int displayRefCount = 0;
...
// Notify the drawable that the displayed state has changed.
// Keep a count to determine when the drawable is no longer displayed.
public void setIsDisplayed(boolean isDisplayed) {
    synchronized (this) {
        if (isDisplayed) {
            displayRefCount++;
            hasBeenDisplayed = true;
        } else {
            displayRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

// Notify the drawable that the cache state has changed.
// Keep a count to determine when the drawable is no longer being cached.
public void setIsCached(boolean isCached) {
    synchronized (this) {
        if (isCached) {
            cacheRefCount++;
        } else {
            cacheRefCount--;
        }
    }
    // 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 (cacheRefCount <= 0 && displayRefCount <= 0 && hasBeenDisplayed
            && hasValidBitmap()) {
        getBitmap().recycle();
    }
}

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

管理 Android 3.0 以上版本的記憶體

Android 3.0 (API 級別 11) 加入了 BitmapFactory.Options.inBitmap 欄位。如果設定此選項，採用 Options 物件的解碼方法會在載入內容時，嘗試重複使用現有的點陣圖。這表示，系統會重複使用點陣圖的記憶體提升效能，並且移除記憶體配置和取消配置。不過，使用 inBitmap 有一些特定的限制。尤其是在 Android 4.4 (API 級別 19) 之前，系統僅支援大小相同的點陣圖。詳情請參閱 inBitmap 說明文件。

儲存點陣圖以供日後使用

下列程式碼片段說明如何儲存現有點陣圖，以供日後在範例應用程式中使用。如果應用程式是在 Android 3.0 以上版本中執行，而點陣圖從 LruCache 移除，則點陣圖的軟參照會放置在 HashSet 中，以便日後與 inBitmap 一起重複使用：

Kotlin

var reusableBitmaps: MutableSet<SoftReference<Bitmap>>? = null
private lateinit var memoryCache: LruCache<String, BitmapDrawable>
// If you're running on Honeycomb or newer, create a
// synchronized HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
    reusableBitmaps = Collections.synchronizedSet(HashSet<SoftReference<Bitmap>>())
}

memoryCache = object : LruCache<String, BitmapDrawable>(cacheParams.memCacheSize) {

    // Notify the removed entry that is no longer being cached.
    override fun entryRemoved(
            evicted: Boolean,
            key: String,
            oldValue: BitmapDrawable,
            newValue: BitmapDrawable
    ) {
        if (oldValue is RecyclingBitmapDrawable) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been removed from the memory cache.
            oldValue.setIsCached(false)
        } else {
            // The removed entry is a standard BitmapDrawable.
            if (Utils.hasHoneycomb()) {
                // We're running on Honeycomb or later, so add the bitmap
                // to a SoftReference set for possible use with inBitmap later.
                reusableBitmaps?.add(SoftReference(oldValue.bitmap))
            }
        }
    }
}

Java

Set<SoftReference<Bitmap>> reusableBitmaps;
private LruCache<String, BitmapDrawable> memoryCache;

// If you're running on Honeycomb or newer, create a
// synchronized HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
    reusableBitmaps =
            Collections.synchronizedSet(new HashSet<SoftReference<Bitmap>>());
}

memoryCache = new LruCache<String, BitmapDrawable>(cacheParams.memCacheSize) {

    // Notify the removed entry that is no longer being cached.
    @Override
    protected void entryRemoved(boolean evicted, String key,
            BitmapDrawable oldValue, BitmapDrawable newValue) {
        if (RecyclingBitmapDrawable.class.isInstance(oldValue)) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been removed from the memory cache.
            ((RecyclingBitmapDrawable) oldValue).setIsCached(false);
        } else {
            // The removed entry is a standard BitmapDrawable.
            if (Utils.hasHoneycomb()) {
                // We're running on Honeycomb or later, so add the bitmap
                // to a SoftReference set for possible use with inBitmap later.
                reusableBitmaps.add
                        (new SoftReference<Bitmap>(oldValue.getBitmap()));
            }
        }
    }
....
}

使用現有點陣圖

在執行中的應用程式，解碼器方法會檢查是否有可用的現有點陣圖。例如：

Kotlin

fun decodeSampledBitmapFromFile(
        filename: String,
        reqWidth: Int,
        reqHeight: Int,
        cache: ImageCache
): Bitmap {

    val options: BitmapFactory.Options = BitmapFactory.Options()
    ...
    BitmapFactory.decodeFile(filename, options)
    ...

    // If we're running on Honeycomb or newer, try to use inBitmap.
    if (Utils.hasHoneycomb()) {
        addInBitmapOptions(options, cache)
    }
    ...
    return BitmapFactory.decodeFile(filename, options)
}

Java

public static Bitmap decodeSampledBitmapFromFile(String filename,
        int reqWidth, int reqHeight, ImageCache cache) {

    final BitmapFactory.Options options = new BitmapFactory.Options();
    ...
    BitmapFactory.decodeFile(filename, options);
    ...

    // If we're running on Honeycomb or newer, try to use inBitmap.
    if (Utils.hasHoneycomb()) {
        addInBitmapOptions(options, cache);
    }
    ...
    return BitmapFactory.decodeFile(filename, options);
}

下一個程式碼片段顯示在上述程式碼片段中呼叫的 addInBitmapOptions() 方法；這種方法是嘗試尋找現有點陣圖做為 inBitmap 的值。請注意，此方法僅會在找到合適的點陣圖時才設定 inBitmap 值，您的程式碼不應假設為一定會找到合適的點陣圖：

Kotlin

private fun addInBitmapOptions(options: BitmapFactory.Options, cache: ImageCache?) {
    // inBitmap only works with mutable bitmaps, so force the decoder to
    // return mutable bitmaps.
    options.inMutable = true

    // Try to find a bitmap to use for inBitmap.
    cache?.getBitmapFromReusableSet(options)?.also { inBitmap ->
        // If a suitable bitmap has been found, set it as the value of
        // inBitmap.
        options.inBitmap = inBitmap
    }
}

// This method iterates through the reusable bitmaps, looking for one
// to use for inBitmap:
fun getBitmapFromReusableSet(options: BitmapFactory.Options): Bitmap? {
    mReusableBitmaps?.takeIf { it.isNotEmpty() }?.let { reusableBitmaps ->
        synchronized(reusableBitmaps) {
            val iterator: MutableIterator<SoftReference<Bitmap>> = reusableBitmaps.iterator()
            while (iterator.hasNext()) {
                iterator.next().get()?.let { item ->
                    if (item.isMutable) {
                        // Check to see it the item can be used for inBitmap.
                        if (canUseForInBitmap(item, options)) {
                            // Remove from reusable set so it can't be used again.
                            iterator.remove()
                            return item
                        }
                    } else {
                        // Remove from the set if the reference has been cleared.
                        iterator.remove()
                    }
                }
            }
        }
    }
    return null
}

Java

private static void addInBitmapOptions(BitmapFactory.Options options,
        ImageCache cache) {
    // inBitmap only works with mutable bitmaps, so force the decoder to
    // return mutable bitmaps.
    options.inMutable = true;

    if (cache != null) {
        // Try to find a bitmap to use for inBitmap.
        Bitmap inBitmap = cache.getBitmapFromReusableSet(options);

        if (inBitmap != null) {
            // If a suitable bitmap has been found, set it as the value of
            // inBitmap.
            options.inBitmap = inBitmap;
        }
    }
}

// This method iterates through the reusable bitmaps, looking for one
// to use for inBitmap:
protected Bitmap getBitmapFromReusableSet(BitmapFactory.Options options) {
        Bitmap bitmap = null;

    if (reusableBitmaps != null && !reusableBitmaps.isEmpty()) {
        synchronized (reusableBitmaps) {
            final Iterator<SoftReference<Bitmap>> iterator
                    = reusableBitmaps.iterator();
            Bitmap item;

            while (iterator.hasNext()) {
                item = iterator.next().get();

                if (null != item && item.isMutable()) {
                    // Check to see it the item can be used for inBitmap.
                    if (canUseForInBitmap(item, options)) {
                        bitmap = item;

                        // Remove from reusable set so it can't be used again.
                        iterator.remove();
                        break;
                    }
                } else {
                    // Remove from the set if the reference has been cleared.
                    iterator.remove();
                }
            }
        }
    }
    return bitmap;
}

最後，這個方法會判斷候選點陣圖的大小是否適合做為 inBitmap：

Kotlin

private fun canUseForInBitmap(candidate: Bitmap, targetOptions: BitmapFactory.Options): Boolean {
    return if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        // From Android 4.4 (KitKat) onward we can re-use if the byte size of
        // the new bitmap is smaller than the reusable bitmap candidate
        // allocation byte count.
        val width = ceil((targetOptions.outWidth * 1.0f / targetOptions.inSampleSize).toDouble()).toInt()
        val height = ceil((targetOptions.outHeight * 1.0f / targetOptions.inSampleSize).toDouble()).toInt()
        val byteCount: Int = width * height * getBytesPerPixel(candidate.config)
        byteCount <= candidate.allocationByteCount
    } else {
        // On earlier versions, the dimensions must match exactly and the inSampleSize must be 1
        candidate.width == targetOptions.outWidth
                && candidate.height == targetOptions.outHeight
                && targetOptions.inSampleSize == 1
    }
}

/**
 * A helper function to return the byte usage per pixel of a bitmap based on its configuration.
 */
private fun getBytesPerPixel(config: Bitmap.Config): Int {
    return when (config) {
        Bitmap.Config.ARGB_8888 -> 4
        Bitmap.Config.RGB_565, Bitmap.Config.ARGB_4444 -> 2
        Bitmap.Config.ALPHA_8 -> 1
        else -> 1
    }
}

Java

static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        // From Android 4.4 (KitKat) onward we can re-use if the byte size of
        // the new bitmap is smaller than the reusable bitmap candidate
        // allocation byte count.
        int width = (int) Math.ceil(targetOptions.outWidth * 1.0f / targetOptions.inSampleSize);
        int height = (int) Math.ceil(targetOptions.outHeight * 1.0f / targetOptions.inSampleSize);
        int byteCount = width * height * getBytesPerPixel(candidate.getConfig());
        return byteCount <= candidate.getAllocationByteCount();
    }

    // On earlier versions, the dimensions must match exactly and the inSampleSize must be 1
    return candidate.getWidth() == targetOptions.outWidth
            && candidate.getHeight() == targetOptions.outHeight
            && targetOptions.inSampleSize == 1;
}

/**
 * A helper function to return the byte usage per pixel of a bitmap based on its configuration.
 */
static int getBytesPerPixel(Config config) {
    if (config == Config.ARGB_8888) {
        return 4;
    } else if (config == Config.RGB_565) {
        return 2;
    } else if (config == Config.ARGB_4444) {
        return 2;
    } else if (config == Config.ALPHA_8) {
        return 1;
    }
    return 1;
}