深刻剖析Android體系中SparseArray的源碼

深刻剖析Android體系中SparseArray的源碼。本站提示廣大學習愛好者：（深刻剖析Android體系中SparseArray的源碼）文章只能為提供參考，不一定能成為您想要的結果。以下是深刻剖析Android體系中SparseArray的源碼正文

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在1.zip中增長一張新圖片

StorageFile jpg = await KnownFolders.PicturesLibrary.GetFileAsync("1.jpg");
            StorageFile zip = await KnownFolders.PicturesLibrary.GetFileAsync("1.zip");

            //把下面這句改成以下就成了緊縮文件
            //StorageFile zip = await KnownFolders.PicturesLibrary.CreateFileAsync(jpg.DisplayName+".zip",CreationCollisionOption.WordStrExisting);
　　
　　
            using (ZipArchive archive = new ZipArchive((await zip.OpenAsync(FileAccessMode.ReadWrite)).AsStream(), ZipArchiveMode.Update))
            {
                ZipArchiveEntry readmeEntry = archive.CreateEntry(jpg.Name);
                byte[] buffer = WindowsRuntimeBufferExtensions.ToArray(await FileIO.ReadBufferAsync(jpg));
                using (var writer = readmeEntry.Open())
                {
                    await writer.WriteAsync(buffer, 0, buffer.Length);
                }
            }

把1.jpg從1.zip中刪除

StorageFile zip = await KnownFolders.PicturesLibrary.GetFileAsync("1.zip");
            using (ZipArchive archive = new ZipArchive((await zip.OpenAsync(FileAccessMode.ReadWrite)).AsStream(), ZipArchiveMode.Update))
            {
                //刪除文件
                archive.GetEntry("1.jpg").Delete();
            }

導出1.jpg,newFile為要到出的文件

StorageFile zip = await KnownFolders.PicturesLibrary.GetFileAsync("1.zip");
            using (ZipArchive archive = new ZipArchive((await zip.OpenAsync(FileAccessMode.ReadWrite)).AsStream(), ZipArchiveMode.Update))
            {
　　　　　　　　　　　　ZipArchiveEntry zipArchiveEntry = archive.GetEntry("1.jpg").
　　　　　　　　　　　　using (Stream fileData = zipArchiveEntry.Open())
                            {
                                StorageFile newFile = await KnownFolders.PicturesLibrary.CreateFileAsync(zipArchiveEntry.FullName, CreationCollisionOption.WordStrExisting);
                                using (IRandomAccessStream newFileStream = await newFile.OpenAsync(FileAccessMode.ReadWrite))
                                {
                                    using (Stream s = newFileStream.AsStreamForWrite())
                                    {
                                        await fileData.CopyToAsync(s);
                                        await s.FlushAsync();
                                    }
                                }
                            }
　　　　　　}

- i != 0) { // Log.e("SparseArray", "move " + (mSize - i)); System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i); System.arraycopy(mValues, i, mValues, i + 1, mSize - i); } mKeys[i] = key; mValues[i] = value; mSize++; } } /** * Returns the number of key-value mappings that this SparseArray * currently stores. */ public int size() { if (mGarbage) { gc(); } return mSize; } /** * Given an index in the range <code>0...size()-1</code>, returns * the key from the <code>index</code>th key-value mapping that this * SparseArray stores. * * <p>The keys corresponding to indices in ascending order are guaranteed to * be in ascending order, e.g., <code>keyAt(0)</code> will return the * smallest key and <code>keyAt(size()-1)</code> will return the largest * key.</p> */ public int keyAt(int index) { if (mGarbage) { gc(); } return mKeys[index]; } /** * Given an index in the range <code>0...size()-1</code>, returns * the value from the <code>index</code>th key-value mapping that this * SparseArray stores. * * <p>The values corresponding to indices in ascending order are guaranteed * to be associated with keys in ascending order, e.g., * <code>valueAt(0)</code> will return the value associated with the * smallest key and <code>valueAt(size()-1)</code> will return the value * associated with the largest key.</p> */ @SuppressWarnings("unchecked") public E valueAt(int index) { if (mGarbage) { gc(); } return (E) mValues[index]; } /** * Given an index in the range <code>0...size()-1</code>, sets a new * value for the <code>index</code>th key-value mapping that this * SparseArray stores. */ public void setValueAt(int index, E value) { if (mGarbage) { gc(); } mValues[index] = value; } /** * Returns the index for which {@link #keyAt} would return the * specified key, or a negative number if the specified * key is not mapped. */ public int indexOfKey(int key) { if (mGarbage) { gc(); } return ContainerHelpers.binarySearch(mKeys, mSize, key); } /** * Returns an index for which {@link #valueAt} would return the * specified key, or a negative number if no keys map to the * specified value. * <p>Beware that this is a linear search, unlike lookups by key, * and that multiple keys can map to the same value and this will * find only one of them. * <p>Note also that unlike most collections' {@code indexOf} methods, * this method compares values using {@code ==} rather than {@code equals}. */ public int indexOfValue(E value) { if (mGarbage) { gc(); } for (int i = 0; i < mSize; i++) if (mValues[i] == value) return i; return -1; } /** * Removes all key-value mappings from this SparseArray. */ public void clear() { int n = mSize; Object[] values = mValues; for (int i = 0; i < n; i++) { values[i] = null; } mSize = 0; mGarbage = false; } /** * Puts a key/value pair into the array, optimizing for the case where * the key is greater than all existing keys in the array. */ public void append(int key, E value) { if (mSize != 0 && key <= mKeys[mSize - 1]) { put(key, value); return; } if (mGarbage && mSize >= mKeys.length) { gc(); } int pos = mSize; if (pos >= mKeys.length) { int n = ArrayUtils.idealIntArraySize(pos + 1); int[] nkeys = new int[n]; Object[] nvalues = new Object[n]; // Log.e("SparseArray", "grow " + mKeys.length + " to " + n); System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length); System.arraycopy(mValues, 0, nvalues, 0, mValues.length); mKeys = nkeys; mValues = nvalues; } mKeys[pos] = key; mValues[pos] = value; mSize = pos + 1; } /** * {@inheritDoc} * * <p>This implementation composes a string by iterating over its mappings. If * this map contains itself as a value, the string "(this Map)" * will appear in its place. */ @Override public String toString() { if (size() <= 0) { return "{}"; } StringBuilder buffer = new StringBuilder(mSize * 28); buffer.append('{'); for (int i=0; i<mSize; i++) { if (i > 0) { buffer.append(", "); } int key = keyAt(i); buffer.append(key); buffer.append('='); Object value = valueAt(i); if (value != this) { buffer.append(value); } else { buffer.append("(this Map)"); } } buffer.append('}'); return buffer.toString(); } }

起首，看一下SparseArray的結構函數：

   

 /** 
   * Creates a new SparseArray containing no mappings. 
   */ 
  public SparseArray() { 
    this(10); 
  } 
   
  /** 
   * Creates a new SparseArray containing no mappings that will not 
   * require any additional memory allocation to store the specified 
   * number of mappings. If you supply an initial capacity of 0, the 
   * sparse array will be initialized with a light-weight representation 
   * not requiring any additional array allocations. 
   */ 
  public SparseArray(int initialCapacity) { 
    if (initialCapacity == 0) { 
      mKeys = ContainerHelpers.EMPTY_INTS; 
      mValues = ContainerHelpers.EMPTY_OBJECTS; 
    } else { 
      initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity); 
      mKeys = new int[initialCapacity]; 
      mValues = new Object[initialCapacity]; 
    } 
    mSize = 0; 
  } 

從結構辦法可以看出，這裡也是事後設置了容器的年夜小，默許年夜小為10。

再來看一下添加數據操作：

  /** 
   * Adds a mapping from the specified key to the specified value, 
   * replacing the previous mapping from the specified key if there 
   * was one. 
   */ 
  public void put(int key, E value) { 
    int i = ContainerHelpers.binarySearch(mKeys, mSize, key); 
   
    if (i >= 0) { 
      mValues[i] = value; 
    } else { 
      i = ~i; 
   
      if (i < mSize && mValues[i] == DELETED) { 
        mKeys[i] = key; 
        mValues[i] = value; 
        return; 
      } 
   
      if (mGarbage && mSize >= mKeys.length) { 
        gc(); 
   
        // Search again because indices may have changed. 
        i = ~ContainerHelpers.binarySearch(mKeys, mSize, key); 
      } 
   
      if (mSize >= mKeys.length) { 
        int n = ArrayUtils.idealIntArraySize(mSize + 1); 
   
        int[] nkeys = new int[n]; 
        Object[] nvalues = new Object[n]; 
   
        // Log.e("SparseArray", "grow " + mKeys.length + " to " + n); 
        System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length); 
        System.arraycopy(mValues, 0, nvalues, 0, mValues.length); 
   
        mKeys = nkeys; 
        mValues = nvalues; 
      } 
   
      if (mSize - i != 0) { 
        // Log.e("SparseArray", "move " + (mSize - i)); 
        System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i); 
        System.arraycopy(mValues, i, mValues, i + 1, mSize - i); 
      } 
   
      mKeys[i] = key; 
      mValues[i] = value; 
      mSize++; 
    } 
  } 

再看查數據的辦法：

  /** 
   * Gets the Object mapped from the specified key, or <code>null</code> 
   * if no such mapping has been made. 
   */ 
  public E get(int key) { 
    return get(key, null); 
  } 
   
  /** 
   * Gets the Object mapped from the specified key, or the specified Object 
   * if no such mapping has been made. 
   */ 
  @SuppressWarnings("unchecked") 
  public E get(int key, E valueIfKeyNotFound) { 
    int i = ContainerHelpers.binarySearch(mKeys, mSize, key); 
   
    if (i < 0 || mValues[i] == DELETED) { 
      return valueIfKeyNotFound; 
    } else { 
      return (E) mValues[i]; 
    } 
  } 

可以看到，在put數據和get數據的進程中，都同一挪用了一個二分查找算法，其實這也就是SparseArray可以或許晉升效力的焦點。

   

 static int binarySearch(int[] array, int size, int value) { 
    int lo = 0; 
    int hi = size - 1; 
   
    while (lo <= hi) { 
      final int mid = (lo + hi) >>> 1; 
      final int midVal = array[mid]; 
   
      if (midVal < value) { 
        lo = mid + 1; 
      } else if (midVal > value) { 
        hi = mid - 1; 
      } else { 
        return mid; // value found 
      } 
    } 
    return ~lo; // value not present 
  } 

小我以為（lo + hi） >>> 1的辦法有些奇異，直接用 lo + (hi - lo) / 2更好一些。