在 Android 應(yīng)用中加載位圖比較復(fù)雜，原因有很多種：

• 位圖很容易就會(huì)耗盡應(yīng)用的內(nèi)存預(yù)算撰洗。例如短条，Pixel 手機(jī)上的相機(jī)拍攝的照片最大可達(dá) 4048x3036 像素（1200 萬(wàn)像素）剑辫。如果使用的位圖配置為 [ARGB_8888](https://developer.android.com/reference/android/graphics/Bitmap.Config)（Android 2.3（API 級(jí)別 9）及更高版本的默認(rèn)設(shè)置）阱飘，將單張照片加載到內(nèi)存大約需要 48MB 內(nèi)存（404830364 字節(jié)）摘昌。如此龐大的內(nèi)存需求可能會(huì)立即耗盡該應(yīng)用的所有可用內(nèi)存速妖。

• 在界面線程中加載位圖會(huì)降低應(yīng)用的性能，導(dǎo)致響應(yīng)速度變慢第焰，甚至?xí)?dǎo)致系統(tǒng)顯示 ANR 消息买优。因此，在使用位圖時(shí)挺举，必須正確地管理線程處理。
  單個(gè)像素的字節(jié)大小
  單個(gè)像素的字節(jié)大小由Bitmap的一個(gè)可配置的參數(shù)Config來(lái)決定烘跺。 Bitmap中湘纵，存在一個(gè)枚舉類Config，定義了Android中支持的Bitmap配置:

  
  
  不同config單個(gè)像素字節(jié)大小.png
  
  

  Bitmap加載方式
  Bitmap 的加載方式有 Resource 資源加載滤淳、本地(SDcard)加載梧喷、網(wǎng)絡(luò)加載等加載方式。

1. 從本地(SDcard)文件讀取
   方式一
   /**

• 獲取縮放后的本地圖片 *
• @param filePath 文件路徑
• @param width 寬
• @param height 高 * @return
  /
  public static Bitmap readBitmapFromFile(String filePath, int width, int height) {
  BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true;
  BitmapFactory.decodeFile(filePath, options);
  float srcWidth = options.outWidth;
  float srcHeight = options.outHeight; int inSampleSize = 1;
  if (srcHeight > height || srcWidth > width) {
  if (srcWidth > srcHeight) {
  inSampleSize = Math.round(srcHeight / height); } else {
  inSampleSize = Math.round(srcWidth / width); }
  }
  options.inJustDecodeBounds = false; options.inSampleSize = inSampleSize;
  options.inSampleSize = inSampleSize;
  return BitmapFactory.decodeFile(filePath, options);
  }
  方式二 (效率高于方式一)
  /*
• 獲取縮放后的本地圖片 *
• @param filePath 文件路徑
• @param width 寬
• @param height 高 * @return
  */
  public static Bitmap readBitmapFromFileDescriptor(String filePath, int
  width, int height) {
  try {
  FileInputStream fis = new FileInputStream(filePath); BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeFileDescriptor(fis.getFD(), null, options); float srcWidth = options.outWidth;
  float srcHeight = options.outHeight;
  int inSampleSize = 1;
  if (srcHeight > height || srcWidth > width) {
  if (srcWidth > srcHeight) {
  inSampleSize = Math.round(srcHeight / height); } else {
  inSampleSize = Math.round(srcWidth / width); }
  }
  options.inJustDecodeBounds = false;
  options.inSampleSize = inSampleSize;
  return BitmapFactory.decodeFileDescriptor(fis.getFD(), null, options);
  } catch (Exception ex) {
  }
  return null;
  }

2. 從輸入流中讀取文件(網(wǎng)絡(luò)加載)
   /**

• 獲取縮放后的本地圖片 *
• @param ins 輸入流
• @param width 寬
• @param height 高 * @return
  /
  public static Bitmap readBitmapFromInputStream(InputStream ins, int width,int height) {
  BitmapFactory.Options options = new BitmapFactory.Options();
  options.inJustDecodeBounds = true;
  BitmapFactory.decodeStream(ins, null, options); float srcWidth = options.outWidth;
  float srcHeight = options.outHeight;
  int inSampleSize = 1;
  if (srcHeight > height || srcWidth > width) {
  if (srcWidth > srcHeight) {
  inSampleSize = Math.round(srcHeight / height); } else {
  inSampleSize = Math.round(srcWidth / width); }
  }
  options.inJustDecodeBounds = false;
  options.inSampleSize = inSampleSize;
  return BitmapFactory.decodeStream(ins, null, options);
  }
  3.Resource資源加載
  Res資源加載方式:
  public static Bitmap readBitmapFromResource(Resources resources, int
  resourcesId, int width, int height) {
  BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeResource(resources, resourcesId, options); float srcWidth = options.outWidth;
  float srcHeight = options.outHeight; int inSampleSize = 1;
  if (srcHeight > height || srcWidth > width) {
  if (srcWidth > srcHeight) {
  inSampleSize = Math.round(srcHeight / height); } else {
  inSampleSize = Math.round(srcWidth / width); }
  }
  options.inJustDecodeBounds = false;
  options.inSampleSize = inSampleSize;
  return BitmapFactory.decodeResource(resources, resourcesId, options);
  }
  此種方式相當(dāng)?shù)暮馁M(fèi)內(nèi)存 建議采用 decodeStream 代替 decodeResource 可以如下形式:
  public static Bitmap readBitmapFromResource(Resources resources, int
  resourcesId, int width, int height) {
  InputStream ins = resources.openRawResource(resourcesId); BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeStream(ins, null, options);
  float srcWidth = options.outWidth; float srcHeight = options.outHeight; int inSampleSize = 1;
  if (srcHeight > height || srcWidth > width) {
  if (srcWidth > srcHeight) {
  inSampleSize = Math.round(srcHeight / height); } else {
  inSampleSize = Math.round(srcWidth / width); }
  }
  options.inJustDecodeBounds = false;
  options.inSampleSize = inSampleSize;
  return BitmapFactory.decodeStream(ins, null, options);
  }
  BitmapFactory.decodeResource 加載的圖片可能會(huì)經(jīng)過(guò)縮放，該縮放目前是放在 java 層做的铺敌，效率 比較低汇歹，而且需要消耗 java 層的內(nèi)存。因此偿凭，如果大量使用該接口加載圖片产弹，容易導(dǎo)致OOM錯(cuò)誤
  BitmapFactory.decodeStream 不會(huì)對(duì)所加載的圖片進(jìn)行縮放，相比之下占用內(nèi)存少弯囊，效率更高痰哨。 這兩個(gè)接口各有用處，如果對(duì)性能要求較高匾嘱，則應(yīng)該使用 decodeStream ;如果對(duì)性能要求不高斤斧，且需 要 Android 自帶的圖片自適應(yīng)縮放功能，則可以使用 decodeResource 霎烙。
  4.Assets資源加載方式:
  /*
• 獲取縮放后的本地圖片
• @param filePath 文件路徑,即文件名稱 * @return
  /
  public static Bitmap readBitmapFromAssetsFile(Context context, String
  filePath) {
  Bitmap image = null;
  AssetManager am = context.getResources().getAssets(); try {
  InputStream is = am.open(filePath); image = BitmapFactory.decodeStream(is); is.close();
  } catch (IOException e) { e.printStackTrace();
  }
  return image;
  }
  5.從二進(jìn)制數(shù)據(jù)讀取圖片
  public static Bitmap readBitmapFromByteArray(byte[] data, int width, int
  height) {
  BitmapFactory.Options options = new BitmapFactory.Options(); options.inJustDecodeBounds = true; BitmapFactory.decodeByteArray(data, 0, data.length, options); float srcWidth = options.outWidth;
  float srcHeight = options.outHeight; int inSampleSize = 1;
  if (srcHeight > height || srcWidth > width) {
  if (srcWidth > srcHeight) {
  inSampleSize = Math.round(srcHeight / height); } else {
  inSampleSize = Math.round(srcWidth / width); }
  }
  options.inJustDecodeBounds = false;
  options.inSampleSize = inSampleSize;
  return BitmapFactory.decodeByteArray(data, 0, data.length, options);
  }
  Bitmap | Drawable | InputStream | Byte[ ] 之間進(jìn)行轉(zhuǎn)換
  1.Drawable轉(zhuǎn)化成Bitmap
  public static Bitmap drawableToBitmap(Drawable drawable) {
  Bitmap bitmap = Bitmap.createBitmap(drawable.getIntrinsicWidth(),
  drawable.getIntrinsicHeight(),
  drawable.getOpacity() != PixelFormat.OPAQUE ?
  Bitmap.Config.ARGB_8888 : Bitmap.Config.RGB_565);
  Canvas canvas = new Canvas(bitmap); drawable.setBounds(0, 0, drawable.getIntrinsicWidth(),
  drawable.getIntrinsicHeight()); drawable.draw(canvas); return bitmap;
  }
  drawable 的獲取方式: Drawable drawable=getResources().getDrawable(R.drawable.ic_launcher);
  2.Bitmap轉(zhuǎn)換成Drawable
  public static Drawable bitmapToDrawable(Resources resources, Bitmap bm) {
  Drawable drawable = new BitmapDrawable(resources, bm);
  return drawable;
  }
  3.Bitmap轉(zhuǎn)換成byte[ ]
  public byte[] bitmap2Bytes(Bitmap bm) { ByteArrayOutputStream baos = new ByteArrayOutputStream(); bm.compress(Bitmap.CompressFormat.PNG, 100, baos);
  return baos.toByteArray();
  }
  4.byte[]轉(zhuǎn)換成Bitmap
  Bitmapbitmap=BitmapFactory.decodeByteArray(byte,0,b.length);
  5.InputStream轉(zhuǎn)換成Bitmap
  InputStreamis=getResources().openRawResource(id); Bitmapbitmap=BitmaoFactory.decodeStream(is);
  6.InputStream轉(zhuǎn)換成byte[]
  InputStream is = getResources().openRawResource(id);//也可以通過(guò)其他方式接收一個(gè) InputStream對(duì)象
  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  byte[] b = new byte[10242];
  int len = 0;
  while ((len = is.read(b, 0, b.length)) != -1) {
  baos.write(b, 0, len);
  baos.flush(); }
  byte[] bytes = baos.toByteArray();
  Bitmap常用操作
  1.將Bitmap保存為本地文件:
  public static void writeBitmapToFile(String filePath, Bitmap b, int quality)
  {
  try {
  File desFile = new File(filePath);
  FileOutputStream fos = new FileOutputStream(desFile); BufferedOutputStream bos = new BufferedOutputStream(fos); b.compress(Bitmap.CompressFormat.JPEG, quality, bos); bos.flush();
  bos.close();
  } catch (IOException e) { e.printStackTrace();
  } }
  2.圖片壓縮:
  private static Bitmap compressImage(Bitmap image) {
  if (image == null) {
  return null;
  }
  ByteArrayOutputStream baos = null;
  try {
  baos = new ByteArrayOutputStream(); image.compress(Bitmap.CompressFormat.JPEG, 50, baos);
  byte[] bytes = baos.toByteArray();
  ByteArrayInputStream isBm = new ByteArrayInputStream(bytes); Bitmap bitmap = BitmapFactory.decodeStream(isBm);
  return bitmap;
  } catch (OutOfMemoryError e) {
  } finally {
  try {
  if (baos != null) {
  baos.close(); }
  } catch (IOException e) {
  } }
  return null;
  }
  3.圖片縮放:
  /**
• 根據(jù)scale生成一張圖片
• @param bitmap
• @param scale 等比縮放值 * @return
  /
  public static Bitmap bitmapScale(Bitmap bitmap, float scale) {
  Matrix matrix = new Matrix();
  matrix.postScale(scale, scale); // 長(zhǎng)和寬放大縮小的比例
  Bitmap resizeBmp = Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(),
  bitmap.getHeight(), matrix, true);
  return resizeBmp;
  }
  4.獲取圖片旋轉(zhuǎn)角度:
  /*
• 讀取照片exif信息中的旋轉(zhuǎn)角度 *
• @param path 照片路徑

• @return角度
  */
  private static int readPictureDegree(String path) { if (TextUtils.isEmpty(path)) {
  return 0; }
  int degree = 0;
  try {
  ExifInterface exifInterface = new ExifInterface(path);
  int orientation = exifInterface.getAttributeInt(ExifInterface.TAG_ORIENTATION,
  ExifInterface.ORIENTATION_NORMAL); switch (orientation) {
  case ExifInterface.ORIENTATION_ROTATE_90: degree = 90;
  break;
  case ExifInterface.ORIENTATION_ROTATE_180:
  degree = 180;
  break;
  case ExifInterface.ORIENTATION_ROTATE_270:
  degree = 270;
  break; }
  } catch (Exception e) {
  }
  return degree;
  }
  5.設(shè)置圖片旋轉(zhuǎn)角度
  private static Bitmap rotateBitmap(Bitmap b, float rotateDegree) {
  if (b == null) {
  return null;
  }
  Matrix matrix = new Matrix(); matrix.postRotate(rotateDegree);
  Bitmap rotaBitmap = Bitmap.createBitmap(b, 0, 0, b.getWidth(),
  b.getHeight(), matrix, true); return rotaBitmap;
  }
  6.通過(guò)圖片id獲得Bitmap:
  Bitmapbitmap=BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
  7.通過(guò) assest 獲取 獲得Drawable bitmap:
  InputStream in = this.getAssets().open("ic_launcher");
  Drawable da = Drawable.createFromStream(in, null);
  Bitmap mm = BitmapFactory.decodeStream(in);
  8.通過(guò) sdcard 獲得 bitmap
  1 Bitmapbit=BitmapFactory.decodeFile("/sdcard/android.jpg");
  9.view轉(zhuǎn)Bitmap
  publicstaticBitmapconvertViewToBitmap(Viewview,intbitmapWidth,int bitmapHeight){
  Bitmap bitmap = Bitmap.createBitmap(bitmapWidth, bitmapHeight, Bitmap.Config.ARGB_8888);
  view.draw(new Canvas(bitmap));
  return bitmap;
  }
  10.將控件轉(zhuǎn)換為bitmap
  public static Bitmap convertViewToBitMap(View view){ // 打開(kāi)圖像緩存
  view.setDrawingCacheEnabled(true);
  // 必須調(diào)用measure和layout方法才能成功保存可視組件的截圖到png圖像文件
  // 測(cè)量View大小
  view.measure(MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED),
  MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED));
  // 發(fā)送位置和尺寸到View及其所有的子View
  view.layout(0, 0, view.getMeasuredWidth(), view.getMeasuredHeight()); // 獲得可視組件的截圖
  Bitmap bitmap = view.getDrawingCache();
  return bitmap;
  }
  public static Bitmap getBitmapFromView(View view){
  Bitmap returnedBitmap = Bitmap.createBitmap(view.getWidth(),
  view.getHeight(), Bitmap.Config.ARGB_8888); Canvas canvas = new Canvas(returnedBitmap); Drawable bgDrawable = view.getBackground(); if (bgDrawable != null)
  bgDrawable.draw(canvas); else
  canvas.drawColor(Color.WHITE); view.draw(canvas);
  return returnedBitmap;
  }
  11.放大縮小圖片
  public static Bitmap zoomBitmap(Bitmap bitmap,int w,int h){ int width = bitmap.getWidth();
  int height = bitmap.getHeight();
  Matrix matrix = new Matrix();
  float scaleWidht = ((float)w / width);
  float scaleHeight = ((float)h / height);
  matrix.postScale(scaleWidht, scaleHeight);
  Bitmap newbmp = Bitmap.createBitmap(bitmap, 0, 0, width, height, matrix,
  true);
  return newbmp;
  }
  12.獲得圓角圖片的方法
  public static Bitmap getRoundedCornerBitmap(Bitmap bitmap,float roundPx){
  Bitmap output = Bitmap.createBitmap(bitmap.getWidth(), bitmap .getHeight(), Config.ARGB_8888);
  Canvas canvas = new Canvas(output);
  final int color = 0xff424242;
  final Paint paint = new Paint();
  final Rect rect = new Rect(0, 0, bitmap.getWidth(), bitmap.getHeight()); final RectF rectF = new RectF(rect);
  paint.setAntiAlias(true);
  canvas.drawARGB(0, 0, 0, 0);
  paint.setColor(color);
  canvas.drawRoundRect(rectF, roundPx, roundPx, paint);
  paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN)); canvas.drawBitmap(bitmap, rect, rect, paint);
  return output;
  }
  13.對(duì) bitmap 進(jìn)行裁剪
  public Bitmap bitmapClip(Context context , int id , int x , int y){
  Bitmap map = BitmapFactory.decodeResource(context.getResources(), id); map = Bitmap.createBitmap(map, x, y, 120, 120);
  return map;
  }
  Bitmap內(nèi)存模型

  
  
  內(nèi)存模型.png
  
  

  Bitmap的內(nèi)存回收

1. 在Android2.3.3之前推薦使用Bitmap.recycle()方法進(jìn)行Bitmap的內(nèi)存回收撬讽。
   備注:只有當(dāng)確定這個(gè)Bitmap不被引用的時(shí)候才能調(diào)用此方法，否則會(huì)有“Canvas: trying to use a recycled bitmap”這個(gè)錯(cuò)誤悬垃。
2. Android3.0之后
   Android3.0之后游昼，并沒(méi)有強(qiáng)調(diào)Bitmap.recycle();而是強(qiáng)調(diào)Bitmap的復(fù)用
   Save a bitmap for later use
   使用LruCache對(duì)Bitmap進(jìn)行緩存**，當(dāng)再次使用到這個(gè)Bitmap的時(shí)候直接獲取盗忱，而不用重走編碼流程酱床。
   Use an existing bitmap
   Android3.0(API 11之后)引入了BitmapFactory.Options.inBitmap字段，設(shè)置此字段之后解 碼方法會(huì)嘗試復(fù)用一張存在的Bitmap趟佃。這意味著B(niǎo)itmap的內(nèi)存被復(fù)用扇谣，避免了內(nèi)存的回收 及申請(qǐng)過(guò)程，顯然性能表現(xiàn)更佳闲昭。不過(guò)罐寨，使用這個(gè)字段有幾點(diǎn)限制:
   聲明可被復(fù)用的Bitmap必須設(shè)置inMutable為true;
   Android4.4(API 19)之前只有格式為jpg、png序矩，同等寬高(要求苛刻)鸯绿，inSampleSize為1的 Bitmap才可以復(fù)用;
   Android4.4(API 19)之前被復(fù)用的Bitmap的inPreferredConfig會(huì)覆蓋待分配內(nèi)存的Bitmap設(shè) 置的inPreferredConfig;
   Android4.4(API 19)之后被復(fù)用的Bitmap的內(nèi)存必須大于需要申請(qǐng)內(nèi)存的Bitmap的內(nèi)存; Android4.4(API 19)之前待加載Bitmap的Options.inSampleSize必須明確指定為1
   獲取Bitmap的大小
3. getByteCount()
   getByteCount()方法是在API12加入的，代表存儲(chǔ)Bitmap的色素需要的最少內(nèi)存簸淀。API19開(kāi)始 getAllocationByteCount()方法代替了getByteCount()瓶蝴。
4. getAllocationByteCount()
   API19之后，Bitmap加了一個(gè)Api:getAllocationByteCount();代表在內(nèi)存中為Bitmap分配的內(nèi)存大
   小租幕。
   public final int getAllocationByteCount() {
   if (mBuffer == null) {
   //mBuffer代表存儲(chǔ)Bitmap像素?cái)?shù)據(jù)的字節(jié)數(shù)組舷手。
   return getByteCount();
   }
   return mBuffer.length; }
5. getByteCount()與getAllocationByteCount()的區(qū)別
   一般情況下兩者是相等的
   通過(guò)復(fù)用Bitmap來(lái)解碼圖片，如果被復(fù)用的Bitmap的內(nèi)存比待分配內(nèi)存的Bitmap大,那么 getByteCount()表示新解碼圖片占用內(nèi)存的大小(并非實(shí)際內(nèi)存大小,實(shí)際大小是復(fù)用的那個(gè) Bitmap的大小)劲绪，getAllocationByteCount()表示被復(fù)用Bitmap真實(shí)占用的內(nèi)存大小(即mBuffer 的長(zhǎng)度)
   Bitmap占用內(nèi)存大小計(jì)算
   Bitmap作為位圖男窟，需要讀入一張圖片每一個(gè)像素點(diǎn)的數(shù)據(jù)盆赤，其主要占用內(nèi)存的地方也正是這些像素?cái)?shù) 據(jù)。對(duì)于像素?cái)?shù)據(jù)總大小歉眷，我們可以猜想為:像素總數(shù)量 × 每個(gè)像素的字節(jié)大小牺六，而像素總數(shù)量在矩形 屏幕表現(xiàn)下，應(yīng)該是:橫向像素?cái)?shù)量 × 縱向像素?cái)?shù)量汗捡，結(jié)合得到:
   Bitmap內(nèi)存占用 ≈ 像素?cái)?shù)據(jù)總大小 = 橫向像素?cái)?shù)量 × 縱向像素?cái)?shù)量 × 每個(gè)像素的字節(jié)大小
   if (env->GetBooleanField(options, gOptions_scaledFieldID)) {
   const int density = env->GetIntField(options, gOptions_densityFieldID);
   const int targetDensity = env->GetIntField(options, gOptions_targetDensityFieldID);
   const int screenDensity = env->GetIntField(options, gOptions_screenDensityFieldID);
   if (density != 0 && targetDensity != 0 && density != screenDensity) {
   scale = (float) targetDensity / density;
   }
   }
   ...
   int scaledWidth = decoded->width();
   int scaledHeight = decoded->height();

if (willScale && mode != SkImageDecoder::kDecodeBounds_Mode) {
scaledWidth = int(scaledWidth * scale + 0.5f);
scaledHeight = int(scaledHeight * scale + 0.5f);
}
...
if (willScale) {
const float sx = scaledWidth / float(decoded->width());
const float sy = scaledHeight / float(decoded->height());
bitmap->setConfig(decoded->getConfig(), scaledWidth, scaledHeight);
bitmap->allocPixels(&javaAllocator, NULL);
bitmap->eraseColor(0);
SkPaint paint;
paint.setFilterBitmap(true);
SkCanvas canvas(bitmap);
canvas.scale(sx, sy);
canvas.drawBitmap(decoded, 0.0f, 0.0f, &paint);
}
從上述代碼中淑际，我們看到bitmap最終通過(guò)canvas繪制出來(lái)，而canvas在繪制之前凉唐，有一個(gè)scale的操作庸追，scale的值由
scale=(float)targetDensity/density;
這一行代碼決定，即縮放的倍率和targetDensity和density相關(guān)台囱，而這兩個(gè)參數(shù)都是從傳入的options中 獲取到的
inDensity:Bitmap位圖自身的密度淡溯、分辨率
inTargetDensity: Bitmap最終繪制的目標(biāo)位置的分辨率
inScreenDensity: 設(shè)備屏幕分辨率
其中inDensity和圖片存放的資源文件的目錄有關(guān)，同一張圖片放置在不同目錄下會(huì)有不同的值:

density.png

1. 圖片放在drawable中簿训，等同于放在drawable-mdpi中咱娶，原因?yàn)?drawable目錄不具有屏幕密度特
   性，所以采用基準(zhǔn)值强品，即mdpi
2. 圖片放在某個(gè)特定drawable中膘侮，比如drawable-hdpi，如果設(shè)備的屏幕密度高于當(dāng)前drawable目
   錄所代表的密度的榛，則圖片會(huì)被放大琼了，否則會(huì)被縮小 放大或縮小比例 = 設(shè)備屏幕密度 / drawable目錄所代表的屏幕密度
   因此，關(guān)于Bitmap占用內(nèi)存大小的公式夫晌，從之前:
   Bitmap內(nèi)存占用 ≈ 像素?cái)?shù)據(jù)總大小 = 橫向像素?cái)?shù)量 × 縱向像素?cái)?shù)量 × 每個(gè)像素的字節(jié)大小
   可以更細(xì)化為:
   Bitmap內(nèi)存占用 ≈ 像素?cái)?shù)據(jù)總大小 = 圖片寬 × 圖片高× (設(shè)備分辨率/資源目錄分辨率)^2 × 每個(gè)像 素的字節(jié)大小
   高效加載大型位圖雕薪，請(qǐng)參考官網(wǎng)
   https://developer.android.com/topic/performance/graphics/load-bitmap
   緩存位圖，請(qǐng)參考官網(wǎng)
   https://developer.android.com/topic/performance/graphics/cache-bitmap
   管理位圖晓淀，請(qǐng)參考官網(wǎng)
   https://developer.android.com/topic/performance/graphics/manage-memory
   本篇參考了http://www.reibang.com/p/3f6f6e4f1c88