Android Bitmap在不加載圖片的前提獲取寬高

代碼參考：

 

 /** 
     * 根據View(主要是ImageView)的寬和高來獲取圖片的縮略圖 
     * @param path 
     * @param viewWidth 
     * @param viewHeight 
     * @return 
     */  
    private Bitmap decodeThumbBitmapForFile(String path, int viewWidth, int viewHeight){  
        BitmapFactory.Options options = new BitmapFactory.Options();  
        //設置為true,表示解析Bitmap對象，該對象不占內存  
        options.inJustDecodeBounds = true;  
        BitmapFactory.decodeFile(path, options);  
        //設置縮放比例  
        options.inSampleSize = computeScale(options, viewWidth, viewHeight);  
          
        //設置為false,解析Bitmap對象加入到內存中  
        options.inJustDecodeBounds = false;  
          
        return BitmapFactory.decodeFile(path, options);  
    }  
      
      
    /** 
     * 根據View(主要是ImageView)的寬和高來計算Bitmap縮放比例。默認不縮放 
     * @param options 
     * @param width 
     * @param height 
     */  
    private int computeScale(BitmapFactory.Options options, int viewWidth, int viewHeight){  
        int inSampleSize = 1;  
        if(viewWidth == 0 || viewHeight == 0){  
            return inSampleSize;  
        }  
        int bitmapWidth = options.outWidth;  
        int bitmapHeight = options.outHeight;  
          
        //假如Bitmap的寬度或高度大於我們設定圖片的View的寬高，則計算縮放比例  
        if(bitmapWidth > viewWidth || bitmapHeight > viewWidth){  
            int widthScale = Math.round((float) bitmapWidth / (float) viewWidth);  
            int heightScale = Math.round((float) bitmapHeight / (float) viewWidth);  
              
            //為了保證圖片不縮放變形，我們取寬高比例最小的那個  
            inSampleSize = widthScale < heightScale ? widthScale : heightScale;  
        }  
        return inSampleSize;  
    }  
      

另外可參照：

 

 

BitmapFactory.Options options = new BitmapFactory.Options();  
         
       /** 
        * 最關鍵在此，把options.inJustDecodeBounds = true; 
        * 這裡再decodeFile()，返回的bitmap為空，但此時調用options.outHeight時，已經包含了圖片的高了 
        */  
       options.inJustDecodeBounds = true;  
       Bitmap bitmap = BitmapFactory.decodeFile("/sdcard/test.jpg", options); // 此時返回的bitmap為null  
       /** 
        *options.outHeight為原始圖片的高 
        */  
       Log.e("Test", "Bitmap Height == " + options.outHeight);  

根據這些方法，可以在實際加載Bitmap圖片之前根據需要設置圖片縮小為原來的（1/2,1/4,1/8,1/16..）

 

下面是Android源碼中關於 inSampleSize的說明:

 

public int inSampleSize

Added in API level 1
If set to a value > 1, requests the decoder to subsample the original image, returning a smaller image to save memory. The sample size is the number of pixels in either dimension that correspond to a single pixel in the decoded bitmap. For example, inSampleSize == 4 returns an image that is 1/4 the width/height of the original, and 1/16 the number of pixels. Any value <= 1 is treated the same as 1. Note: the decoder uses a final value based on powers of 2, any other value will be rounded down to the nearest power of 2.