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Loading Large Bitmaps Efficiently

2024-11-19 09:16:02   202人阅读

NOTE:看来Android文档过来做个笔记，并没有详细去写这些东西。

BitmapFactory提供的decode方法直接去decode的话，会造成oom异常。

要设置BitmapFactory.Options 的inJustDecodeBounds为true。将图片信息decode出来，此时并没有真正的加载图片，只是获取了边界信息和图片mimetype

BitmapFactory.Options options = new BitmapFactory.Options();options.inJustDecodeBounds = true;BitmapFactory.decodeResource(getResources(), R.id.myimage, options);int imageHeight = options.outHeight;int imageWidth = options.outWidth;String imageType = options.outMimeType;

　往往一个Imageview很小，但是一个图片很大，这时加载时就需要将图片压缩到ImageView差不多大小加载能有效的节约内存：

public static int calculateInSampleSize(            BitmapFactory.Options options, int reqWidth, int reqHeight) {    // Raw height and width of image    final int height = options.outHeight;    final int width = options.outWidth;    int inSampleSize = 1;    if (height > reqHeight || width > reqWidth) {        final int halfHeight = height / 2;        final int halfWidth = width / 2;        // Calculate the largest inSampleSize value that is a power of 2 and keeps both        // height and width larger than the requested height and width.        while ((halfHeight / inSampleSize) > reqHeight                && (halfWidth / inSampleSize) > reqWidth) {            inSampleSize *= 2;        }    }    return inSampleSize;}

缩放的因子是2，Android文档中是这么描述inSampleSize的：

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.

意思就是说inSample不管你算出来是多少，最后都被四舍五入到了2的倍数。

加载图片时就可以这么加载了：

首先获取到图片的实际大小，然后输入你实际想要的大小。获取到缩放倍数，再将inJustDecodeBounds设置为false才是真正的加载图片

public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,        int reqWidth, int reqHeight) {    // First decode with inJustDecodeBounds=true to check dimensions    final BitmapFactory.Options options = new BitmapFactory.Options();    options.inJustDecodeBounds = true;    BitmapFactory.decodeResource(res, resId, options);    // Calculate inSampleSize    options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);    // Decode bitmap with inSampleSize set    options.inJustDecodeBounds = false;    return BitmapFactory.decodeResource(res, resId, options);}

举个栗子：加载一张图片到100×100的ImageView中就可以这么做

mImageView.setImageBitmap(    decodeSampledBitmapFromResource(getResources(), R.id.myimage, 100, 100));

 

 

Loading Large Bitmaps Efficiently






























        

            
                
                  
                
                

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