Android BitMap的优化

 接使用ImageView显示bitmap会占用较多资源，特别是图片较大的时候，可能导致崩溃。

使用BitmapFactory.Options设置inSampleSize, 这样做可以减少对系统资源的要求。

属性值inSampleSize表示缩略图大小为原始图片大小的几分之一，即如果这个值为2，则取出的缩略图的宽和高都是原始图片的1/2，图片大小就为原始大小的1/4。

Options中有个属性inJustDecodeBounds，SDK中是这么说的

If set to true, the decoder will return null (no bitmap), but the out... fields will still be set, allowing the caller to query the bitmap without having to allocate the memory for its pixels.

通过设置inJustDecodeBounds为true，获取到outHeight（图片原始高度）和outWidth（图片的原始宽度），然后计算一个inSampleSize（缩放值）

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/hello"
/>
<ImageView
android:id="@+id/imageview"
android:layout_gravity="center"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:scaleType="center"
/>
</LinearLayout>

java源码

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.widget.ImageView;
import android.widget.Toast;
 
public class AndroidImage extends Activity {
 
private String imageFile = "/sdcard/AndroidSharedPreferencesEditor.png";
/** Called when the activity is first created. */
 
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
   
ImageView myImageView = (ImageView)findViewById(R.id.imageview);
//Bitmap bitmap = BitmapFactory.decodeFile(imageFile);
//myImageView.setImageBitmap(bitmap);
 
Bitmap bitmap;
float imagew = 300;
float imageh = 300;
 
BitmapFactory.Options bitmapFactoryOptions = new BitmapFactory.Options();
bitmapFactoryOptions.inJustDecodeBounds = true;
bitmap = BitmapFactory.decodeFile(imageFile, bitmapFactoryOptions);
 
int yRatio = (int)Math.ceil(bitmapFactoryOptions.outHeight/imageh);
int xRatio = (int)Math.ceil(bitmapFactoryOptions.outWidth/imagew);
 
if (yRatio > 1 || xRatio > 1){
 if (yRatio > xRatio) {
  bitmapFactoryOptions.inSampleSize = yRatio;
  Toast.makeText(this,
    "yRatio = " + String.valueOf(yRatio),
    Toast.LENGTH_LONG).show();
 }
 else {
  bitmapFactoryOptions.inSampleSize = xRatio;
  Toast.makeText(this,
    "xRatio = " + String.valueOf(xRatio),
    Toast.LENGTH_LONG).show();
 }
}
else{
 Toast.makeText(this,
   "inSampleSize = 1",
   Toast.LENGTH_LONG).show();
}
 
bitmapFactoryOptions.inJustDecodeBounds = false;
bitmap = BitmapFactory.decodeFile(imageFile, bitmapFactoryOptions);
myImageView.setImageBitmap(bitmap);
}
 
}