利用BitmapFactory.Options的inSampleSize對bitmap進行優化

在我的舊文章"從內部文件夾加載圖像視圖(ImageView)", "從SD Card加載圖像視圖(ImageView), 使用BitmapFactory"和"從網絡加載圖像視圖(ImageView)"中, 我展示了如何顯示位圖(bitmap), 但都是把位圖直接顯示在ImageView上. 這樣做有一個問題: 就是對系統資源很"大食", 特別是當源位圖是非常大時, 甚至可以使應用程序崩潰.

這篇文章以"從SD Card加載圖像視圖(ImageView)"為例, 首先檢查源位圖的大小, 通過BitmapFactory.Options設置inSampleSize, 這樣做可以減少對系統資源的要求.



先把較大的圖片文件複製到SD Card中.

main.xml

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<?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源代碼

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package com.AndroidImage;   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); }   }