android 加载大图的原理和实现

package com.alleged.imageLoad;import android.support.v7.app.ActionBarActivity;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.graphics.BitmapFactory.Options;import android.os.Bundle;import android.view.Menu;import android.view.MenuItem;import android.view.View;import android.view.WindowManager;import android.widget.ImageView;public class MainActivity extends ActionBarActivity {private ImageView image;    /*实现逻辑 * 第一步得到当前屏幕的分辨率 * 第二步得到要加载图片的信息 * 第三步计算缩放比例 * 第四步加载目标图片 * */    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        image = (ImageView)findViewById(R.id.showImage);    }    public void loadImage(View v){        //得到当前屏幕的分辨率        //创建WindowManger类对象        WindowManager window = (WindowManager)getSystemService(WINDOW_SERVICE);        //screenWidth代表屏幕的宽 screenHeight代表屏幕的高        int screenWidth = window.getDefaultDisplay().getWidth();        int screenHight = window.getDefaultDisplay().getHeight();        //获取图片的信息        BitmapFactory.Options options = new Options();        //只加载信息 不加载内容        options.inJustDecodeBounds = true;        //把一个图片文件 解码到bitmap中 只加载了文件信息        BitmapFactory.decodeFile("/mnt/sdcard/6.jpg",options);        //得到图片的宽 高        int imageWidth = options.outWidth;        int imageHeight = options.outHeight;        //计算缩放比        int scale = 1;        int scalex = imageWidth/screenWidth;        int scaley = imageHeight/screenHight;        if(scalex>scale&&scalex>scaley){            scale =scalex;        }else if(scaley>scale&&scaley>scalex){            scale =scaley;        }else{            scale = 1;        }        System.out.println(scale);        //正式加载图片        //设置缩放比        options.inSampleSize=scale;        options.inJustDecodeBounds = false;        Bitmap bitmap = BitmapFactory.decodeFile("/mnt/sdcard/6.jpg",options);        image.setImageBitmap(bitmap);    }    @Override    public boolean onCreateOptionsMenu(Menu menu) {        // Inflate the menu; this adds items to the action bar if it is present.        getMenuInflater().inflate(R.menu.main, menu);        return true;    }    @Override    public boolean onOptionsItemSelected(MenuItem item) {        // Handle action bar item clicks here. The action bar will        // automatically handle clicks on the Home/Up button, so long        // as you specify a parent activity in AndroidManifest.xml.        int id = item.getItemId();        if (id == R.id.action_settings) {            return true;        }        return super.onOptionsItemSelected(item);    }}

布局文件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:paddingBottom="@dimen/activity_vertical_margin"    android:paddingLeft="@dimen/activity_horizontal_margin"    android:paddingRight="@dimen/activity_horizontal_margin"    android:paddingTop="@dimen/activity_vertical_margin"    android:orientation="vertical"    android:gravity="center_horizontal"    tools:context="com.alleged.imageLoad.MainActivity" >    <Button         android:id="@+id/imageLoad"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="加载"        android:onClick="loadImage"/>    <ImageView         android:id="@+id/showImage"        android:layout_width="wrap_content"        android:layout_height="wrap_content"/></LinearLayout>