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>