Android to achieve long picture loading effect

A key class bitmapregiondoder is used for long graph loading. Bitmap memory reuse is used for long graph loading. For example, the view size is 440 * 654 and the width and height of the picture is 440 * 12000. At this time, the width and height of the picture should be obtained. Compared with the width and height of the view, a reduction ratio will be obtained. Then, a width ratio and a height ratio will be obtained, Use the large scale as the zoom factor, and then slide the long graph with the gesture

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapRegionDecoder;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Rect;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.GestureDetector;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Scroller;

import java.io.IOException;
import java.io.InputStream;

public class BigView extends View implements GestureDetector.OnGestureListener,View.OnTouchListener {
  private static final String TAG = "BigView";
  private Scroller mScroller;
  private GestureDetector mGestureDetector;
  private BitmapFactory.Options mOptions;
  private Rect mRect;
  private int mImageWidth;
  private int mImageHeight;
  private BitmapRegionDecoder mDecoder;
  private int mViewWidth;
  private int mViewHeight;
  private float mScale;
  private Bitmap bitmap;

  public BigView(Context context) {
    this(context,null,0);
  }

  public BigView(Context context,@Nullable AttributeSet attrs) {
    this(context,attrs,@Nullable AttributeSet attrs,int defStyleAttr) {
    super(context,defStyleAttr);
    //指定要加载的矩形区域
    mRect = new Rect();
    //解码图片的配置
    mOptions = new BitmapFactory.Options();
    //手势
    mGestureDetector = new GestureDetector(context,this);
    setOnTouchListener(this);
    // 滑动帮助
    mScroller = new Scroller(context);
  }

  /**
   * 由使用者输入一张图片 输入流
   *
   * @param is
   */
  public void setImage(InputStream is) {
    //先读取原图片的 宽、高
    mOptions.inJustDecodeBounds = true;
    BitmapFactory.decodeStream(is,mOptions);
    mImageWidth = mOptions.outWidth;
    mImageHeight = mOptions.outHeight;
    //复用 内存复用
    mOptions.inMutable = true;
    //设置像素格式为 rgb565
    mOptions.inPreferredConfig = Bitmap.Config.RGB_565;
    mOptions.inJustDecodeBounds = false;
    //创建区域解码器 用于区域解码图片
    try {
      mDecoder = BitmapRegionDecoder.newInstance(is,false);
    } catch (IOException e) {
      e.printStackTrace();
    }
    requestLayout();
  }

  /**
   * 测量 view的大小
   *
   * @param widthMeasureSpec
   * @param heightMeasureSpec
   */
  @Override
  protected void onMeasure(int widthMeasureSpec,int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec,heightMeasureSpec);
    //获得测量的view的大小
    mViewWidth = getMeasuredWidth();
    mViewHeight = getMeasuredHeight();
    //如果解码器是null 表示没有设置过要现实的图片
    if (null == mDecoder) {
      return;
    }
    //确定要加载的图片的区域
    mRect.left = 0;
    mRect.top = 0;
    mRect.right = mImageWidth;
//    Log.e(TAG,"缩放因子="+(mViewWidth*1.0f/mImageWidth*1.0f));
//    Log.e(TAG,"缩放因子="+(mViewHeight*1.0f/mImageHeight*1.0f));
    //获得缩放因子
    mScale = mViewWidth / (float) mImageWidth;

    // 需要加载的高 * 缩放因子 = 视图view的高
    // x * mScale = mViewHeight
    mRect.bottom = (int) (mViewHeight / mScale);
    Log.e(TAG,"l="+mRect.left);
    Log.e(TAG,"t="+mRect.top);
    Log.e(TAG,"r="+mRect.right);
    Log.e(TAG,"b="+mRect.bottom);
  }

  /**
   * 把图片画上去
   *
   * @param canvas
   */
  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    // 如果解码器是null 表示没有设置过要现实的图片
    if (null == mDecoder) {
      return;
    }
    //复用上一张bitmap
    Log.e(TAG,"复用上一张bitmap="+bitmap);
    mOptions.inBitmap = bitmap;
    //解码指定区域
    bitmap = mDecoder.decodeRegion(mRect,mOptions);
    //使用矩阵 对图片进行 缩放
    Matrix matrix = new Matrix();
    matrix.setScale(mScale,mScale);
    //画出来

    canvas.drawBitmap(bitmap,matrix,null);
  }

  /**
   * 手指按下屏幕的回调
   * @param e
   * @return
   */
  @Override
  public boolean onDown(MotionEvent e) {
    //如果滑动还没有停止 强制停止
    if (!mScroller.isFinished()){
      mScroller.forceFinished(true);
    }
    //继续接收后续事件
    return true;
  }

  @Override
  public void onShowPress(MotionEvent e) {

  }

  @Override
  public boolean onSingleTapUp(MotionEvent e) {
    return false;
  }

  @Override
  public void onLongPress(MotionEvent e) {

  }

  /**
   * 手指 不离开屏幕 拖动
   * @param e1 手指按下去 的事件 -- 获取开始的坐标
   * @param e2 当前手势事件 -- 获取当前的坐标
   * @param distanceX x轴 方向移动的距离
   * @param distanceY y方向移动的距离
   * @return
   */
  @Override
  public boolean onScroll(MotionEvent e1,MotionEvent e2,float distanceX,float distanceY) {
    // 手指从下往上 图片也要往上 distanceY是负数,top 和 bottom 在减
    // 手指从上往下 图片也要往下 distanceY是正数,top 和 bottom 在加
    //改变加载图片的区域
    mRect.offset(0,(int) distanceY);
    //bottom大于图片高了， 或者 top小于0了
    if (mRect.bottom > mImageHeight){
      mRect.bottom = mImageHeight;
      mRect.top = mImageHeight-(int) (mViewHeight / mScale);
    }
    if (mRect.top < 0){
      mRect.top = 0;
      mRect.bottom = (int) (mViewHeight / mScale);
    }
    //重绘
    invalidate();
    return false;
  }

  /**
   * 手指离开屏幕 滑动 惯性
   * @param e1
   * @param e2
   * @param veLocityX 速度 每秒x方向 移动的像素
   * @param veLocityY y
   * @return
   */
  @Override
  public boolean onFling(MotionEvent e1,float veLocityX,float veLocityY) {
    /**
     * startX: 滑动开始的x坐标
     * startY: 滑动开始的y坐标
     * 两个速度
     * minX: x方向的最小值
     * max 最大
     * y
     */
    //计算器
    mScroller.fling(0,mRect.top,(int)-veLocityY,mImageHeight - (int) (mViewHeight / mScale));
    return false;
  }

  //获取计算结果并且重绘
  @Override
  public void computeScroll() {
    //已经计算结束 return
    if (mScroller.isFinished()){
      return;
    }
    //true 表示当前动画未结束
    if (mScroller.computeScrollOffset()){
      //
      mRect.top = mScroller.getCurrY();
      mRect.bottom = mRect.top+ (int) (mViewHeight / mScale);
      invalidate();
    }
  }

  @Override
  public boolean onTouch(View v,MotionEvent event) {
    //交由手势处理
    return mGestureDetector.onTouchEvent(event);
  }
}

If it is the second key point of the interview, the first one is to say this class, and the second one is to know that memory reuse is used

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