像音樂播放App同樣移動背景

假設你經常聽歌。你會發現歌曲app的背景會隨着音樂移動的，從左到右或者從上到下。這樣的動畫儘管簡單，但是這裏有一個技巧。

假設你還不明確這樣的動效看看如下的demo

(不少其它具體請參考：https://github.com/flavienlaurent/PanningView）
一，使用setImageMatrix播放圖片動畫
如下是官方文檔給出的解釋

你可以看到這裏的解釋很是easy。就是取代ImageView的圖像矩陣。而後configureBounds和invalidate被調用。
在java代碼中咱們可以設置Matrix的scaleType

mImageView.setScaleType(ScaleType.MATRIX)

或者在xml文件裏設置

android:scaleType="matrix"

如下是ImageView的初始矩陣（matrix)

在x和y方向上放大2倍

final Matrix matrix = new Matrix();
matrix.postScale(2, 2);
imageView.setImageMatrix(matrix);

final Matrix matrix = new Matrix();
matrix.postScale(2, 2);
matrix.postRotate(15);
imageView.setImageMatrix(matrix);

二，使你的圖片移動
首先咱們需要計算ImageView當前方向（水平。縱向）和圖片當前方向的比例，就比方水平方向吧，咱們就要讓圖片和view的高度一樣，橫向放大或者縮小。

float scaleFactor = (float)imageView.getHeight() / (float) drawable.getIntrinsicHeight();
mMatrix.postScale(scaleFactor, scaleFactor);

這樣咱們就能保證圖片的高和ImageView一樣，並且填充滿ImageView.
接下來咱們就讓ImageView的圖片移動，咱們要用到一個強大的Android動畫框架：ValueAnimator,其原理就是利用ImageView的圖像矩陣在x軸方向變換移動。

mAnimator = ValueAnimator.ofFloat(0, 100);
mAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
  @Override
  public void onAnimationUpdate(ValueAnimator animation) {
      float value = (Float) animation.getAnimatedValue();
      matrix.reset();
      matrix.postScale(scaleFactor, scaleFactor);
      matrix.postTranslate(-value, 0);
      imageView.setImageMatrix(matrix);

  }
});
mAnimator.setDuration(5000);
mAnimator.start();

整個代碼例如如下：

package com.testimageview;
import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.ValueAnimator;
import android.app.Activity;
import android.graphics.Matrix;
import android.graphics.RectF;
import android.os.Bundle;
import android.widget.ImageView;

public class MainActivity extends Activity{
    private static final int RightToLeft = 1;
    private static final int LeftToRight = 2;
    private static final int DURATION = 5000;

    private ValueAnimator mCurrentAnimator;
    private final Matrix mMatrix = new Matrix();
    private ImageView mImageView;
    private float mScaleFactor;
    private int mDirection = RightToLeft;
    private RectF mDisplayRect = new RectF();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

      mImageView = (ImageView) findViewById(R.id.imageView);

      mImageView.post(new Runnable() {
          @Override
          public void run() {
              mScaleFactor = (float)  mImageView.getHeight() 
                      / (float) mImageView.getDrawable().getIntrinsicHeight();
              mMatrix.postScale(mScaleFactor, mScaleFactor);
              mImageView.setImageMatrix(mMatrix);
              animate();
          }
      });

    }

    private void animate() {
      updateDisplayRect();
      if(mDirection == RightToLeft) {
          animate(mDisplayRect.left, mDisplayRect.left
                  - (mDisplayRect.right - mImageView.getWidth()));
      } else {
          animate(mDisplayRect.left, 0.0f);
      }
    }

    private void animate(float from, float to) {
      mCurrentAnimator = ValueAnimator.ofFloat(from, to);
      mCurrentAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
          @Override
          public void onAnimationUpdate(ValueAnimator animation) {
              float value = (Float) animation.getAnimatedValue();

              mMatrix.reset();
              mMatrix.postScale(mScaleFactor, mScaleFactor);
              mMatrix.postTranslate(value, 0);

              mImageView.setImageMatrix(mMatrix);

          }
      });
      mCurrentAnimator.setDuration(DURATION);
      mCurrentAnimator.addListener(new AnimatorListenerAdapter() {
          @Override
          public void onAnimationEnd(Animator animation) {
              if(mDirection == RightToLeft)
                  mDirection = LeftToRight;
              else
                  mDirection = RightToLeft;

              animate();
          }
      });
      mCurrentAnimator.start();
    }

    private void updateDisplayRect() {
      mDisplayRect.set(0, 0, mImageView.getDrawable().getIntrinsicWidth(),
              mImageView.getDrawable().getIntrinsicHeight());
      mMatrix.mapRect(mDisplayRect);
    }
}