像音乐播放App一样移动背景

假设你常常听歌。你会发现歌曲app的背景会随着音乐移动的，从左到右或者从上到下。这样的动画尽管简单，可是这里有一个技巧。

假设你还不明确这样的动效看看以下的demo

(很多其它具体请參考：https://github.com/flavienlaurent/PanningView）

一，使用setImageMatrix播放图片动画

以下是官方文档给出的解释

你能够看到这里的解释非常easy。就是取代ImageView的图像矩阵。然后configureBounds和invalidate被调用。

在java代码中我们能够设置Matrix的scaleType

mImageView.setScaleType(ScaleType.MATRIX)

或者在xml文件里设置

android:scaleType="matrix"

以下是ImageView的初始矩阵（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，使你的图片移动

首先我们须要计算ImageView当前方向（水平。纵向）和图片当前方向的比例，就比方水平方向吧，我们就要让图片和view的高度同样，横向放大或者缩小。

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

这样我们就能保证图片的高和ImageView同样，而且填充满ImageView.

接下来我们就让ImageView的图片移动，我们要用到一个强大的Android动画框架：ValueAnimator,其原理就是利用ImageView的图像矩阵在x轴方向变换移动。

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();

整个代码例如以下：

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);
    }
}