像音乐播放App一样移动背景

如果你经常听歌，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，这种动画虽然简单，但是这里有一个技巧。如果你还不明白这种动效看看下面的demo



(更多详细请参考：https://github.com/flavienlaurent/PanningView）

一，使用setImageMatrix播放图片动画

下面是官方文档给出的解释



你可以看到这里的解释很简单，就是代替ImageView的图像矩阵，然后configureBounds和invalidate被调用。

在java代码中我们可以设置Matrix的scaleType

mImageView.setScaleType(ScaleType.MATRIX)

或者在xml文件中设置

android:scaleType="matrix"

下面是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

首先我们需要计算ImageView当前方向（水平，纵向）和图片当前方向的比例，就比如水平方向吧，我们就要让图片和view的高度相同，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相同，并且填充满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);
}
}