使用ObjectAnimator实现视图的动画效果（平移）

该小程序是实现一个ImageView在布局FrameLayout中的位移的。

布局文件如下：

实际效果图如下：

其中的ImageView是一个圆形（sun.xml）：

我们使用fragment来实现布局并且通过ObjectAnimator实现sun的平移：

public class SunsetFragment extends Fragment{

    private View mSceneView;
    private View mSunView;
    private View mSkyView;

    public static SunsetFragment newInstance() {
        return new SunsetFragment();
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_sunset, container, false);

        mSceneView = view;
        mSunView = view.findViewById(R.id.sun);
        mSkyView = view.findViewById(R.id.sky);
        mSceneView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startAnimation();
            }
        });
        return view;
    }

    //太阳的开始和结束的坐标
    private void startAnimation() {
        float sunYStart = mSunView.getTop();
        float sunYEnd = mSkyView.getHeight();

        ObjectAnimator heightAnimator = ObjectAnimator.ofFloat(mSunView, "y", sunYStart, sunYEnd).setDuration(3000);
        heightAnimator.start();
    }
}

ObjectAnimator.ofFloat(mSunView, "y", sunYStart, sunYEnd)表示对mSunView进行"Y"轴方向的平移，平移的开始点是sunYStart，结束点是sunYEnd。

我们进入ObjectAnimator.java看下这个方法：

/**
 * Constructs and returns an ObjectAnimator that animates between float values. A single
 * value implies that that value is the one being animated to, in which case the start value
 * will be derived from the property being animated and the target object when {@link #start()}
 * is called for the first time. Two values imply starting and ending values. More than two
 * values imply a starting value, values to animate through along the way, and an ending value
 * (these values will be distributed evenly across the duration of the animation).
 *
 * @param target The object whose property is to be animated. This object should
 * have a public method on it called setName(), where name is
 * the value of the propertyName parameter.
 * @param propertyName The name of the property being animated.
 * @param values A set of values that the animation will animate between over time.
 * @return An ObjectAnimator object that is set up to animate between the given values.
 */
public static ObjectAnimator ofFloat(Object target, String propertyName, float... values) {
    ObjectAnimator anim = new ObjectAnimator(target, propertyName);
    anim.setFloatValues(values);
    return anim;
}

这段英文的意思大家应该很清楚了，主要注意的是这个propertyName。这个propertyName是需要target这个对象有setpropertyName方法的。

第一句创建anim不用说了，第二句anim.setFloatValues(values)可以看下：

@Override
public void setFloatValues(float... values) {
    if (mValues == null || mValues.length == 0) {
        // No values yet - this animator is being constructed piecemeal. Init the values with
        // whatever the current propertyName is
        if (mProperty != null) {
            setValues(PropertyValuesHolder.ofFloat(mProperty, values));
        } else {
            setValues(PropertyValuesHolder.ofFloat(mPropertyName, values));
        }
    } else {
        super.setFloatValues(values);
    }
}

其中 mValues 来源于 ValueAnimator.java

/**
 * The property/value sets being animated.
 */
PropertyValuesHolder[] mValues;

PropertyValuesHolder由来源于android.animation.PropertyValuesHolder.java

/**
 * This class holds information about a property and the values that that property
 * should take on during an animation. PropertyValuesHolder objects can be used to create
 * animations with ValueAnimator or ObjectAnimator that operate on several different properties
 * in parallel.
 */
public class PropertyValuesHolder implements Cloneable