animation过渡效果

References:

http://developer.android.com/training/animation/index.html

http://developer.android.com/reference/android/view/ViewGroup.html#attr_android:animateLayoutChanges

动画效果可以微妙地提升用户体验。特别是当屏幕状态发生改变时，比如新的内容添加进来或者新的动作产生。本文将以developer的training教程Adding Animations为基础，分析Android里面的动画使用。

layout过渡动画

如果layout界面有view的简单怎加，删减，可以用addView()与removeView(), 此时可以对这个过程怎加一个动画效果。ViewGroup有一个android:animateLayoutChanges的属性可以设置，这个属性的作用就在于，当layout发生变化时是否触发过度动画效果。因此只需设置为true，就可以开启默认的过渡动画。

addBtn = (Button) findViewById(R.id.button1);
        container = (LinearLayout) findViewById(R.id.container);
        addBtn.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                final TextView tv = new TextView(MainActivity.this);
                tv.setText("new item");
                container.addView(tv, 0);
                tv.setOnClickListener(new OnClickListener() {
                    
                    @Override
                    public void onClick(View v) {
                       container.removeView(tv);
                    }
                });
            }
        });

View的渐变
从API level 12以后基类Veiw增加了animate(),可以返回一个ViewPropertyAnimator实例，作用是用来控制view本身的动画属性。
API level 12指的是Android 3.1.X以上，也就是所2.X是不支持的。
这个ViewPropertyAnimator 的接口非常多，基本的如透明度alpha,动画时长duration，坐标，渐变方式，动画结束时的回调借口等等，training重的案例使用到了alpha属性，使得进度条和显示内容的切换有一淡入淡出的效果。

mContentView.setAlpha(0f);
    mContentView.setVisibility(View.VISIBLE);

    // Animate the content view to 100% opacity, and clear any animation
    // listener set on the view.
    mContentView.animate()
            .alpha(1f)
            .setDuration(mShortAnimationDuration)
            .setListener(null);

    // Animate the loading view to 0% opacity. After the animation ends,
    // set its visibility to GONE as an optimization step (it won't
    // participate in layout passes, etc.)
    mHideView.animate()
            .alpha(0f)
            .setDuration(mShortAnimationDuration)
            .setListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    mHideView.setVisibility(View.GONE);
                }
            });

滑动屏幕动画
这个就是Viewpager的效果，在上下也滑动之间有一个动画效果，平时使用viewpager是一般不设置，实际上也可以通过实现自己的ViewPager.PageTransformer 来做出完全不同的切换动画效果。官方给出的缩放效果和深度效果的ViewPager.PageTransformer与默认的效果很不同。
自己实现动画关键是理解transformPage(View page, float position)的参数并实现他。page当然值得就是滑动中德那个view，position这里是float，不是平时理解的int位置信息，而是当前滑动状态的一个表示，比如当滑动到正全屏时，position是0，而向左滑动，使得右边刚好有一部被进入屏幕时，position是1，如果前一夜和下一页基本各在屏幕占一半时，前一页的position是-0.5，后一页的posiotn是0.5，所以根据position的值我们就可以自行设置需要的alpha，x/y信息。

Zoom-out page transformer

public class ZoomOutPageTransformer implements ViewPager.PageTransformer {
    private static float MIN_SCALE = 0.85f;
    private static float MIN_ALPHA = 0.5f;

    public void transformPage(View view, float position) {
        int pageWidth = view.getWidth();
        int pageHeight = view.getHeight();

        if (position < -1) { // [-Infinity,-1)
            // This page is way off-screen to the left.
            view.setAlpha(0);

        } else if (position <= 1) { // [-1,1]
            // Modify the default slide transition to shrink the page as well
            float scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position));
            float vertMargin = pageHeight * (1 - scaleFactor) / 2;
            float horzMargin = pageWidth * (1 - scaleFactor) / 2;
            if (position < 0) {
                view.setTranslationX(horzMargin - vertMargin / 2);
            } else {
                view.setTranslationX(-horzMargin + vertMargin / 2);
            }

            // Scale the page down (between MIN_SCALE and 1)
            view.setScaleX(scaleFactor);
            view.setScaleY(scaleFactor);

            // Fade the page relative to its size.
            view.setAlpha(MIN_ALPHA +
                    (scaleFactor - MIN_SCALE) /
                    (1 - MIN_SCALE) * (1 - MIN_ALPHA));

        } else { // (1,+Infinity]
            // This page is way off-screen to the right.
            view.setAlpha(0);
        }
    }
}

Depth page transformer

public class DepthPageTransformer implements ViewPager.PageTransformer {
    private static float MIN_SCALE = 0.75f;

    public void transformPage(View view, float position) {
        int pageWidth = view.getWidth();

        if (position < -1) { // [-Infinity,-1)
            // This page is way off-screen to the left.
            view.setAlpha(0);

        } else if (position <= 0) { // [-1,0]
            // Use the default slide transition when moving to the left page
            view.setAlpha(1);
            view.setTranslationX(0);
            view.setScaleX(1);
            view.setScaleY(1);

        } else if (position <= 1) { // (0,1]
            // Fade the page out.
            view.setAlpha(1 - position);

            // Counteract the default slide transition
            view.setTranslationX(pageWidth * -position);

            // Scale the page down (between MIN_SCALE and 1)
            float scaleFactor = MIN_SCALE
                    + (1 - MIN_SCALE) * (1 - Math.abs(position));
            view.setScaleX(scaleFactor);
            view.setScaleY(scaleFactor);

        } else { // (1,+Infinity]
            // This page is way off-screen to the right.
            view.setAlpha(0);
        }
    }
}