StretchAnimation伸缩动画.

原理是继承animation  然后改变他的margintop  和marginbottom  形成2个效果

ExpandTopAnimation

public class ExpandTopAnimation extends Animation {
	private View mAnimatedView;
	private LayoutParams mViewLayoutParams;
	private int mMarginStart, mMarginEnd;
	private boolean mIsVisibleAfter = false;
	private boolean mWasEndedAlready = false;

	public ExpandTopAnimation(View view, int duration) {

		setDuration(duration);
		mAnimatedView = view;
		mViewLayoutParams = (LayoutParams) view.getLayoutParams();

		// if the bottom margin is 0,
		// then after the animation will end it‘ll be negative, and invisible.
		mIsVisibleAfter = (mViewLayoutParams.topMargin == 0);

		mMarginStart = mViewLayoutParams.topMargin;
		mMarginEnd = (mMarginStart == 0 ? (0 - view.getHeight()) : 0);

		view.setVisibility(View.VISIBLE);
	}

	@Override
	protected void applyTransformation(float interpolatedTime, Transformation t) {
		super.applyTransformation(interpolatedTime, t);

		if (interpolatedTime < 1.0f) {

			// Calculating the new bottom margin, and setting it
			mViewLayoutParams.topMargin = mMarginStart
					+ (int) ((mMarginEnd - mMarginStart) * interpolatedTime);

			// Invalidating the layout, making us seeing the changes we made
			mAnimatedView.requestLayout();

			// Making sure we didn‘t run the ending before (it happens!)
		} else if (!mWasEndedAlready) {
			mViewLayoutParams.topMargin = mMarginEnd;
			mAnimatedView.requestLayout();

			if (mIsVisibleAfter) {
				mAnimatedView.setVisibility(View.GONE);
			}
			mWasEndedAlready = true;
		}
	}
}

ExpandBottomAnimation

public class ExpandBottomAnimation extends Animation {
    private View mAnimatedView;
    private LayoutParams mViewLayoutParams;
    private int mMarginStart, mMarginEnd;
    private boolean mIsVisibleAfter = false;
    private boolean mWasEndedAlready = false;

    public ExpandBottomAnimation(View view, int duration) {

        setDuration(duration);
        mAnimatedView = view;
        mViewLayoutParams = (LayoutParams) view.getLayoutParams();

        // if the bottom margin is 0,
        // then after the animation will end it‘ll be negative, and invisible.
        mIsVisibleAfter = (mViewLayoutParams.bottomMargin == 0);

        mMarginStart = mViewLayoutParams.bottomMargin;
        mMarginEnd = (mMarginStart == 0 ? (0 - view.getHeight()) : 0);

        view.setVisibility(View.VISIBLE);
    }

    @Override
    protected void applyTransformation(float interpolatedTime, Transformation t) {
        super.applyTransformation(interpolatedTime, t);

        if (interpolatedTime < 1.0f) {

            // Calculating the new bottom margin, and setting it
            mViewLayoutParams.bottomMargin = mMarginStart
                    + (int) ((mMarginEnd - mMarginStart) * interpolatedTime);

            // Invalidating the layout, making us seeing the changes we made
            mAnimatedView.requestLayout();

            // Making sure we didn‘t run the ending before (it happens!)
        } else if (!mWasEndedAlready) {
            mViewLayoutParams.bottomMargin = mMarginEnd;
            mAnimatedView.requestLayout();

            if (mIsVisibleAfter) {
                mAnimatedView.setVisibility(View.GONE);
            }
            mWasEndedAlready = true;
        }
    }
}

MainActivity

public class MainActivity extends Activity {

    private ListView lv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lv = (ListView) findViewById(R.id.lv);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.lv_item);

        for (int i = 0; i < 30; i++) {
            adapter.add("wiki"+i);
        }

        lv.setAdapter(adapter);

    }

    public void top(View view){
//        trans(lv);

        ExpandTopAnimation ea = new ExpandTopAnimation(lv, 200);
        view.startAnimation(ea);
    }

    public void bottom(View view){

        ExpandBottomAnimation ea = new ExpandBottomAnimation(lv, 200);
        view.startAnimation(ea);
    }

}

top效果

bottom效果