Android 下拉展开动画

使用ValueAnimator实现一个匀速的下拉动画

点击显示部分可以匀速的下拉出隐藏部分，再次点击隐藏部分匀速收回。

public class DropTest extends Activity {

	private LinearLayout mHiddenView;
	private float mDensity;
	private int mHiddenViewMeasuredHeight;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.drop);
		mHiddenView = (LinearLayout) findViewById(R.id.hidden_view);
		// 获取像素密度
		mDensity = getResources().getDisplayMetrics().density;
		// 获取布局的高度
		mHiddenViewMeasuredHeight = (int) (mDensity * 40 + 0.5);
	}

	public void llClick(View view) {
		if (mHiddenView.getVisibility() == View.GONE) {
			// 打开动画
			animateOpen(mHiddenView);
		} else {
			// 关闭动画
			animateClose(mHiddenView);
		}
	}

	@SuppressLint("NewApi")
	private void animateOpen(final View view) {
		view.setVisibility(View.VISIBLE);
		ValueAnimator animator = createDropAnimator(view, 0,
				mHiddenViewMeasuredHeight);
		animator.start();
	}

	@SuppressLint("NewApi")
	private void animateClose(final View view) {
		int origHeight = view.getHeight();
		ValueAnimator animator = createDropAnimator(view, origHeight, 0);
		animator.addListener(new AnimatorListenerAdapter() {
			public void onAnimationEnd(Animator animation) {
				// 动画结束时隐藏view
				view.setVisibility(View.GONE);
			}
		});
		animator.start();
	}

	@SuppressLint("NewApi")
	private ValueAnimator createDropAnimator(final View view, int start, int end) {
		// 创建一个数值发生器
		ValueAnimator animator = ValueAnimator.ofInt(start, end);
		animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {

			@Override
			public void onAnimationUpdate(ValueAnimator valueAnimator) {
				int value = (Integer) valueAnimator.getAnimatedValue();
				ViewGroup.LayoutParams layoutParams = view.getLayoutParams();
				layoutParams.height = value;
				view.setLayoutParams(layoutParams);
			}
		});
		return animator;
	}
}