开发中经常需要实现可点击的图片、文案、布局，本文是通过对组件的透明度变化实现简单的点击事件，简单而且实用。
以ImageView为例（TextView和RelativeLayout类似）直接上代码：

/**
 * 可点击的ImageView，具有点击态效果
 *
 * @author luninggithub
 */
public class AlphaImageView extends AppCompatImageView {

    private ObjectAnimator mAnimatorOut;

    private boolean mIsClick = false;

    public AlphaImageView(Context context) {
        super(context);
        init();
    }

    public AlphaImageView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public AlphaImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() { // 1. 初始化animator
        mAnimatorOut = ObjectAnimator.ofFloat(this, "alpha", 0.4f, 1f);
        mAnimatorOut.setDuration(100);
    }

    @Override
    public void setOnClickListener(@Nullable OnClickListener l) {
        mIsClick = l != null;
        super.setOnClickListener(l);
    }

    @SuppressLint("ClickableViewAccessibility")
    @Override
    public boolean onTouchEvent(MotionEvent event) { // 2. 重写Touch事件
        if (mIsClick && isEnabled()) {
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    this.setAlpha(0.4f);
                    break;
                case MotionEvent.ACTION_UP:
                case MotionEvent.ACTION_CANCEL:
                    mAnimatorOut.start();
                    break;
            }
        }
        return super.onTouchEvent(event);
    }

}

1. 声明Animator对象mAnimatorOut且初始化时设置透明度渐变范围0.4f~1.0f，渐变周期为100ms

mAnimatorOut = ObjectAnimator.ofFloat(this, "alpha", 0.4f, 1f);
mAnimatorOut.setDuration(100);

2. 重写onTouchEvent事件，判断当前是否设置了listener且可点击。DOWN事件时设置初始透明度0.4f，UP和CANCEL事件时Animator start

switch (event.getAction()) {
      case MotionEvent.ACTION_DOWN:
             this.setAlpha(0.4f);
             break;
      case MotionEvent.ACTION_UP:
      case MotionEvent.ACTION_CANCEL:
             mAnimatorOut.start();
             break;

Demo地址：
AlphaImageView
AlphaTextView
AlphaRelativeLayout