Android View中getTop，getY，getTranslationY之間的關係

首先View源碼中沒有Y這個成員變量。

View源碼中的成員變量只有 mTop和mTransformationInfo（和translationY有關）。

Y只是作爲一個抽象概念存在，它的計算關係如以下代碼所示。

/**
     * The visual y position of this view, in pixels. This is equivalent to the
     * {@link #setTranslationY(float) translationY} property plus the current
     * {@link #getTop() top} property.
     *
     * @return The visual y position of this view, in pixels.
     */
    @ViewDebug.ExportedProperty(category = "drawing")
    public float getY() {
        return mTop + (mTransformationInfo != null ? mTransformationInfo.mTranslationY : 0);
    }

    /**
     * Sets the visual y position of this view, in pixels. This is equivalent to setting the
     * {@link #setTranslationY(float) translationY} property to be the difference between
     * the y value passed in and the current {@link #getTop() top} property.
     *
     * @param y The visual y position of this view, in pixels.
     */
    public void setY(float y) {
        setTranslationY(y - mTop);
    }

    /**
     * Sets the vertical location of this view relative to its {@link #getTop() top} position.
     * This effectively positions the object post-layout, in addition to wherever the object's
     * layout placed it.
     *
     * @param translationY The vertical position of this view relative to its top position,
     * in pixels.
     *
     * @attr ref android.R.styleable#View_translationY
     */
    public void setTranslationY(float translationY) {
        ensureTransformationInfo();
        final TransformationInfo info = mTransformationInfo;
        if (info.mTranslationY != translationY) {
            invalidateViewProperty(true, false);
            info.mTranslationY = translationY;
            info.mMatrixDirty = true;
            invalidateViewProperty(false, true);
            if (mDisplayList != null) {
                mDisplayList.setTranslationY(translationY);
            }
            if ((mPrivateFlags2 & PFLAG2_VIEW_QUICK_REJECTED) == PFLAG2_VIEW_QUICK_REJECTED) {
                // View was rejected last time it was drawn by its parent; this may have changed
                invalidateParentIfNeeded();
            }
        }
    }

源碼基於Android 4.3