android中的scroll一般是调用View.scrollTo()函数实现的,另外有一个View.scrollBy()其实现也是View.scrollTo()。关于View.scrollTo(),下面有一些问题来探究一下,本文所查看的代码是android4.2.2的源代码。
- 问:View.scrollTo()的原理是什么?
答:在android.view.View.java中有两个变量mScrollX和mScrollY,两个变量的定义如下:
/**
* The offset, in pixels, by which the content of this view is scrolled
* horizontally.
*/
protected int mScrollX;
/**
* The offset, in pixels, by which the content of this view is scrolled
* vertically.
*/
protected int mScrollY;
在View的绘制过程中,会根据这两个值对里面的内容进行偏移。View.draw()中的代码片段如下:
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
在View.scrollTo()的作用就是就是改变这两个变量,从而引起View的content位置发生变化。
- 问:Scroll是否引起Layout的改变?
答:Scroll只是改变View.mScrollx和View.mScrollY,并且在draw()函数中,根据这两个偏移量进行绘制。所以只是影响draw(),不会引起Layout的改变。 - 问:如果scroll只是在绘制的时候进行偏移,那么对touch事件的派发是否产生影响,View事件的响应是在scroll前的位置还是scroll后的位置?
答:事件的分派是在ViewGroup.dispatchTouchEvent()中,查看源码,发现在寻找响应事件的子View时,会算上mScrollX和mScrollY的偏移。所以scroll是会对事件派发产生影响的。View事件的响应是在scroll后的位置。关键代码如下:
protected boolean isTransformedTouchPointInView(float x, float y, View child,
PointF outLocalPoint) {
float localX = x + mScrollX - child.mLeft;
float localY = y + mScrollY - child.mTop;
if (! child.hasIdentityMatrix() && mAttachInfo != null) {
final float[] localXY = mAttachInfo.mTmpTransformLocation;
localXY[0] = localX;
localXY[1] = localY;
child.getInverseMatrix().mapPoints(localXY);
localX = localXY[0];
localY = localXY[1];
}
final boolean isInView = child.pointInView(localX, localY);
if (isInView && outLocalPoint != null) {
outLocalPoint.set(localX, localY);
}
return isInView;
}
- 问:Scroller的作用是什么?
答:Scroller的作用是帮助Scroll更加顺滑的。如果我们不是想在一段时间内,有A点顺滑的scroll到B点。这样我们就要计算每次draw应该偏移多少,才能达到顺滑的效果。Scroller就是用来帮助计算的。具体的计算方式是在Scroller.computeScrollOffset()函数中。代码如下:
public boolean computeScrollOffset() {
if (mFinished) {
return false;
}
int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
if (timePassed < mDuration) {
switch (mMode) {
case SCROLL_MODE:
float x = timePassed * mDurationReciprocal;
if (mInterpolator == null)
x = viscousFluid(x);
else
x = mInterpolator.getInterpolation(x);
mCurrX = mStartX + Math.round(x * mDeltaX);
mCurrY = mStartY + Math.round(x * mDeltaY);
break;
case FLING_MODE:
final float t = (float) timePassed / mDuration;
final int index = (int) (NB_SAMPLES * t);
float distanceCoef = 1.f;
float velocityCoef = 0.f;
if (index < NB_SAMPLES) {
final float t_inf = (float) index / NB_SAMPLES;
final float t_sup = (float) (index + 1) / NB_SAMPLES;
final float d_inf = SPLINE_POSITION[index];
final float d_sup = SPLINE_POSITION[index + 1];
velocityCoef = (d_sup - d_inf) / (t_sup - t_inf);
distanceCoef = d_inf + (t - t_inf) * velocityCoef;
}
mCurrVelocity = velocityCoef * mDistance / mDuration * 1000.0f;
mCurrX = mStartX + Math.round(distanceCoef * (mFinalX - mStartX));
// Pin to mMinX <= mCurrX <= mMaxX
mCurrX = Math.min(mCurrX, mMaxX);
mCurrX = Math.max(mCurrX, mMinX);
mCurrY = mStartY + Math.round(distanceCoef * (mFinalY - mStartY));
// Pin to mMinY <= mCurrY <= mMaxY
mCurrY = Math.min(mCurrY, mMaxY);
mCurrY = Math.max(mCurrY, mMinY);
if (mCurrX == mFinalX && mCurrY == mFinalY) {
mFinished = true;
}
break;
}
}
else {
mCurrX = mFinalX;
mCurrY = mFinalY;
mFinished = true;
}
return true;
}
- 问:Scroll和ValueAnimator的区别是什么?
答:ValueAnimator的本质是在一段时间内,按照一定的规律去改变一个值。而scroll就是改变mScrollX和mScrollY的值。所以理论上也可以用ValueAnimator来改变mScrollX和mScrollY,从而达到scroll的目的。 - 问:Scroll和Animation的区别是什么?
答:animation也是通过在draw函数里面实现的,但是animation只是改变了绘制的位置,并没有影响到事件的响应。所以当一个View从A点通过Animation动画移动到B点,View的事件继续在A点才能响应。