前言

我想開頭先說說

a.度娘一下什么都知道了，整理這些東西有什么用？還費(fèi)時(shí)費(fèi)力的！如何才能完全掌握一個(gè)知識(shí)？當(dāng)你能把它講清楚的時(shí)候，你才算掌握了他。

想完全把mScrollX和mScrollY，scrollTo()和scrollBy()，smoothScrollTo和smoothScrollBy弄明白并不容易，但查閱他們的源代碼基本就能明白個(gè)大概，這篇文章就是從源碼分析他們究竟有什么作用和區(qū)別，讀懂了基本就會(huì)用！

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公眾號(hào)

目錄

• mScrollX和mScrollY
• scrollTo()和scrollBy()
• smoothScrollBy和smoothScrollTo
• view和viewgroup

一.mScrollX和mScrollY

/**

* The offset, in pixels, by which the content of this view is scrolled

* horizontally.

* {@hide}

*/

@ViewDebug.ExportedProperty(category = "scrolling")

protected int mScrollX;

/**

* The offset, in pixels, by which the content of this view is scrolled

* vertically.

* {@hide}

*/

@ViewDebug.ExportedProperty(category = "scrolling")

protected int mScrollY;

/**

* Return the scrolled left position of this view. This is the left edge of

* the displayed part of your view. You do not need to draw any pixels

* farther left, since those are outside of the frame of your view on

* screen.

*

* @return The left edge of the displayed part of your view, in pixels.

*/

public final int getScrollX() {

return mScrollX;

}

/**

* Return the scrolled top position of this view. This is the top edge of

* the displayed part of your view. You do not need to draw any pixels above

* it, since those are outside of the frame of your view on screen.

*

* @return The top edge of the displayed part of your view, in pixels.

*/

public final int getScrollY() {

return mScrollY;

}

直接翻譯就可以得知

mScrollX：表示離視圖起始位置的x水平方向的偏移量

mScrollY：表示離視圖起始位置的y垂直方向的偏移量

通過getScrollX() 和getScrollY()方法獲這兄弟倆。注意：mScrollX和mScrollY指的并不是坐標(biāo)，而是偏移量。

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二.scrollTo()和scrollBy()

<span style="font-family:SimSun;font-size:14px;"> /**

* Set the scrolled position of your view. This will cause a call to

* {@link #onScrollChanged(int, int, int, int)} and the view will be

* invalidated.

* @param x the x position to scroll to

* @param y the y position to scroll to

*/

public void scrollTo(int x, int y) {

if (mScrollX != x || mScrollY != y) {

int oldX = mScrollX;

int oldY = mScrollY;

mScrollX = x;

mScrollY = y;

invalidateParentCaches();

onScrollChanged(mScrollX, mScrollY, oldX, oldY);

if (!awakenScrollBars()) {

postInvalidateOnAnimation();

}

}

}

/**

* Move the scrolled position of your view. This will cause a call to

* {@link #onScrollChanged(int, int, int, int)} and the view will be

* invalidated.

* @param x the amount of pixels to scroll by horizontally

* @param y the amount of pixels to scroll by vertically

*/

public void scrollBy(int x, int y) {

scrollTo(mScrollX + x, mScrollY + y);

}</span>

源碼可以看出，scrollTo 和 scrollBy區(qū)別，其實(shí)2者的效果是一樣的，只是過程不同而已。

scrollTo（int x,int y）：

如果偏移位置發(fā)生了改變，就會(huì)給mScrollX和mScrollY賦新值，改變當(dāng)前位置。注意：x,y代表的不是坐標(biāo)點(diǎn)，而是偏移量。例如：我要移動(dòng)view到坐標(biāo)點(diǎn)（100，100），那么我的偏移量就是(0,，0) - （100，100） = （-100 ，-100） ，我就要執(zhí)行view.scrollTo(-100,-100),達(dá)到這個(gè)效果。

scrollBy（int x,int y）：

從源碼中看出，它實(shí)際上是調(diào)用了scrollTo(mScrollX + x, mScrollY + y);mScrollX + x和mScrollY + y，即表示在原先偏移的基礎(chǔ)上在發(fā)生偏移，通俗的說就是相對(duì)我們當(dāng)前位置偏移。根據(jù)父類VIEW里面移動(dòng)，如果移動(dòng)到了超出的地方，就不會(huì)顯示。

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三.smoothScrollTo和smoothScrollBy

ScrollView.smoothScrollBy和smoothScrollTo的源碼

/**

* Like {@link View#scrollBy}, but scroll smoothly instead of immediately.

*

* @param dx the number of pixels to scroll by on the X axis

* @param dy the number of pixels to scroll by on the Y axis

*/

public final void smoothScrollBy(int dx, int dy) {

if (getChildCount() == 0) {

// Nothing to do.

return;

}

long duration = AnimationUtils.currentAnimationTimeMillis() - mLastScroll;

if (duration > ANIMATED_SCROLL_GAP) {

final int height = getHeight() - mPaddingBottom - mPaddingTop; //獲取部件高度

final int bottom = getChildAt(0).getHeight(); //獲取當(dāng)前能看到的item高度

final int maxY = Math.max(0, bottom - height);

final int scrollY = mScrollY;

dy = Math.max(0, Math.min(scrollY + dy, maxY)) - scrollY;

mScroller.startScroll(mScrollX, scrollY, 0, dy);

postInvalidateOnAnimation();

} else {

if (!mScroller.isFinished()) {

mScroller.abortAnimation();

if (mFlingStrictSpan != null) {

mFlingStrictSpan.finish();

mFlingStrictSpan = null;

}

}

scrollBy(dx, dy);

}

mLastScroll = AnimationUtils.currentAnimationTimeMillis();

}

/**

* Like {@link #scrollTo}, but scroll smoothly instead of immediately.

*

* @param x the position where to scroll on the X axis

* @param y the position where to scroll on the Y axis

*/

public final void smoothScrollTo(int x, int y) {

smoothScrollBy(x - mScrollX, y - mScrollY);

}

源碼可以看出，是ScrollBy和ScrollTo增加滾動(dòng)動(dòng)畫的升級(jí)方案；

moothScrollBy(int dx, int dy) ：

?dy = Math.max(0, Math.min(scrollY + dy, maxY)) - scrollY;為方法的核心，比較繞，大致為計(jì)算當(dāng)前滑動(dòng)狀態(tài)下可劃動(dòng)距離；

if(duration > ANIMATEDSCROLLGAP) 這句是判斷當(dāng)前是否在滾動(dòng)，當(dāng)還在滾動(dòng)狀態(tài)下，執(zhí)行mScroller.startScroll(mScrollX, scrollY, 0, dy)；當(dāng)不在滾動(dòng)，就立馬打斷。

smoothScrollTo(int x, int y)：

?它實(shí)際上是調(diào)用了smoothScrollBy(x - mScrollX, y - mScrollY);方法根據(jù)x,y的值來計(jì)算剩余可滾動(dòng)的位移量；

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四.view和viewgroup

最后講講view和viewgroup這倆，完全吃透有點(diǎn)難，通俗講，Android的UI界面都是由View和ViewGroup及其派生類組合而成的，View是所有UI組件的基類，而ViewGroup是容納這些組件的容器；View類是ViewGroup的父類，ViewGroup具有View的所有特性，ViewGroup主要用來充當(dāng)View的容器，將其中的View作為自己孩子，并對(duì)其進(jìn)行管理，當(dāng)然孩子也可以是ViewGroup類型。

View派生出的直接子類有：AnalogClock,ImageView,KeyboardView, ProgressBar,SurfaceView,TextView,ViewGroup,ViewStubView派生出的間接子類有：AbsListView,AbsSeekBar, AbsSpinner, AbsoluteLayout, AdapterView,AdapterViewAnimator, AdapterViewFlipper, AppWidgetHostView, AutoCompleteTextView,Button,CalendarView, CheckBox, CheckedTextView, Chronometer, CompoundButton

ViewGroup派生出的直接子類有：AbsoluteLayout,AdapterView,FragmentBreadCrumbs,FrameLayout,LinearLayout,RelativeLayout,SlidingDrawer；

ViewGroup派生出的間接子類有：AbsListView,AbsSpinner, AdapterViewAnimator, AdapterViewFlipper, AppWidgetHostView, CalendarView, DatePicker, DialerFilter, ExpandableListView, Gallery, GestureOverlayView,GridView,HorizontalScrollView, ImageSwitcher,ListView,

當(dāng)然，隨著Android版本不斷地更新，這些派生出來的子類也是在不段增加的！

END

轉(zhuǎn)載于:https://www.cnblogs.com/xiangevan/p/7818150.html

超強(qiáng)干貨來襲 云風(fēng)專訪：近40年碼齡，通宵達(dá)旦的技術(shù)人生

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