Android開(kāi)發(fā)中View的滑動(dòng)處理源碼，該怎么看焊夸？
Android 帶你從源碼的角度解析Scroller的滾動(dòng)實(shí)現(xiàn)原理，
今天給大家講解的是Scroller類的滾動(dòng)實(shí)現(xiàn)原理，可能很多朋友不太了解該類是用來(lái)干嘛的舞萄，但是研究Launcher的朋友應(yīng)該對(duì)他很熟悉，Scroller類是滾動(dòng)的一個(gè)封裝類低滩，可以實(shí)現(xiàn)View的平滑滾動(dòng)效果缤言，什么是實(shí)現(xiàn)View的平滑滾動(dòng)效果呢，舉個(gè)簡(jiǎn)單的例子审胚，一個(gè)View從在我們指定的時(shí)間內(nèi)從一個(gè)位置滾動(dòng)到另外一個(gè)位置匈勋，我們利用Scroller類可以實(shí)現(xiàn)勻速滾動(dòng)，可以先加速后減速膳叨，可以先減速后加速等等效果洽洁，而不是瞬間的移動(dòng)的效果，所以Scroller可以幫我們實(shí)現(xiàn)很多滑動(dòng)的效果菲嘴。
在介紹Scroller類之前饿自，我們先去了解View的scrollBy() 和scrollTo()方法的區(qū)別，在區(qū)分這兩個(gè)方法的之前龄坪，我們要先理解View 里面的兩個(gè)成員變量mScrollX昭雌， mScrollY，X軸方向的偏移量和Y軸方向的偏移量健田，這個(gè)是一個(gè)相對(duì)距離烛卧，相對(duì)的不是屏幕的原點(diǎn)，而是View的左邊緣妓局，舉個(gè)通俗易懂的例子唱星，一列火車從吉安到深圳，途中經(jīng)過(guò)贛州跟磨，那么原點(diǎn)就是贛州间聊，偏移量就是 負(fù)的吉安到贛州的距離，大家從getScrollX()方法中的注釋中就能看出答案來(lái)
/**
* 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;
}
現(xiàn)在我們知道了向右滑動(dòng) mScrollX就為負(fù)數(shù)抵拘，向左滑動(dòng)mScrollX為正數(shù)哎榴，接下來(lái)我們先來(lái)看看scrollTo()方法的源碼
注 : 此處應(yīng)該參考: 藝術(shù)探索 里所說(shuō),移動(dòng)的是 蓋板
/**
* 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();
}
}
}
從該方法中我們可以看出，先判斷傳進(jìn)來(lái)的(x, y)值是否和View的X, Y偏移量相等，如果不相等尚蝌，就調(diào)用onScrollChanged()方法來(lái)通知界面發(fā)生改變迎变，然后重繪界面，所以這樣子就實(shí)現(xiàn)了移動(dòng)效果啦飘言， 現(xiàn)在我們知道了scrollTo()方法是滾動(dòng)到(x, y)這個(gè)偏移量的點(diǎn)衣形，他是相對(duì)于View的開(kāi)始位置來(lái)滾動(dòng)的。在看看scrollBy()這個(gè)方法的代碼
/**
* 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);
}
原來(lái)他里面調(diào)用了scrollTo()方法姿鸿，那就好辦了谆吴，他就是相對(duì)于View上一個(gè)位置根據(jù)(x, y)來(lái)進(jìn)行滾動(dòng)，可能大家腦海中對(duì)這兩個(gè)方法還有點(diǎn)模糊苛预，沒(méi)關(guān)系句狼，還是舉個(gè)通俗的例子幫大家理解下，假如一個(gè)View,調(diào)用兩次scrollTo(-10, 0)热某，第一次向右滾動(dòng)10腻菇，第二次就不滾動(dòng)了，因?yàn)閙ScrollX和x相等了昔馋，當(dāng)我們調(diào)用兩次scrollBy(-10, 0),第一次向右滾動(dòng)10筹吐，第二次再向右滾動(dòng)10，他是相對(duì)View的上一個(gè)位置來(lái)滾動(dòng)的秘遏。
對(duì)于scrollTo()和scrollBy()方法還有一點(diǎn)需要注意骏令，這點(diǎn)也很重要，假如你給一個(gè)LinearLayout調(diào)用scrollTo()方法垄提，并不是LinearLayout滾動(dòng)榔袋，而是LinearLayout里面的內(nèi)容進(jìn)行滾動(dòng)，比如你想對(duì)一個(gè)按鈕進(jìn)行滾動(dòng)铡俐，直接用Button調(diào)用scrollTo()一定達(dá)不到你的需求凰兑，大家可以試一試，如果真要對(duì)某個(gè)按鈕進(jìn)行scrollTo()滾動(dòng)的話审丘，我們可以在Button外面包裹一層Layout吏够，然后對(duì)Layout調(diào)用scrollTo()方法。
了解了scrollTo()和scrollBy()方法之后我們就了解下Scroller類了滩报，先看其構(gòu)造方法
/**
* Create a Scroller with the default duration and interpolator.
*/
public Scroller(Context context) {
this(context, null);
}
/**
* Create a Scroller with the specified interpolator. If the interpolator is
* null, the default (viscous) interpolator will be used. "Flywheel" behavior will
* be in effect for apps targeting Honeycomb or newer.
*/
public Scroller(Context context, Interpolator interpolator) {
this(context, interpolator,
context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.HONEYCOMB);
}
/**
* Create a Scroller with the specified interpolator. If the interpolator is
* null, the default (viscous) interpolator will be used. Specify whether or
* not to support progressive "flywheel" behavior in flinging.
*/
public Scroller(Context context, Interpolator interpolator, boolean flywheel) {
mFinished = true;
if (interpolator == null) {
mInterpolator = new ViscousFluidInterpolator();
} else {
mInterpolator = interpolator;
}
mPpi = context.getResources().getDisplayMetrics().density * 160.0f;
mDeceleration = computeDeceleration(ViewConfiguration.getScrollFriction());
mFlywheel = flywheel;
mPhysicalCoeff = computeDeceleration(0.84f); // look and feel tuning
}
共計(jì)三個(gè)構(gòu)造方法锅知，第一個(gè)只有一個(gè)Context參數(shù)，第二個(gè)構(gòu)造方法中指定了Interpolator脓钾，什么Interpolator呢?中文意思插補(bǔ)器售睹，了解Android動(dòng)畫的朋友都應(yīng)該熟悉
Interpolator，他指定了動(dòng)畫的變化率可训，比如說(shuō)勻速變化昌妹，先加速后減速捶枢，正弦變化等等，不同的Interpolator可以做出不同的效果出來(lái)飞崖，第一個(gè)使用默認(rèn)的Interpolator(viscous)
第三個(gè)構(gòu)造方法,是對(duì)滑動(dòng)的 "慣性輪"特性的支持.
接下來(lái)我們就要在Scroller類里面找滾動(dòng)的方法烂叔，我們從名字上面可以看出startScroll()應(yīng)該是個(gè)滾動(dòng)的方法，我們來(lái)看看其源碼吧
/**
* Start scrolling by providing a starting point, the distance to travel,
* and the duration of the scroll.
*
* @param startX Starting horizontal scroll offset in pixels. Positive
* numbers will scroll the content to the left.
* @param startY Starting vertical scroll offset in pixels. Positive numbers
* will scroll the content up.
* @param dx Horizontal distance to travel. Positive numbers will scroll the
* content to the left.
* @param dy Vertical distance to travel. Positive numbers will scroll the
* content up.
* @param duration Duration of the scroll in milliseconds.
*/
public void startScroll(int startX, int startY, int dx, int dy, int duration) {
mMode = SCROLL_MODE;
mFinished = false;
mDuration = duration;
mStartTime = AnimationUtils.currentAnimationTimeMillis();
mStartX = startX;
mStartY = startY;
mFinalX = startX + dx;
mFinalY = startY + dy;
mDeltaX = dx;
mDeltaY = dy;
mDurationReciprocal = 1.0f / (float) mDuration;
}
?
在這個(gè)方法中我們只看到了對(duì)一些滾動(dòng)的基本設(shè)置動(dòng)作固歪，比如設(shè)置滾動(dòng)模式蒜鸡，開(kāi)始時(shí)間，持續(xù)時(shí)間等等牢裳，并沒(méi)有任何對(duì)View的滾動(dòng)操作逢防，也許你正納悶，不是滾動(dòng)的方法干嘛還叫做startScroll(),稍安勿躁贰健，既然叫開(kāi)始滾動(dòng)，那就是對(duì)滾動(dòng)的滾動(dòng)之前的基本設(shè)置咯恬汁。
接下來(lái)是computeScrollOffset()方法:
/**
* Call this when you want to know the new location. If it returns true,
* the animation is not yet finished.
*/
public boolean computeScrollOffset() {
if (mFinished) {
return false;
}
int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis() - mStartTime);
if (timePassed < mDuration) {
switch (mMode) {
case SCROLL_MODE:
final float x = mInterpolator.getInterpolation(timePassed * mDurationReciprocal);
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;
}
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《注：》這里想補(bǔ)充一句個(gè)人理解：isFinish()方法返回的就是該方法里面的mFinished伶椿。當(dāng)時(shí)間截止或者路程走完都會(huì)使mFinished =true，但是computeScrollOffset()的返回值是依賴于mFinished的
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我們?cè)趕tartScroll()方法的時(shí)候獲取了當(dāng)前的動(dòng)畫毫秒賦值給了mStartTime氓侧，在computeScrollOffset()中再一次調(diào)用AnimationUtils.currentAnimationTimeMillis()來(lái)獲取動(dòng)畫毫秒減去mStartTime就是動(dòng)畫已過(guò)時(shí)間了脊另，然后進(jìn)去if判斷，如果動(dòng)畫已過(guò)時(shí)間時(shí)間小于我們?cè)O(shè)置的滾動(dòng)持續(xù)時(shí)間mDuration约巷，進(jìn)去switch的SCROLL_MODE偎痛，然后根據(jù)Interpolator來(lái)計(jì)算出在該時(shí)間段里面移動(dòng)的距離，賦值給mCurrX独郎， mCurrY踩麦， 所以該方法的作用是，計(jì)算在0到mDuration時(shí)間段內(nèi)滾動(dòng)的偏移量氓癌，并且判斷滾動(dòng)是否結(jié)束谓谦，true代表還沒(méi)結(jié)束，false則表示滾動(dòng)介紹了贪婉，Scroller類的其他的方法我就不提了反粥，大都是一些get(), set()方法。
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看了這么多疲迂，到底要怎么才能觸發(fā)滾動(dòng)才顿，你心里肯定有很多疑惑，在說(shuō)滾動(dòng)之前我要先提另外一個(gè)方法View 的 computeScroll()尤蒿，該方法是滑動(dòng)的控制方法郑气，在繪制View時(shí)，會(huì)在draw()過(guò)程調(diào)用該方法腰池。我們先看看computeScroll()的源碼:
/**
* Called by a parent to request that a child update its values for mScrollX
* and mScrollY if necessary. This will typically be done if the child is
* animating a scroll using a {@link android.widget.Scroller Scroller}
* object.
*/
public void computeScroll() {
}
沒(méi)錯(cuò)竣贪，他是一個(gè)空的方法军洼，需要子類去重寫該方法來(lái)實(shí)現(xiàn)邏輯，到底該方法在哪里被觸發(fā)呢演怎。我們繼續(xù)看看View的繪制方法draw()
/**
* This method is called by ViewGroup.drawChild() to have each child view draw itself.
*
* This is where the View specializes rendering behavior based on layer type,
* and hardware acceleration.
*/
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
final boolean hardwareAcceleratedCanvas = canvas.isHardwareAccelerated();
/* If an attached view draws to a HW canvas, it may use its RenderNode + DisplayList.
*
* If a view is dettached, its DisplayList shouldn't exist. If the canvas isn't
* HW accelerated, it can't handle drawing RenderNodes.
*/
boolean drawingWithRenderNode = mAttachInfo != null
&& mAttachInfo.mHardwareAccelerated
&& hardwareAcceleratedCanvas;
boolean more = false;
final boolean childHasIdentityMatrix = hasIdentityMatrix();
final int parentFlags = parent.mGroupFlags;
if ((parentFlags & ViewGroup.FLAG_CLEAR_TRANSFORMATION) != 0) {
parent.getChildTransformation().clear();
parent.mGroupFlags &= ~ViewGroup.FLAG_CLEAR_TRANSFORMATION;
}
Transformation transformToApply = null;
boolean concatMatrix = false;
final boolean scalingRequired = mAttachInfo != null && mAttachInfo.mScalingRequired;
final Animation a = getAnimation();
if (a != null) {
more = applyLegacyAnimation(parent, drawingTime, a, scalingRequired);
concatMatrix = a.willChangeTransformationMatrix();
if (concatMatrix) {
mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
}
transformToApply = parent.getChildTransformation();
} else {
if ((mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_TRANSFORM) != 0) {
// No longer animating: clear out old animation matrix
mRenderNode.setAnimationMatrix(null);
mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_TRANSFORM;
}
if (!drawingWithRenderNode
&& (parentFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
final Transformation t = parent.getChildTransformation();
final boolean hasTransform = parent.getChildStaticTransformation(this, t);
if (hasTransform) {
final int transformType = t.getTransformationType();
transformToApply = transformType != Transformation.TYPE_IDENTITY ? t : null;
concatMatrix = (transformType & Transformation.TYPE_MATRIX) != 0;
}
}
}
concatMatrix |= !childHasIdentityMatrix;
// Sets the flag as early as possible to allow draw() implementations
// to call invalidate() successfully when doing animations
mPrivateFlags |= PFLAG_DRAWN;
if (!concatMatrix &&
(parentFlags & (ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS |
ViewGroup.FLAG_CLIP_CHILDREN)) == ViewGroup.FLAG_CLIP_CHILDREN &&
canvas.quickReject(mLeft, mTop, mRight, mBottom, Canvas.EdgeType.BW) &&
(mPrivateFlags & PFLAG_DRAW_ANIMATION) == 0) {
mPrivateFlags2 |= PFLAG2_VIEW_QUICK_REJECTED;
return more;
}
mPrivateFlags2 &= ~PFLAG2_VIEW_QUICK_REJECTED;
if (hardwareAcceleratedCanvas) {
// Clear INVALIDATED flag to allow invalidation to occur during rendering, but
// retain the flag's value temporarily in the mRecreateDisplayList flag
mRecreateDisplayList = (mPrivateFlags & PFLAG_INVALIDATED) != 0;
mPrivateFlags &= ~PFLAG_INVALIDATED;
}
RenderNode renderNode = null;
Bitmap cache = null;
int layerType = getLayerType(); // TODO: signify cache state with just 'cache' local
if (layerType == LAYER_TYPE_SOFTWARE || !drawingWithRenderNode) {
if (layerType != LAYER_TYPE_NONE) {
// If not drawing with RenderNode, treat HW layers as SW
layerType = LAYER_TYPE_SOFTWARE;
buildDrawingCache(true);
}
cache = getDrawingCache(true);
}
if (drawingWithRenderNode) {
// Delay getting the display list until animation-driven alpha values are
// set up and possibly passed on to the view
renderNode = updateDisplayListIfDirty();
if (!renderNode.isValid()) {
// Uncommon, but possible. If a view is removed from the hierarchy during the call
// to getDisplayList(), the display list will be marked invalid and we should not
// try to use it again.
renderNode = null;
drawingWithRenderNode = false;
}
}
int sx = 0;
int sy = 0;
if (!drawingWithRenderNode) {
//******************************************
computeScroll();
//******************************************
sx = mScrollX;
sy = mScrollY;
}
final boolean drawingWithDrawingCache = cache != null && !drawingWithRenderNode;
final boolean offsetForScroll = cache == null && !drawingWithRenderNode;
int restoreTo = -1;
if (!drawingWithRenderNode || transformToApply != null) {
restoreTo = canvas.save();
}
if (offsetForScroll) {
canvas.translate(mLeft - sx, mTop - sy);
} else {
if (!drawingWithRenderNode) {
canvas.translate(mLeft, mTop);
}
if (scalingRequired) {
if (drawingWithRenderNode) {
// TODO: Might not need this if we put everything inside the DL
restoreTo = canvas.save();
}
// mAttachInfo cannot be null, otherwise scalingRequired == false
final float scale = 1.0f / mAttachInfo.mApplicationScale;
canvas.scale(scale, scale);
}
}
float alpha = drawingWithRenderNode ? 1 : (getAlpha() * getTransitionAlpha());
if (transformToApply != null
|| alpha < 1
|| !hasIdentityMatrix()
|| (mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_ALPHA) != 0) {
if (transformToApply != null || !childHasIdentityMatrix) {
int transX = 0;
int transY = 0;
if (offsetForScroll) {
transX = -sx;
transY = -sy;
}
if (transformToApply != null) {
if (concatMatrix) {
if (drawingWithRenderNode) {
renderNode.setAnimationMatrix(transformToApply.getMatrix());
} else {
// Undo the scroll translation, apply the transformation matrix,
// then redo the scroll translate to get the correct result.
canvas.translate(-transX, -transY);
canvas.concat(transformToApply.getMatrix());
canvas.translate(transX, transY);
}
parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
}
float transformAlpha = transformToApply.getAlpha();
if (transformAlpha < 1) {
alpha *= transformAlpha;
parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
}
}
if (!childHasIdentityMatrix && !drawingWithRenderNode) {
canvas.translate(-transX, -transY);
canvas.concat(getMatrix());
canvas.translate(transX, transY);
}
}
// Deal with alpha if it is or used to be <1
if (alpha < 1 || (mPrivateFlags3 & PFLAG3_VIEW_IS_ANIMATING_ALPHA) != 0) {
if (alpha < 1) {
mPrivateFlags3 |= PFLAG3_VIEW_IS_ANIMATING_ALPHA;
} else {
mPrivateFlags3 &= ~PFLAG3_VIEW_IS_ANIMATING_ALPHA;
}
parent.mGroupFlags |= ViewGroup.FLAG_CLEAR_TRANSFORMATION;
if (!drawingWithDrawingCache) {
final int multipliedAlpha = (int) (255 * alpha);
if (!onSetAlpha(multipliedAlpha)) {
if (drawingWithRenderNode) {
renderNode.setAlpha(alpha * getAlpha() * getTransitionAlpha());
} else if (layerType == LAYER_TYPE_NONE) {
canvas.saveLayerAlpha(sx, sy, sx + getWidth(), sy + getHeight(),
multipliedAlpha);
}
} else {
// Alpha is handled by the child directly, clobber the layer's alpha
mPrivateFlags |= PFLAG_ALPHA_SET;
}
}
}
} else if ((mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
onSetAlpha(255);
mPrivateFlags &= ~PFLAG_ALPHA_SET;
}
if (!drawingWithRenderNode) {
// apply clips directly, since RenderNode won't do it for this draw
if ((parentFlags & ViewGroup.FLAG_CLIP_CHILDREN) != 0 && cache == null) {
if (offsetForScroll) {
canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
} else {
if (!scalingRequired || cache == null) {
canvas.clipRect(0, 0, getWidth(), getHeight());
} else {
canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
}
}
}
if (mClipBounds != null) {
// clip bounds ignore scroll
canvas.clipRect(mClipBounds);
}
}
if (!drawingWithDrawingCache) {
if (drawingWithRenderNode) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
} else {
// Fast path for layouts with no backgrounds
if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
//******************************************
dispatchDraw(canvas);
//******************************************
} else {
draw(canvas);
}
}
} else if (cache != null) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
// no layer paint, use temporary paint to draw bitmap
Paint cachePaint = parent.mCachePaint;
if (cachePaint == null) {
cachePaint = new Paint();
cachePaint.setDither(false);
parent.mCachePaint = cachePaint;
}
cachePaint.setAlpha((int) (alpha * 255));
canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
} else {
// use layer paint to draw the bitmap, merging the two alphas, but also restore
int layerPaintAlpha = mLayerPaint.getAlpha();
if (alpha < 1) {
mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
}
canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
if (alpha < 1) {
mLayerPaint.setAlpha(layerPaintAlpha);
}
}
}
if (restoreTo >= 0) {
canvas.restoreToCount(restoreTo);
}
if (a != null && !more) {
if (!hardwareAcceleratedCanvas && !a.getFillAfter()) {
onSetAlpha(255);
}
parent.finishAnimatingView(this, a);
}
if (more && hardwareAcceleratedCanvas) {
if (a.hasAlpha() && (mPrivateFlags & PFLAG_ALPHA_SET) == PFLAG_ALPHA_SET) {
// alpha animations should cause the child to recreate its display list
invalidate(true);
}
}
mRecreateDisplayList = false;
return more;
}
這上面星號(hào)標(biāo)注的部分,寫出了繪制一個(gè)View的幾個(gè)步驟匕争，我們看看第四步繪制孩子的時(shí)候會(huì)觸發(fā)dispatchDraw()這個(gè)方法，來(lái)看看源碼是什么內(nèi)容
@Override
protected void dispatchDraw(Canvas canvas) {
boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
int flags = mGroupFlags;
if ((flags & FLAG_RUN_ANIMATION) != 0 && canAnimate()) {
final boolean buildCache = !isHardwareAccelerated();
for (int i = 0; i < childrenCount; i++) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
final LayoutParams params = child.getLayoutParams();
attachLayoutAnimationParameters(child, params, i, childrenCount);
bindLayoutAnimation(child);
}
}
final LayoutAnimationController controller = mLayoutAnimationController;
if (controller.willOverlap()) {
mGroupFlags |= FLAG_OPTIMIZE_INVALIDATE;
}
controller.start();
mGroupFlags &= ~FLAG_RUN_ANIMATION;
mGroupFlags &= ~FLAG_ANIMATION_DONE;
if (mAnimationListener != null) {
mAnimationListener.onAnimationStart(controller.getAnimation());
}
}
int clipSaveCount = 0;
final boolean clipToPadding = (flags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;
if (clipToPadding) {
clipSaveCount = canvas.save(Canvas.CLIP_SAVE_FLAG);
canvas.clipRect(mScrollX + mPaddingLeft, mScrollY + mPaddingTop,
mScrollX + mRight - mLeft - mPaddingRight,
mScrollY + mBottom - mTop - mPaddingBottom);
}
// We will draw our child's animation, let's reset the flag
mPrivateFlags &= ~PFLAG_DRAW_ANIMATION;
mGroupFlags &= ~FLAG_INVALIDATE_REQUIRED;
boolean more = false;
final long drawingTime = getDrawingTime();
if (usingRenderNodeProperties) canvas.insertReorderBarrier();
final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size();
int transientIndex = transientCount != 0 ? 0 : -1;
// Only use the preordered list if not HW accelerated, since the HW pipeline will do the
// draw reordering internally
final ArrayList<View> preorderedList = usingRenderNodeProperties
? null : buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
while (transientIndex >= 0) {
// there may be additional transient views after the normal views
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
break;
}
}
if (preorderedList != null) preorderedList.clear();
// Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
final ArrayList<View> disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View child = disappearingChildren.get(i);
//*************************
more |= drawChild(canvas, child, drawingTime);
//*************************
}
}
if (usingRenderNodeProperties) canvas.insertInorderBarrier();
if (debugDraw()) {
onDebugDraw(canvas);
}
if (clipToPadding) {
canvas.restoreToCount(clipSaveCount);
}
// mGroupFlags might have been updated by drawChild()
flags = mGroupFlags;
if ((flags & FLAG_INVALIDATE_REQUIRED) == FLAG_INVALIDATE_REQUIRED) {
invalidate(true);
}
if ((flags & FLAG_ANIMATION_DONE) == 0 && (flags & FLAG_NOTIFY_ANIMATION_LISTENER) == 0 &&
mLayoutAnimationController.isDone() && !more) {
// We want to erase the drawing cache and notify the listener after the
// next frame is drawn because one extra invalidate() is caused by
// drawChild() after the animation is over
mGroupFlags |= FLAG_NOTIFY_ANIMATION_LISTENER;
final Runnable end = new Runnable() {
@Override
public void run() {
notifyAnimationListener();
}
};
post(end);
}
}
?
這個(gè)方法代碼有點(diǎn)多爷耀，但是我們還是挑重點(diǎn)看吧甘桑，從星號(hào)標(biāo)注的行可以看出 在dispatchDraw()里面會(huì)對(duì)ViewGroup里面的子View調(diào)用drawChild()來(lái)進(jìn)行繪制，接下來(lái)我們來(lái)看看drawChild()方法的代碼
/**
* Draw one child of this View Group. This method is responsible for getting
* the canvas in the right state. This includes clipping, translating so
* that the child's scrolled origin is at 0, 0, and applying any animation
* transformations.
*
* @param canvas The canvas on which to draw the child
* @param child Who to draw
* @param drawingTime The time at which draw is occurring
* @return True if an invalidate() was issued
*/
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
最終又回到了View的draw() 方法,我們可以看到如下代碼:
int sx = 0;
int sy = 0;
if (!drawingWithRenderNode) {
//******************************************
computeScroll();
//******************************************
sx = mScrollX;
sy = mScrollY;
}
看到computeScroll()你大概明白什么了吧歹叮，轉(zhuǎn)了老半天終于找到了computeScroll()方法被觸發(fā)跑杭，就是ViewGroup在分發(fā)繪制自己的孩子的時(shí)候，會(huì)對(duì)其子View調(diào)用computeScroll()方法
因此我們?cè)谑褂肧croller可以這么寫:
Scroller scroller = new Scroller(getContext());
//緩慢滑動(dòng)到指定位置
private void smoothScrollXTo(int targetX) {
int scrollX = getScrollX();
int deltaX
= targetX - scrollX;
//1000ms內(nèi) 緩慢 滑動(dòng)到targetX處
scroller.startScroll(scrollX, 0, deltaX, 0, 1000);
invalidate();
}
@Override
public void computeScroll() {
//動(dòng)畫未完成
if (scroller.computeScrollOffset()) {
scrollTo(scroller.getCurrX(), scroller.getCurrY());
//觸發(fā)draw()
postInvalidate();
}
}
整理下思路咆耿，來(lái)看看View滾動(dòng)的實(shí)現(xiàn)原理德谅，我們先調(diào)用Scroller的startScroll()方法來(lái)進(jìn)行一些滾動(dòng)的初始化設(shè)置，然后迫使View進(jìn)行繪制萨螺，我們調(diào)用View的invalidate()或postInvalidate()就可以重新繪制View窄做，繪制View的時(shí)候會(huì)觸發(fā)computeScroll()方法，我們重寫computeScroll()慰技，在computeScroll()里面先調(diào)用Scroller的computeScrollOffset()方法來(lái)判斷滾動(dòng)有沒(méi)有結(jié)束椭盏，如果滾動(dòng)沒(méi)有結(jié)束我們就調(diào)用scrollTo()方法來(lái)進(jìn)行滾動(dòng)，該scrollTo()方法雖然會(huì)重新繪制View,但是我們還是要手動(dòng)調(diào)用下invalidate()或者postInvalidate()來(lái)觸發(fā)界面重繪吻商，重新繪制View又觸發(fā)computeScroll()掏颊，所以就進(jìn)入一個(gè)循環(huán)階段，這樣子就實(shí)現(xiàn)了在某個(gè)時(shí)間段里面滾動(dòng)某段距離的一個(gè)平滑的滾動(dòng)效果也許有人會(huì)問(wèn)艾帐，干嘛還要調(diào)用來(lái)調(diào)用去最后在調(diào)用scrollTo()方法乌叶，還不如直接調(diào)用scrollTo()方法來(lái)實(shí)現(xiàn)滾動(dòng)，其實(shí)直接調(diào)用是可以柒爸，只不過(guò)scrollTo()是瞬間滾動(dòng)的枉昏，給人的用戶體驗(yàn)不太好，所以Android提供了Scroller類實(shí)現(xiàn)平滑滾動(dòng)的效果揍鸟。為了方面大家理解兄裂，我畫了一個(gè)簡(jiǎn)單的調(diào)用示意圖
好了，講到這里就已經(jīng)講完了Scroller類的滾動(dòng)實(shí)現(xiàn)原理啦阳藻，我們來(lái)總結(jié)一下：
（1）Scroller類能夠幫助我們實(shí)現(xiàn)高級(jí)的滑動(dòng)功能晰奖，如手指抬起后的慣性滑動(dòng)功能。使用流程為腥泥，首先通過(guò)Scroller類的startScroll()+invalidate()觸發(fā)View的computeScroll()匾南，在computeScroll()中讓Scroller類去計(jì)算最新的坐標(biāo)信息，拿到最新的坐標(biāo)偏移信息后還是要調(diào)用View的scrollTo來(lái)實(shí)現(xiàn)滑動(dòng)蛔外∏悖可以看到溯乒，使用Scroller的整個(gè)流程比較簡(jiǎn)單，關(guān)鍵的是控制滑動(dòng)的一些邏輯計(jì)算豹爹，比如上面例子中的計(jì)算什么時(shí)候該往哪一頁(yè)滑動(dòng)...（2）Android后面推出了OverScroller類裆悄，OverScroller在整體功能上和Scroller類似，使用也相同臂聋。OverScroller類可以完全代替Scroller光稼，相比Scroller，OverScroller主要是增加了對(duì)滑動(dòng)到邊界的一些控制孩等，如增加一些回彈效果等艾君，功能更加強(qiáng)大。如果對(duì)android技術(shù)比較感興趣肄方，可以關(guān)注一下微信公眾號(hào)：終端研發(fā)部冰垄，和我一塊交流和學(xué)習(xí)。終端研發(fā)部是一個(gè)以技術(shù)為主的學(xué)習(xí)交流技術(shù)號(hào)权她，談的是技術(shù)虹茶，是產(chǎn)品，更是我們的人生伴奥。做東半球最會(huì)思考写烤，最有味道的互聯(lián)網(wǎng)開(kāi)發(fā)者
作者：終端研發(fā)部
鏈接：https://www.zhihu.com/question/278916471/answer/403341655
來(lái)源：知乎
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