View的measure()方法是final的,是以我們無法在子類中去重寫這個方法,在該方法内部會調用onMeasure()方法。是以我們自定義view時,要重寫也是重寫onMeasure方法。同理,layout、draw也是final的,我們根據需要重寫onLayout和onDraw方法即可。
1、measure過程
measure過程要分成兩種情況來看:
- 1、如果隻是一個view,那麼直接調用其measure方法即可完成測量;
- 2、如果是一個ViewGroup,除了完成自己的測量外,還會周遊調用所有子view的measure方法;
首先看第一種情況,也就是View的情況。
1.1、 View的Measure過程
View的measure()方法是final的,我們無法在子類中去重寫這個方法,在該方法内部會調用onMeasure()方法,如果自定義時需要,可以重寫此方法, 源碼如下所示:
// (frameworks\base\core\java\android\view\View.java)
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
上述代碼很簡潔,就是給view設定寬高,都是通過getDefaultSize方法設定的:
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
邏輯很簡單,如果是UNSPECIFIED,直接傳回系統給的建議值size, 如果是AT_MOST或EXACTLY,就傳回在MeasureSpec中的實際測量值,而實際測量值的來源有三種情況:
(1)如果View設定了固定寬高,傳回的就是設定的大小;
(2)如果設定了MATCH_PARENT,View的測量模式會有兩種情況,不過不管是哪一種,傳回最大值的都是父容器剩餘大小;
(3)如果是自定義view,設定了WRAP_CONTENT,需要自己在onMeasure中重寫代碼,計算此種情況下應該是什麼大小,否則WRAP_CONTENT的效果可能會失效;
第一二點都好了解,第三點通常發生在自定義view,因為從上述代碼可見,如果沒自己進行計算,則最後效果MATCH_PARENT是一緻的,都是得到父容器剩餘大小。常見的解決方式就是在onMeasure裡針對WRAP_CONTENT屬性直接通過setMeasuredDimension()方法為其指定一個預設的寬高。
1.2 ViewGroup的Measure過程
ViewGroup類是沒有onMeasure()方法的,因為不同的ViewGroup子類布局都不一樣,是以這個方法是交給子類去實作的。
ViewGroup中定義了一個measureChildren()方法,其中通過for循環來周遊測量子視圖的大小,然後就可以确定自己的大小:
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
周遊過程中去調用measureChild, 去調用每個子view的measure方法:
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
2、号外:何時去擷取View的寬高是最合适的?
通常來說,在measure後,view的大小就測量出來了,就可擷取view的寬高了,但是measure是三大流程中最複雜的一個,有的情況下可能需要多次measure後才能最終确定寬高,是以,一種比較好的方式就是在onLayout中去擷取View的最終測量寬高。
此外,如果我們不是在view内部的onLayout中去擷取寬高,而是在Activity中想去擷取某個View的寬高,應該怎麼做呢? 因為view的繪制流程和Activity的生命周期無關,是以不能保證在哪個Activity的生命周期時,View繪制完畢,此時我們又以下四種方法來解決:
- 1、Activity/View的 onWindowFocusChanged方法: 回調這個方法時,就表示View已經初始化完畢,但onWindowFocusChanged可能會被回調多次,是以我們要做好相應的邏輯判斷;
- 2、view.post(Runnable): 通過view的post方法将一個Runnable加入到消息隊列,然後Looper調用Runnable時,view也就完成初始化了;
- 3、通過ViewTreeObserver監聽View的變化:當View樹的狀态或可見性改變,會回調給它的onGloableLayout方法,但也會回調多次,需要處理好多次的邏輯;
- 4、調用view.measure:手動調用view的measure方法,此種方法比較複雜,這裡不進行擴充;
3、layout過程
layout的作用就是ViewGroup用來确定子view的位置,當ViewGroup的位置确定後,它會在onLayout中周遊所有子view并調用其layout方法,然後layout方法又會調用子view的onLayout方法,layout方法确定view自己的位置,onLayout方法确定所有子view的位置, 以下是ViewGroup的layout:
public final void layout(int l, int t, int r, int b) {
if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
if (mTransition != null) {
mTransition.layoutChange(this);
}
super.layout(l, t, r, b);
} else {
// record the fact that we noop'd it; request layout when transition finishes
mLayoutCalledWhileSuppressed = true;
}
}
ViewGroup的layout什麼都沒做,直接調用了view的layout:
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);//注釋1
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);//注釋2
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
}
上述代碼中:
- 注釋1: 通過setFrame,完成對View四個頂點的初始化,也就确定了view相對父view的位置,并且會判斷視圖的大小是否發生過變化,避免未發生變化時進行重繪操作;
- 注釋2:調用view的onLayout方法,完成真正的layout,onLayout在View和ViewGroup中均是抽象方法,需要子view實作;
4、draw過程
measure和layout的過程都結束後,就進入到對View進行繪制的draw過程了。
首先會建立出一個Canvas對象,然後調用View的draw()方法來執行具體的繪制工作,大緻可以分為四步:
- 1、繪制背景background.draw(canvas);
- 2、繪制自己 onDraw;
- 3、繪制childview(dispatchDraw);
- 4、繪制裝飾(onDrawScrollBars);
代碼很長,但邏輯很清楚:
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
}
View的繪制流程就講完了,如果您覺得還可以,麻煩點個贊吧~~