1、繪製流程源碼路徑
一、Activity加載ViewRootImpl
ActivityThread.handleResumeActivity() --> WindowManagerImpl.addView(decorView, layoutParams) --> WindowManagerGlobal.addView()
二、ViewRootImpl啓動View樹的遍歷
ViewRootImpl.setView(decorView, layoutParams, parentView) -->ViewRootImpl.requestLayout() -->scheduleTraversals() -->TraversalRunnable.run() -->doTraversal() -->performTraversals()(performMeasure、performLayout、performDraw)
2、View繪製流程
一、measure
(1)MeasureSpec是什麼?
重寫過onMeasure()方法都知道,測量須要用到MeasureSpec類獲取View的測量模式和大小,那麼這個類是怎樣存儲這兩個信息呢?html
留心觀察的話會發現,onMeasure方法的兩個參數實際是32位int類型數據,即:canvas
00 000000 00000000 00000000 00000000
而其結構爲 mode + size ,前2位爲mode,然後30位爲size。app
==> getMode()方法(measureSpec --> mode):
private static final int MODE_SHIFT = 30; // 0x3轉換爲二進制即爲:11 // 左移30位後:11000000 00000000 00000000 00000000 private static final int MODE_MASK = 0x3 << MODE_SHIFT; public static int getMode(int measureSpec) { // 與MODE_MASK按位與運算後,即將低30位清零,結果爲mode左移30位後的值 return (measureSpec & MODE_MASK); }
getSize()方法同理。ide
==> makeMeasureSpec()方法(mode + size --> measureSpec):
public static int makeMeasureSpec( @IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size, @MeasureSpecMode int mode) { if (sUseBrokenMakeMeasureSpec) { return size + mode; } else { return (size & ~MODE_MASK) | (mode & MODE_MASK); } }
這裏解釋一下,按位或左側爲size的高2位清零後的結果,右側爲mode的低30位清零後的結果,二者按位或運算的結果正好爲高2位mode、低30位size,例:佈局
01000000 00000000 00000000 00000000 | 00001000 00001011 11110101 10101101 = 01001000 00001011 11110101 10101101
二進制計算規則可參考:http://www.javashuo.com/article/p-mskvidun-ek.htmlspa
==> 測量模式:
public static final int UNSPECIFIED = 0 << MODE_SHIFT; public static final int EXACTLY = 1 << MODE_SHIFT; public static final int AT_MOST = 2 << MODE_SHIFT;
UNSPECIFIED:父容器不對View做任何限制,系統內部使用。rest
EXACTLY:精確模式,父容器檢測出View大小,即爲SpecSize;對應LayoutParams中的match_parent和指定大小的狀況。code
AT_MOST:最大模式,父容器指定可用大小,View的大小不能超出這個值;對應wrap_content。orm
(2)ViewGroup的測量流程
回到ViewRootImpl的performMeasure方法,這裏傳入的參數爲頂層DecorView的測量規格,其測量方式爲:htm
private static int getRootMeasureSpec(int windowSize, int rootDimension) { int measureSpec; switch (rootDimension) { case ViewGroup.LayoutParams.MATCH_PARENT: measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; }
match_parent和具體數值大小爲EXACTLY模式,wrap_content則爲AT_MOST模式。
往下走,performMeasure方法中調用了DecorView的onMeasure方法,而DecorView繼承自FrameLayout,能夠看到FL的onMeasure方法中調用了measureChildWithMargins方法,並傳入自身的測量規格:
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) { final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams(); final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin + widthUsed, lp.width); final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin + heightUsed, lp.height); child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }
即測量子控件的大小,測量規則詳情可看getChildMeasureSpec方法,總結以下:
childLayoutParams\parentSpecMode | EXACTLY | AT_MOST | UNSPECIFIED |
---|---|---|---|
dp | EXACTLY/childSize | EXACTLY/childSize | EXCATLY/childSize |
match_parent | EXACTLY/parentSize | AT_MOST/parentSize | UNSPECIFIED/0 |
wrap_content | AT_MOST/parentSize | AT_MOST/parentSize | UNSPECIFIED/0 |
回到onMeasure方法,測完子控件以後,ViewGroup會通過一些計算,得出自身大小:
// 加上padding maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground(); maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground(); // 檢查是否小於最小寬度、最小高度 maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight()); maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth()); // 檢查Drawable的最小高度和寬度 final Drawable drawable = getForeground(); if (drawable != null) { maxHeight = Math.max(maxHeight, drawable.getMinimumHeight()); maxWidth = Math.max(maxWidth, drawable.getMinimumWidth()); } setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState), resolveSizeAndState(maxHeight, heightMeasureSpec, childState << MEASURED_HEIGHT_STATE_SHIFT));
綜上,ViewGroup的測量須要先測量子View的大小,然後結合padding等屬性計算得出自身大小。
(3)View的測量流程
View.performMeasure() -->onMeasure(int widthMeasureSpec, int heightMeasureSpec) -->setMeasuredDimension(int measuredWidth, int measuredHeight) -->setMeasuredDimensionRaw(int measuredWidth, int measuredHeight)
能夠看到setMeasuredDimensionRaw()方法:
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) { // 存儲測量結果 mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; // 設置測量完成的標誌位 mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; }
View不須要考慮子View的大小,根據內容測量得出自身大小便可。
另外,View中的onMeasure方法中調用到getDefaultSize方法:
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } 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: // 最終測量的結果都是父容器的大小 result = specSize; break; } return result; }
這裏看到精確模式和最大模式,最終測量的結果都是父容器的大小,即佈局中的wrap_content、match_parent以及數值大小效果都同樣,這也就是自定義View必定要重寫onMeasure方法的緣由。
二、layout
佈局相對測量而言要簡單許多,從ViewRootImpl的performLayout方法出發,能夠看到其中調用了DecorView的layout方法:
// 實則爲DecorView的left, top, right, bottom四個信息 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
進入layout方法,發現l、t、r、b被傳遞到了setFrame方法中,並設置給了成員變量:
mLeft = left; mTop = top; mRight = right; mBottom = bottom;
因此,佈局實際爲調用View的layout方法,設置自身的l、t、r、b值。另外,layout方法中往下走,能夠看到調用了onLayout方法,進入後發現爲空方法。於是查看FrameLayout的onLayout方法:
@Override protected void onLayout(boolean changed, int left, int top, int right, int bottom) { layoutChildren(left, top, right, bottom, false /* no force left gravity */); } void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) { final int count = getChildCount(); // 省略 for (int i = 0; i < count; i++) { final View child = getChildAt(i); if (child.getVisibility() != GONE) { final LayoutParams lp = (LayoutParams) child.getLayoutParams(); // 省略 child.layout(childLeft, childTop, childLeft + width, childTop + height); } } }
能夠看到,進行一系列計算後,調用了child的layout方法,對子控件進行佈局,同時子控件又會繼續往下對本身的子控件佈局,從而實現遍歷。
綜上,佈局實際爲調用layout方法設置View位置,ViewGroup則須要另外實現onLayout方法擺放子控件。
三、draw
(1)繪製過程入口
ViewRootImpl.performDraw() -->ViewRootImpl.draw() -->ViewRootImpl.drawSoftware() -->View.draw()
(2)繪製步驟
進入到View的draw方法中,能夠看到如下一段註釋:
/* * 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) */
結合draw方法的源碼,繪製過程的關鍵步驟以下:
==> 繪製背景:drawBackground(canvas)
==> 繪製本身:onDraw(canvas)
==> 繪製子view:dispatchDraw(canvas)
==> 繪製滾動條、前景等裝飾:onDrawForeground(canvas)
原文出處:https://www.cnblogs.com/joahyau/p/11294970.html