performTravseral.png

• DecorView 是界面上的頂層view. ViewRoot是連接WindowManager和DecorView的紐帶.view的繪制流程從ViewRoot的perfromTraversals開始.
• ViewRoot作為頂級View,通常包括一個豎直方向的LinerLayout.這個LinerLayout分兩部分.上邊是標(biāo)題欄.就是屏幕顯示時間的那一行,下邊是內(nèi)容欄.id為content,我們平時設(shè)置的setContentView(),就是在想contentView里添加子類.通過ViewGroup content =(ViewGroup)findViewById(android.R.id.content);拿到contentView. content.getChildAt(0);拿到我們設(shè)置的View;
• MeasureSpace 用一個32位的int代表測量的數(shù)據(jù)和類型,高二位代表SpecMode,表示測量模式,低30位 SpecSize表示測量大小.
• MeasureSpace的三種模式
  • UNSPECIFIED:父容器不做限制,通常用于系統(tǒng)內(nèi)部.
  • EXACTLY 指定大小,view此時的大小就是SpecSize的值.它對應(yīng)于LayoutParams中的march_parent和具體指定的值(多少dp,sp).
  • At_MOST 父容器指定一個可用大小即SpecSize,View不能大于這個值.對于wrap_content.
• MeasureSpace 決定view的寬高,但MeasureSpace由父類和view自己的layoutParams共同決定.
• DecorView的MeasureSpace決定方式

3.png

2.png

-接著看vp如何測設(shè)計子類的MeasureSpace

 //parentWidthMeasureSpec,parentHeightMeasureSpec 是vp自己的measureSpace寬高值
    //widthUsed,heightUsed 是被vp額外用掉的寬高值
    protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {

        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        
        //viewgroup自己的measureSpace,第二個參數(shù)是vp所使用的值
        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);
        //調(diào)用子類進(jìn)行測量
        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }
    //得到子類的MeasureSpace 通過父類的specMode和子類Layoutparams的組合得到子類最終的MeasureSpace
    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        vp的MeasureSpec
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);
        
        //vp自己的尺寸減去自己的padding.margin等 最后得到子類能用的最大尺寸
        int size = Math.max(0, specSize - padding);
        
        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY: //vp是精確模式
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST: //父類是最大模式
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:  //父類是不確定模式
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
     }

-最終的匹配結(jié)果如下,parentSize值最終父類可用大小,既getChildMeasureSpec中的 size

4.png

-measure 過程

-view 的measures過程

view 的measure方法是final,既子類無法重寫該方法,在measure 中又調(diào)用了onMeasure方法,只需要看onMeasure方法就可以.

//widthMeasureSpec,heightMeasureSpec為父類vp傳過來的子類的MeasureSpace 寬高值
//getSuggestedMinimumWidth()  是建議的最小寬度,一般為view背景的大小和屬性android:minWidth大小的最大值.
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
//子類可以重寫這個方法來決定view的measure大小.但是必須調(diào)用setMeasuredDimension()方法,不然會拋出異常.
setMeasuredDimension()方法會設(shè)置view寬高的測量值.

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;
}
//UNSPECIFIED的情況可以忽略.看剩下兩種情況.所以 getDefaultSize的取值其實就是vp傳過來的measureSpace的specSize的值.

• 經(jīng)過以上代碼我們得出結(jié)論:直接繼承View的自定義空間要重寫onMeasure方法并設(shè)置wrap_content時自身大小,不然設(shè)置wrap_content和設(shè)置march_parent的效果一樣.
• 因為view 設(shè)置為wrap_content, 對應(yīng)的MeasureSpec 為SpecMode=AT_MOST,SpecSize=parentSize,在onMeasure中就會把父類可用空間的值設(shè)置給view.解決辦法如下

5.png

-ViewGroup 的measures過程

• viewgroup 出來測量自己,還要遍歷子veiw測量大小.vp通過measureChildren方法來遍歷測量子類大小

  //Gone的子view不會被測量
   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);
          }
      }
  }
  //這個方法和之前的 measureChildWithMarging 方法差不多,只是少了widthUsed,heighUsed參數(shù)
   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);
   }
  

• viewgroup并沒有實現(xiàn)具體測量過程,因為他是一個抽象類,需要子類例如LinerLayout等自行實現(xiàn)測量過程.

• 四種方法拿到view的寬高

  1. Activity/View#onWindowFocusChanged,這個方法的含義標(biāo)識窗口的交點變化時回調(diào),此時view已經(jīng)初始化完畢,寬高已經(jīng)準(zhǔn)備好了.此方法會回調(diào)多次

  public void onWindowFocusChanged(boolean hasFocus){
      super.onWindowFocusChanged(hasFocus);
      if(hasFocus){
          int width=view.getMeasuredWidth();
          int hei=view.getMeasuredHeigh();
      }
  }
  

  2. view.post(runnable) 發(fā)送一個消息到消息隊列的尾部,等Lopper調(diào)用此消息時,view已經(jīng)初始化完成

     view.post(new Runnable{

     @Override
     public void run(){
          int width=view.getMeasuredWidth();
          int hei=view.getMeasuredHeigh();
     }
     

     });

  3. ViewTreeObserver#onGlobalLayoutListener,當(dāng)view樹的狀態(tài)或者view樹的子view可見性發(fā)生改變時會回調(diào),方法會被回調(diào)多次

     ViewTreeObserver observer=view.getViewTreeObserver();
     observer.addonGlobalLayoutListener(new onGlobalLayoutListener(){

     publci void onGlobalLayout(){
         view.getViewTreeObserver().removeGlobalLayoutListener(this);//先移除監(jiān)聽
         int width=view.getMeasuredWidth();
         int hei=view.getMeasuredHeigh();
     }
     

     });

  4. view.measure(int,int);需要根據(jù)view 的layoutParam 來區(qū)分

     march_parent :此狀態(tài)無法得到結(jié)果,因為此時父view的測量寬高也不知道
     具體數(shù)值 :比如寬高為100

      int width =MeasureSpec.makeMeasureSpec(100,MeasursSpec.EACTLY)
      int heigh =MeasureSpec.makeMeasureSpec(100,MeasursSpec.EACTLY)
      view.measurd(width,heigh);
     

     wrap_content

      int width =MeasureSpec.makeMeasureSpec((1<<30)-1,MeasursSpec.EACTLY)
      int heigh =MeasureSpec.makeMeasureSpec((1<<30)-1,MeasursSpec.EACTLY)
      view.measurd(width,heigh);
     

     View 的最大尺寸為30位二進(jìn)制,既 2的30次方減1 既(1<<30)-1,在最大化模式下,我們用view理論上的最大值去構(gòu)造MeasureSpec是合理的.

-layout 過程

• layout 方法確定view本身的位置,在onLayout方法則會確定所有子元素的位置.

• viewGroup 在位置被確定后,會在onLayout中便利所有子類比調(diào)用其layout方法,在layout方法中onLayout方法又被調(diào)用.

• view 的layout方法中, 通過setFrame(l,t,r,b)來設(shè)定view的四個定點的位置.父容器通過onLayout方法來確定子view的位置.

• view和viewgroup均沒實現(xiàn)onLayout方法,需要看具體的layout.

• view 的layout過程

    public void layout(int l, int t, int r, int b) {
        //如果需要在layout前進(jìn)行一次measure
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) { 
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }
        //之前的位置 左上右下
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        //主要部分,設(shè)置view的左上右下位置.
        boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
        
        //通知監(jiān)聽器,界面layout發(fā)生變化
        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            onLayout(changed, l, t, r, b); //空實現(xiàn)
            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;
    }
  

-vieGroup layout過程

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); //其實還是調(diào)用了父類,最終調(diào)用view 的layout
        } else {
            // record the fact that we noop'd it; request layout when transition finishes
            mLayoutCalledWhileSuppressed = true;
        }
}

• getwidth getMeasuredWidth 區(qū)別

• getMeasuredWith 形成于measure過程.getWidth 形成于layout過程.

• 一般情況兩者是相同的.除非如下情況

  public void layout (int l,int t,int r,int b){
      super.layout(l,t,r+100,b+100);
  }     
  

-draw 過程

1罐柳、對View的背景進(jìn)行繪制
2、保存當(dāng)前的圖層信息(可跳過)
3老客、繪制View的內(nèi)容
4、對View的子View進(jìn)行繪制(如果有子View)
5、繪制View的褪色的邊緣蛛壳，類似于陰影效果(可跳過)
6吸占、繪制View的裝飾（例如：滾動條)

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); //繪制背景
 }
 
 // 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); //空實現(xiàn),需要子類實現(xiàn)
 
  // Step 4, draw the children
  dispatchDraw(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);
 
  // we're done...
  return;
 }
 ...
}


分布講解
1繪制背景
private void drawBackground(Canvas canvas) {

     //mBackground是該View的背景參數(shù)馁筐，比如背景顏色,沒有背景就不繪制
     final Drawable background = mBackground;
     if (background == null) {
      return;
     }
     
     //根據(jù)View四個布局參數(shù)來確定背景的邊界 mBackground.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);
     setBackgroundBounds();
     
     ...
     
     //獲取當(dāng)前View的mScrollX和mScrollY值
     final int scrollX = mScrollX;
     final int scrollY = mScrollY;
     if ((scrollX | scrollY) == 0) {
      background.draw(canvas);
     } else {
      //如果scrollX和scrollY有值调限，則對canvas的坐標(biāo)進(jìn)行偏移舟陆，再繪制背景
      canvas.translate(scrollX, scrollY);
      background.draw(canvas);
      canvas.translate(-scrollX, -scrollY);
     }
}
    3.繪制子View,view中這個方法為空,viewgroup實現(xiàn)了這個方法
    protected void dispatchDraw(Canvas canvas) {
        boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
        final int childrenCount = mChildrenCount;
        final View[] children = mChildren;
        int flags = mGroupFlags;
            
        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); //這句是重點
                }
                transientIndex++;
                if (transientIndex >= transientCount) {
                    transientIndex = -1;
                }
            }
            int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i;
            final View child = (preorderedList == null)? children[childIndex] : preorderedList.get(childIndex);
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
                more |= drawChild(canvas, child, drawingTime);//這句是重點
            }
        }
            //省略...
    }

    drawChild(canvas, transientChild, drawingTime)的實現(xiàn),調(diào)用了子view的繪制方法
    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
         return child.draw(canvas, this, drawingTime);
    }

• view 的draw(Canvas canvas, ViewGroup parent, long drawingTime)
• 我們主要來看核心部分，首先判斷是否已經(jīng)有緩存旧噪，即之前是否已經(jīng)繪制過一次了吨娜，如果沒有脓匿，則會調(diào)用draw(canvas)方法淘钟，開始正常的繪制，即上面所說的六個步驟陪毡，否則利用緩存來顯示米母。
• 這一步也可以歸納為ViewGroup繪制過程，它對子View進(jìn)行了繪制毡琉，而子View又會調(diào)用自身的draw方法來繪制自身铁瞒，這樣不斷遍歷子View及子View的不斷對自身的繪制，從而使得View樹完成繪制

•   Skip 6 繪制裝飾,指View除了背景桅滋、內(nèi)容慧耍、子View的其余部分，例如滾動條等丐谋，我們看View#onDrawForeground:
    public void onDrawForeground(Canvas canvas) {
     onDrawScrollIndicators(canvas);
     onDrawScrollBars(canvas);
     
     final Drawable foreground = mForegroundInfo != null ? mForegroundInfo.mDrawable : null;
     if (foreground != null) {
      if (mForegroundInfo.mBoundsChanged) {
       mForegroundInfo.mBoundsChanged = false;
       final Rect selfBounds = mForegroundInfo.mSelfBounds;
       final Rect overlayBounds = mForegroundInfo.mOverlayBounds;
     
       if (mForegroundInfo.mInsidePadding) {
        selfBounds.set(0, 0, getWidth(), getHeight());
       } else {
        selfBounds.set(getPaddingLeft(), getPaddingTop(),
          getWidth() - getPaddingRight(), getHeight() - getPaddingBottom());
       }
     
       final int ld = getLayoutDirection();
       Gravity.apply(mForegroundInfo.mGravity, foreground.getIntrinsicWidth(),
         foreground.getIntrinsicHeight(), selfBounds, overlayBounds, ld);
       foreground.setBounds(overlayBounds);
      }
     
      foreground.draw(canvas);
     }
    }
  

自定義view,寬高寫為wrap_content時,如果不處理,和寫成 march_parent是一樣的,處理規(guī)則如下,mWidth,mHeigh為自定義寬高

11.png