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 Android教程網 >> Android技術 >> 關於Android編程 >> android View繪制源碼分析

android View繪制源碼分析

編輯：關於Android編程

在開發過程中我們經常要進行view的自定義。如果熟練掌握自定義技巧的話就能做出很多控件出來。這篇博客來講講view繪制背後發生的那些事。

一, view的基礎知識

view的繪制概括

首先先說說view繪制的整體過程。
View繪制的源碼分析 ，它的三大流程都是在ViewRootImpl中完成的，從ViewRootImpl中的performTraversals開始，有三個方法performMeasure,performLayout,prformDraw分別對measure,layout,draw三個方法。在onMeasure對所有子元素進行measure過程 ，這時measure就從父容器傳遞到子元素。子元素重復父元素的過程。layout與draw類似，只是draw通過diapatchDraw來實現 。
measure完成後可以通過getMeasureWidth,getMeasureHeight分別獲取View測量後的寬高。在實際情況下幾乎所有情況它都等於最終寬高。layout過程決定view的四個頂點的坐標和實際view的寬高，完成之後可以通過getTop,getBottom,getLeft,getRight來拿 到view的四個頂點位置。並通過getWidth()和getHeight()來拿到最終寬高。draw決定了view的顯示，只有完成才能顯示在屏幕上。

MeasureSpec

在測量過程中系統會將View的LayoutParams根據容器所施加的規則轉換成對應的MeasureSpec，然後再根據這個測量出view。
Measure是一個32位的int,高2位代表SpecMode，低30位代表SpecSize。SpecMode表示測量模式，SpecSize指在某種測量模式下規格的大小。其代碼如下：

public static class MeasureSpec {
    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

    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;

public static int makeMeasureSpec(int size, int mode) {
    if (sUseBrokenMakeMeasureSpec) {
        return size + mode;
    } else {
        return (size & ~MODE_MASK) | (mode & MODE_MASK);
    }
}
public static int getMode(int measureSpec) {
    return (measureSpec & MODE_MASK) ;
}
public static int getSize(int measureSpec) {
    return (measureSpec & ~MODE_MASK) ;
}

其實MeasureSpec中源碼很值得我們學習。他用一個32位的int來表示模式和大小，節省了空間，也更直觀。MeasureSpec通過將specMode和specSize打包成一個int來避免過多的對象內存分配。以上是MeasureSpec的打包和解包過程。
specMode有三種狀態：UNSPECIFIED,EXACTLY（相當於match_parent和精確值這兩種模式）,AT_MOST（wrap_content）。

LayoutParams

對於一般容器，它的MeasureSpec是由父容器的MeasureSpec和自身的LayoutParams共同決定的。上篇博客LayoutInflater源碼解析 我們己經介紹了android view的結構，PhoneWindow包了一層DecorView,DecorView裡才是title和我們的content view。所以行分析DecorView。
先來看下DecorView的產生源碼：

 childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

再看下getRootMeasureSpec方法：

private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.//自定義
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

這裡很清楚，分別分MatchPraent和wrap_content和自定義來計算寬高。再來看下普通的view,在ViewGroup的measureChildWIthMargins中：

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);
}

再看下getChildMeasureSpec：

public static int getChildMeasureSpec(int spec, int padding , int childDimension) {
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec. getSize(spec) ;

    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:
        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:
        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:
        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 = 0;
            resultMode = MeasureSpec. UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = 0;
            resultMode = MeasureSpec. UNSPECIFIED;
        }
        break;
    }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode) ;
}

以上表明，如果父是EXACTLY,parentSize，那麼子如果是EXACTLY，
1）具體的值size：那子的MeasureSpec就是EXACTLY,size;
2)MATCH_PARENT:那子的MeasureSpec就是EXACTLY,parentSize;
3)WRAP_CONTENT:那子的MeasureSpec就是AT_MOST,parentSize;

如果父是ATMOST,parentSize，那麼子如果是EXACTLY，
1）具體的值size：那子的MeasureSpec就是EXACTLY,size;
2)MATCH_PARENT:那子的MeasureSpec就是AT_MOST,parentSize;
3)WRAP_CONTENT:那子的MeasureSpec就是AT_MOST,parentSize;
總結：對於普通View的MeasureSpec由父容器的MeasureSpec和自身的LayoutParams來共同決定。

二，三大過程源碼分析

OnMeasure

measure。如果是view,measure繪制其自身。如果是VIewGroup，measure繪制自身外，還要繪制其子元素。先看View的measure方法，measure是一個final方法，不能重寫： 
if (cacheIndex < 0 |if (cacheIndex < 0 || sIgnoreMeasureCache ) {
    // measure ourselves, this should set the measured dimension flag back
    onMeasure(widthMeasureSpec , heightMeasureSpec);
    mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}| sIgnoreMeasureCache ) {
    // measure ourselves, this should set the measured dimension flag back
    onMeasure(widthMeasureSpec , heightMeasureSpec);
    mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}

調用了onMeasure()，來看下它的源碼：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth() , widthMeasureSpec) ,
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)) ;
}

看下getSuggestedMinimumWidth()，它就是獲取背景大小和mMinWidth的較大值：

protected int getSuggestedMinimumWidth () {
    return (mBackground == null) ? mMinWidth : max(mMinWidth , mBackground .getMinimumWidth());
}

那麼mMinWidth是什麼呢，mMinWidth就是設置的android:minWidth的屬性，沒設置就等於0。不信，看如下代碼：

case R.styleable.View_minWidth:
    mMinWidth = a.getDimensionPixelSize(attr , 0) ;
    break;

getMinimumWidth()表示的是獲取背景圖大小，它位於Drawable下：

public int getMinimumHeight() {
    final int intrinsicHeight = getIntrinsicHeight() ;
    return intrinsicHeight > 0 ? intrinsicHeight : 0 ;
}

看下getDefaultSize方法：

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;
}

它返回了specSize，它是測量後的大小。由上面的分析可知，view的寬高由specSize決定，而如果直接繼承View的控件需要重寫onMeasure方法並設置wrap_content的自身大小。否則wrap_content就相當 於Match_parent了。一般一重寫方法如下：

 protected void onMeasure( int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
        int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
        if (widthSpecMode == MeasureSpec. AT_MOST
                && heightSpecMode == MeasureSpec. AT_MOST) {
            setMeasuredDimension(200, 200);
        } else if (widthSpecMode == MeasureSpec. AT_MOST) {
            setMeasuredDimension(200, heightSpecSize);
        } else if (heightSpecMode == MeasureSpec. AT_MOST) {
            setMeasuredDimension(widthSpecSize, 200);
        }
    }

上面的200是指定的一個默認寬高。

2.ViewGroup的measure過程，它沒有重寫onMeasure，它會調用measureChildren如下：

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);
        }
    }
}

分別繪制child，進入measureChild：

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);
}

獲取LayoutParams，通過getChildMeasureSpec來創建子無素的MeasureSpec，調用child.measure，因為ViewGroup有不同的特性，所以無法實現統一的onMeasure。

Layout的過程

viewGroup會遍歷所有子元素並調用 其layout方法，layout方法來確定子元素的位置。viewgroup如下：

protected abstract void onLayout(boolean changed,
        int l , int t, int r, int b) ;

需要子類自己實現。看下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);

    if (changed || ( mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED ) {
        onLayout(changed, l, t , r, b);
        mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLayoutChangeListeners != null) {
            ArrayList listenersCopy =
                    (ArrayList)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;
}

在setFrame中確定了view的四個頂點坐標。mleft等。onLayout view也沒有具體實現，要看具體的。以LinearLayout為例：

protected void onLayout(boolean changed, int l , int t, int r, int b) {
    if (mOrientation == VERTICAL) {
        layoutVertical(l, t, r , b);
    } else {
        layoutHorizontal(l, t, r , b);
    }
}

以layoutVertical為例：

void layoutVertical(int left, int top , int right, int bottom) {
    final int paddingLeft = mPaddingLeft ;

    int childTop ;
    int childLeft ;

    // Where right end of child should go
    final int width = right - left;
    int childRight = width - mPaddingRight ;

    // Space available for child
    int childSpace = width - paddingLeft - mPaddingRight ;

    final int count = getVirtualChildCount() ;

    final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
    final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

    switch (majorGravity) {
       case Gravity.BOTTOM:
           // mTotalLength contains the padding already
           childTop = mPaddingTop + bottom - top - mTotalLength;
           break;

           // mTotalLength contains the padding already
       case Gravity.CENTER_VERTICAL:
           childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
           break;

       case Gravity. TOP:
       default :
           childTop = mPaddingTop;
           break;
    }

    for (int i = 0; i < count ; i++) {
        final View child = getVirtualChildAt(i) ;
        if (child == null) {
            childTop += measureNullChild(i);
        } else if (child.getVisibility() != GONE) {
            final int childWidth = child.getMeasuredWidth() ;
            final int childHeight = child.getMeasuredHeight();

            final LinearLayout.LayoutParams lp =
                    (LinearLayout.LayoutParams) child.getLayoutParams() ;

            int gravity = lp. gravity;
            if (gravity < 0) {
                gravity = minorGravity;
            }
            final int layoutDirection = getLayoutDirection() ;
            final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection) ;
            switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                case Gravity. CENTER_HORIZONTAL :
                    childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                            + lp. leftMargin - lp.rightMargin ;
                    break;

                case Gravity.RIGHT:
                    childLeft = childRight - childWidth - lp. rightMargin;
                    break;

                case Gravity.LEFT:
                default:
                    childLeft = paddingLeft + lp. leftMargin;
                    break;
            }

            if (hasDividerBeforeChildAt(i)) {
                childTop += mDividerHeight;
            }

            childTop += lp.topMargin;
            setChildFrame(child , childLeft, childTop + getLocationOffset(child) ,
                    childWidth, childHeight);
            childTop += childHeight + lp. bottomMargin + getNextLocationOffset(child);

            i += getChildrenSkipCount(child , i);
        }
    }
}

主要看以下代碼：

final int childWidth = child.getMeasuredWidth() ;
final int childHeight = child.getMeasuredHeight() ;
setChildFrame(child , childLeft, childTop + getLocationOffset(child) ,
        childWidth , childHeight);
childTop += childHeight + lp. bottomMargin + getNextLocationOffset(child);

top會逐漸增大，所以會往下排。setChildFrame僅僅是調用子元素的layout方法。

private void setChildFrame(View child, int left, int top , int width, int height) {       
    child.layout(left, top, left + width , top + height);
}

通過子元素的layout來確定自身。

draw過程

它有以下幾步：

繪制背景，(canvas) 繪制自己。(onDraw) 繪制children(dispatchDraw) 繪制裝飾（onDrawScrollBars）
看下view的draw源碼： 
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) ;

        // Step 6, draw decorations (scrollbars)
        onDrawScrollBars(canvas) ;

        if ( mOverlay != null && !mOverlay.isEmpty()) {
            mOverlay .getOverlayView().dispatchDraw(canvas) ;
        }

        // we're done...
        return;
    }

viewgroup中的dispatchDraw用於遍歷子view並調用子view的draw方法。這樣就一層層的傳下去。
到此源碼分析就結束了。在繪制view的時候經常會在activity中獲得view的寬高，因為activity的生命周期和view不同步，在oncreate中無法獲取到view的寬高，接下來講講activity中如何獲取view。

三，view寬高確定

onWindowFocusChanged:view己經初始化完畢，寬高己經准備好。當Activity得到焦點和失去焦點均會被調用，所以它會調用多次。 通過view.post，將一個runnable投弟到消息隊列尾部，等待looper調用時，view己經初始化好。 
protected void onStart(){
super.onStart();
view.post(new Runnable(){
public void run(){
int width = view.getMeasuredWidth();
int height = new .getMeasuredHeight();
}
})
}
ViewTreeObserver:
使用ViewTreeObserver眾多回調接口來完成，如OnGlobalLayoutListener，當view樹狀態發生改變時或內部view可見性發生改變時會回調。 
ViewObserver obserber = view.getViewObserver ();
obserber.addOnGlobalLayoutListener(new OnGlobalLayoutListener(){
public void onGlobalLayout(){
obserber.removeOnGlobalLayoutListener(this);
int width = view.getMeasuredWidth();
int height = new .getMeasuredHeight();
}
})
通過view進行measure來得到view的寬高。 
int width = MeasureSpec.makeMeasureSpec(100,Measure.EXACTLY);//確定值
int height= MeasureSpec.makeMeasureSpec(100,Measure.EXACTLY);//確定值
view.measure(width,height);
對於wrap_content:
int width = MeasureSpec.makeMeasureSpec((1<<30)-1,Measure.AT_MOST);//wrap_content
int height= MeasureSpec.makeMeasureSpec((1<<30)-1,Measure.AT_MOST);//wrap_content
view.measure(width,height);

四，自定義view中注意事項

自定義View需要注意的事項：

如果是繼承view或者viewGroup，讓view支持wrap_content。

如果有必要，讓view支持padding。

view中如果有動畫或者線程，要在onDetachedFromWindow中及時停止。當view的Activity退出或者當前view被remove時，調用它。

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