Android中的任何一个布局、任何一个控件其实都是直接或间接继承自View实现的,当然也包括我们在平时开发中所写的各种炫酷的自定义控件了,所以学习View的工作原理对于我们来说显得格外重要,本篇博客,我们将一起深入学习Android中View的工作原理。

ViewRoot和DecorView

1.ViewRoot对应于ViewRootImpl类,是连接WindowManager和DecorView的纽带,View的三大流程均是通过ViewRoot来完成的。在ActivityThread中,当Activity对象被创建完毕后,会将DecorView添加到Window中,同时会创建ViewRootImpl对象,并将ViewRootImpl对象和DecorView建立关联。

2.View的绘制流程从ViewRoot的performTraversals开始,经过measure、layout和draw三个过程才可以把一个View绘制出来,其中measure用来测量View的宽高,layout用来确定View在父容器中的放置位置,而draw则负责将View绘制到屏幕上。

3.performTraversals会依次调用performMeasure、performLayout和performDraw三个方法,这三个方法分别完成顶级View的measure、layout和draw这三大流程。其中performMeasure中会调用measure方法,在measure方法中又会调用onMeasure方法,在onMeasure方法中则会对所有子元素进行measure过程,这样就完成了一次measure过程;子元素会重复父容器的measure过程,如此反复完成了整个View数的遍历。

这里写图片描述

measure过程决定了View的宽/高,完成后可通过getMeasuredWidth/getMeasureHeight方法来获取View测量后的宽/高。Layout过程决定了View的四个顶点的坐标和实际View的宽高,完成后可通过getTop、getBotton、getLeft和getRight拿到View的四个定点坐标。Draw过程决定了View的显示,完成后View的内容才能呈现到屏幕上。

DecorView作为顶级View,一般情况下它内部包含了一个竖直方向的LinearLayout,里面分为两个部分(具体情况和Android版本和主题有关),上面是标题栏,下面是内容栏。在Activity通过setContextView所设置的布局文件其实就是被加载到内容栏之中的。

//获取内容栏 
ViewGroup content = findViewById(R.android.id.content); 
//获取我们设置的Viewcontext.getChildAt(0); 
DecorView其实是一个FrameLayout,View层的事件都先经过DecorView,然后才传给我们的View。

这里写图片描述

MeasureSpec

1.MeasureSpec很大程度上决定一个View的尺寸规格,测量过程中,系统会将View的layoutParams根据父容器所施加的规则转换成对应的MeasureSpec,再根据这个measureSpec来测量出View的宽/高。

2.MeasureSpec代表一个32位的int值,高2位为SpecMode,低30位为SpecSize,SpecMode是指测量模式,SpecSize是指在某种测量模式下的规格大小。

MpecMode有三类;

1.UNSPECIFIED 父容器不对View进行任何限制,要多大给多大,一般用于系统内部

2.EXACTLY 父容器检测到View所需要的精确大小,这时候View的最终大小就是SpecSize所指定的值,对应LayoutParams中的match_parent和具体数值这两种模式。

3.AT_MOST 父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值,不同View实现不同,对应LayoutParams中的wrap_content。

当View采用固定宽/高的时候,不管父容器的MeasureSpec的是什么,View的MeasureSpec都是精确模式兵其大小遵循Layoutparams的大小。 当View的宽/高是match_parent时,如果他的父容器的模式是精确模式,那View也是精确模式并且大小是父容器的剩余空间;如果父容器是最大模式,那么View也是最大模式并且起大小不会超过父容器的剩余空间。 当View的宽/高是wrap_content时,不管父容器的模式是精确还是最大化,View的模式总是最大化并且不能超过父容器的剩余空间。

对于DecorView,它的MeasureSpec由Window的尺寸和其自身的LayoutParams来共同确定,对于普通的View,其MeasureSpec由父容器的MeasureSpec和自身的Layoutparams来共同确定。

对于 DecorView,在ViewRootImpl源码中的measureHierarchy有如下一段代码:

......... 
if (baseSize != 0 && desiredWindowWidth > baseSize) { 
                childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width); 
                childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height); 
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); 
                if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured (" 
                        + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")"); 
                if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) { 
                    goodMeasure = true; 
.........

我们查看一下getRootMeasureSpec方法的源码:

 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. 
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); 
            break; 
        } 
        return measureSpec; 
    }

从上面的代码中就可以很容理解DecorView的MeasureSpec是如何产生的,rootDimension就是DecorView自身的LayoutParams,然后会根据这个值进行判断
LayoutParams.MATCH_PARENT:DecorView的MeasureSpec被赋值为精确模式,DecorView的大小就是Window的大小

ViewGroup.LayoutParams.WRAP_CONTENT:DecorView的MeasureSpec被赋值为最大模式,DecorView的大小不定,但是不能超过Window的大小

默认情况:DecorView的MeasureSpec被赋值为精确模式,DecorView的大小为自身LayoutParams设置的值,也就是rootDimension

接着是对于普通的View,也就是布局中的View,它的Measure过程由ViewGroup传递而来,其中有一个方法是measureChildWithMargins

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); 
    }

在对子view进行measure之前会先调用getChildMeasureSpec方法来获取子view的MeasureSpec,从这段代码就可以看出来子view的MeasureSpec的确定与父容器的MeasureSpec(parentWidthMeasureSpec)还有自身的LayoutParams(lp.height和lp.width),还有View自己的Margin和Padding有关

接下来查看getChildMeasureSpec方法源码:

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 = 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; 
        } 
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode); 
    }

这里参数中的padding是指父容器的padding,这里是父容器所占用的空间,所以子view能使用的空间要减去这个padding的值。同时这个方法内部其实就是根据父容器的MeasureSpec结合子view的LayoutParams来确定子view的MeasureSpec

View的绘制流程

measure的过程

如果只是一个View,那么通过measure方法就完成了其测量的过程,如果是一个ViewGroup,除了测量自身外,还会调用子孩子的measure方法

1.View的measure过程

View的measure过程由其measure方法完成,其中有下面一段内容

......... 
int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 : 
                    mMeasureCache.indexOfKey(key); 
            if (cacheIndex < 0 || sIgnoreMeasureCache) { 
                // measure ourselves, this should set the measured dimension flag back 
                onMeasure(widthMeasureSpec, heightMeasureSpec); 
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; 
            } else { 
                long value = mMeasureCache.valueAt(cacheIndex); 
                // Casting a long to int drops the high 32 bits, no mask needed 
                setMeasuredDimensionRaw((int) (value >> 32), (int) value); 
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; 
            } 
.........

可以知道View的measure方法内,其实调用了自身的onMeasure方法

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), 
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); 
} 
//里面有一个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; 
    }

一般我们只需要看MeasureSpec.AT_MOST和MeasureSpec.EXACTLY两种情况,这两种情况返回的result其实都是measureSpec中取得的specSize,这个specSize就是View测量后的大小,这里之所以是View测量后的大小,是因为View的最终大小是在layout阶段确定的,所以要加已区分,一般情况下View测量大小和最终大小是一样的。

UNSPECIFIED情况下,result的值就是getSuggestedMinimumWidth()方法和getSuggestedMinimumHeight()返回的值,查看这两个方法

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

从getSuggestedMinimumWidth代码可以看出,如果View没有设置背景,那么宽度就为mMinWidth,这个值对应android:minWidth这个属性所设定的值,如果View设置了背景,则为max(mMinWidth, mBackground.getMinimumWidth())

public int getMinimumWidth() { 
        final int intrinsicWidth = getIntrinsicWidth(); 
        return intrinsicWidth > 0 ? intrinsicWidth : 0; 
    }

查看mBackground.getMinimumWidth()方法,它其实是Drawable的方法,如果intrinsicHeight也就是原始的宽度不为0,就返回它,如果为0,就返回0。

从View的getDefaultSize方法可以得出结论:View的宽高由specSize决定,如果我们通过继承View来自定义控件需要重写onMeasure方法,并设置WRAP_CONTENT时的大小,否则在布局中使用WRAP_CONTENT相当于使用MATCH_PARENT

原因:因为View在布局中使用WRAP_CONTENT就相当于specMode为AT_MOST,而这种情况下,result = specSize,这个specSize的大小为parentSize, parentSize就是父容器目前可用的大小,也就是父容器当前剩余空间的大小,那这时候和在布局中使用MATCH_PARENT效果是一样的

所以在AT_MOST模式下,我们一般都会给View设定默认的内部宽高,并在WRAP_CONTENT时设置此宽高即可。
可以通过查看TextView、ImageView的源码,可以得知在WRAP_CONTENT下,onMeasure方法均做了特殊的处理,下面是TextView的onMeasure中的一段内容

if (widthMode == MeasureSpec.AT_MOST) { 
                width = Math.min(widthSize, width); 
            }

2.ViewGroup的measure流程

ViewGroup是一个抽象类,它没有重写View的onMeasure方法,而是自己提供了一个measureChildren方法

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); 
            } 
        } 
    }

里面会对子元素进行遍历,然后调用measureChild方法去测量每一个子元素的宽高

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); 
}

在对子view进行measure之前会先调用getChildMeasureSpec方法来获取子孩子的MeasureSpec,从这段代码就可以看出来子view的MeasureSpec的确定与父容器的MeasureSpec(parentWidthMeasureSpec和parentHeightMeasureSpec)还有自身的LayoutParams(lp.height和lp.width),还有View自己的Margin和Padding有关,最后就是调用子view的measure方法

ViewGroup并没有去定义测量的具体过程,这是因为ViewGroup是一个抽象类,其onMeasure方法需要各个子类去实现,因为每个ViewGroup的实现类,例如LinearLayout,RelativeLayout等的布局方式都是不同的,所以不可能一概而论的来写onMeasure方法。

接下来分析LinearLayout的onMeasure方法:

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 
        if (mOrientation == VERTICAL) { 
            measureVertical(widthMeasureSpec, heightMeasureSpec); 
        } else { 
            measureHorizontal(widthMeasureSpec, heightMeasureSpec); 
        } 
}

查看measureVertical方法

// See how tall everyone is. Also remember max width. 
        for (int i = 0; i < count; ++i) { 
            final View child = getVirtualChildAt(i); 
 
            if (child == null) { 
                mTotalLength += measureNullChild(i); 
                continue; 
            } 
 
            if (child.getVisibility() == View.GONE) { 
               i += getChildrenSkipCount(child, i); 
               continue; 
            } 
 
            if (hasDividerBeforeChildAt(i)) { 
                mTotalLength += mDividerHeight; 
            } 
 
            LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams(); 
 
            totalWeight += lp.weight; 
 
            if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) { 
                // Optimization: don't bother measuring children who are going to use 
                // leftover space. These views will get measured again down below if 
                // there is any leftover space. 
                final int totalLength = mTotalLength; 
                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin); 
                skippedMeasure = true; 
            } else { 
                int oldHeight = Integer.MIN_VALUE; 
 
                if (lp.height == 0 && lp.weight > 0) { 
                    // heightMode is either UNSPECIFIED or AT_MOST, and this 
                    // child wanted to stretch to fill available space. 
                    // Translate that to WRAP_CONTENT so that it does not end up 
                    // with a height of 0 
                    oldHeight = 0; 
                    lp.height = LayoutParams.WRAP_CONTENT; 
                } 
 
                // Determine how big this child would like to be. If this or 
                // previous children have given a weight, then we allow it to 
                // use all available space (and we will shrink things later 
                // if needed). 
                measureChildBeforeLayout( 
                       child, i, widthMeasureSpec, 0, heightMeasureSpec, 
                       totalWeight == 0 ? mTotalLength : 0); 
 
                if (oldHeight != Integer.MIN_VALUE) { 
                   lp.height = oldHeight; 
                } 
 
                final int childHeight = child.getMeasuredHeight(); 
                final int totalLength = mTotalLength; 
                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin + 
                       lp.bottomMargin + getNextLocationOffset(child)); 
 
                if (useLargestChild) { 
                    largestChildHeight = Math.max(childHeight, largestChildHeight); 
                } 
            } 
 
            /** 
             * If applicable, compute the additional offset to the child's baseline 
             * we'll need later when asked [email protected] #getBaseline}. 
             */ 
            if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) { 
               mBaselineChildTop = mTotalLength; 
            } 
 
            // if we are trying to use a child index for our baseline, the above 
            // book keeping only works if there are no children above it with 
            // weight.  fail fast to aid the developer. 
            if (i < baselineChildIndex && lp.weight > 0) { 
                throw new RuntimeException("A child of LinearLayout with index " 
                        + "less than mBaselineAlignedChildIndex has weight > 0, which " 
                        + "won't work.  Either remove the weight, or don't set " 
                        + "mBaselineAlignedChildIndex."); 
            } 
 
            boolean matchWidthLocally = false; 
            if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) { 
                // The width of the linear layout will scale, and at least one 
                // child said it wanted to match our width. Set a flag 
                // indicating that we need to remeasure at least that view when 
                // we know our width. 
                matchWidth = true; 
                matchWidthLocally = true; 
            } 
 
            final int margin = lp.leftMargin + lp.rightMargin; 
            final int measuredWidth = child.getMeasuredWidth() + margin; 
            maxWidth = Math.max(maxWidth, measuredWidth); 
            childState = combineMeasuredStates(childState, child.getMeasuredState()); 
 
            allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT; 
            if (lp.weight > 0) { 
                /* 
                 * Widths of weighted Views are bogus if we end up 
                 * remeasuring, so keep them separate. 
                 */ 
                weightedMaxWidth = Math.max(weightedMaxWidth, 
                        matchWidthLocally ? margin : measuredWidth); 
            } else { 
                alternativeMaxWidth = Math.max(alternativeMaxWidth, 
                        matchWidthLocally ? margin : measuredWidth); 
            } 
 
            i += getChildrenSkipCount(child, i); 
        }

遍历子元素,调用他们的measureChildBeforeLayout方法,这个方法内会测量子孩子的宽高,并且有一个mTotalLength来记录LinearLayout 在竖直方向的初步高度,每测量一次子元素,mTotalLength都会增加,增加部分包括子元素的高度以及子元素竖直方向的margin

void measureChildBeforeLayout(View child, int childIndex, 
            int widthMeasureSpec, int totalWidth, int heightMeasureSpec, 
            int totalHeight) { 
        measureChildWithMargins(child, widthMeasureSpec, totalWidth, 
                heightMeasureSpec, totalHeight); 
    } 
里面调用了child.measure方法,也就是子孩子的measure方法 
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); 
}

当子元素测量完毕后,LinearLayout会测量自身的大小,对于竖直的LinearLayout,它在水平方向上的测量过程,遵循View的测量过程,在竖直方向上,如果采用的是match_parent或者具体的数值,那么它的测量过程和View的一致,即高度为specSize;如果它的布局中高度采用wrap_content,那么高度是子元素所占用的高度总和,但这个和不能超过父容器的剩余空间,当然还要考虑padding,竖直方向的结论可以从下面代码得知:

public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) { 
        final int specMode = MeasureSpec.getMode(measureSpec); 
        final int specSize = MeasureSpec.getSize(measureSpec); 
        final int result; 
        switch (specMode) { 
            case MeasureSpec.AT_MOST: 
                if (specSize < size) { 
                    result = specSize | MEASURED_STATE_TOO_SMALL; 
                } else { 
                    result = size; 
                } 
                break; 
            case MeasureSpec.EXACTLY: 
                result = specSize; 
                break; 
            case MeasureSpec.UNSPECIFIED: 
            default: 
                result = size; 
        } 
        return result | (childMeasuredState & MEASURED_STATE_MASK); 
}

有时候onMeasure中拿到的测量宽高可能是不准确的,比较好的习惯是在onLayout中去获取View的测量宽高和最终宽高

在Activity中,在onCreate,onStart,onResume中均无法正确获得View的宽高信息,这是因为measure和Activity的生命周期是不同步的,所以很可能View没有测量完毕,获得的宽高是0.

measure总结

1.measure过程主要就是从顶层父View向子View递归调用view.measure方法(measure中又回调onMeasure方法)的过程。具体measure核心主要有如下几点:

2.MeasureSpec(View的内部类)测量规格为int型,值由高2位规格模式specMode和低30位具体尺寸specSize组成。其中specMode只有三种值:

MeasureSpec.EXACTLY //确定模式,父View希望子View的大小是确定的,由specSize决定; 
MeasureSpec.AT_MOST //最多模式,父View希望子View的大小最多是specSize指定的值; 
MeasureSpec.UNSPECIFIED //未指定模式,父View完全依据子View的设计值来决定;

3.View的measure方法是final的,不允许重载,View子类只能重载onMeasure来完成自己的测量逻辑。

4.最顶层DecorView测量时的MeasureSpec是由ViewRootImpl中getRootMeasureSpec方法确定的(LayoutParams宽高参数均为MATCH_PARENT,specMode是EXACTLY,specSize为物理屏幕大小)。

5.ViewGroup类提供了measureChild,measureChild和measureChildWithMargins方法,简化了父子View的尺寸计算。

6.只要是ViewGroup的子类就必须要求LayoutParams继承子MarginLayoutParams,否则无法使用layout_margin参数。

7.View的布局大小由父View和子View共同决定。

8.使用View的getMeasuredWidth()和getMeasuredHeight()方法来获取View测量的宽高,必须保证这两个方法在onMeasure流程之后被调用才能返回有效值。

layout的过程

ViewGroup的位置确定后,它在onLayout中会遍历所有的子元素并调用子元素layout方法,子元素layout方法中又会调用onLayout方法,View的layout方法确定自身的位置,而onLayout方法方法确定子孩子的位置

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<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; 
}

layout方法的大致流程如下:首先会通过setFrame方法来确定mLeft;mTop;mBottom;

mRight;只要这四个点一旦确定,那么View在父容器中的位置就确定了,接着会调用onLayout方法,该方法目的是父容器来确定子元素的位置,无论是View还是ViewGroup都没有实现onLayout方法,我们查看LinearLayout的onLayout方法

@Override 
    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); 
        } 
}

查看layoutVertical中关键代码

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); 
            } 
        }

这个方法会遍历所有的子元素并调用setChildFrame方法来为子元素指定对应的位置,其中childTop的数值会不断的增大,这意味着后面的子元素还位于靠下的位置,刚好符合竖直的LinearLayout的特性,setChildFrame方法中不过是调用了子元素的Layout方法而已

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

同时,会发现setChildFrame中的width和height实际上就是子元素的测量宽高

final int childWidth = child.getMeasuredWidth(); 
             final int childHeight = child.getMeasuredHeight();

View的layout方法中会通过setFrame方法去设置子元素四个顶点的位置,这样子元素的位置就可以确定

int oldWidth = mRight - mLeft; 
            int oldHeight = mBottom - mTop; 
            int newWidth = right - left; 
            int newHeight = bottom - top; 
            boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight); 
 
            // Invalidate our old position 
            invalidate(sizeChanged); 
 
            mLeft = left; 
            mTop = top; 
            mRight = right; 
            mBottom = bottom; 
            mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);

接下来是View的getWidth和getHeight方法,结合里面的实现,可以发现他们分别返回的就是View测量的宽度和高度

@ViewDebug.ExportedProperty(category = "layout") 
    public final int getWidth() { 
        return mRight - mLeft; 
    } 
 
    /** 
     * Return the height of your view. 
     * 
     * @return The height of your view, in pixels. 
     */ 
    @ViewDebug.ExportedProperty(category = "layout") 
    public final int getHeight() { 
        return mBottom - mTop; 
}

layout总结

1.layout也是从顶层父View向子View的递归调用view.layout方法的过程,即父View根据上一步measure子View所得到的布局大小和布局参数,将子View放在合适的位置上。

2.View.layout方法可被重载,ViewGroup.layout为final的不可重载,ViewGroup.onLayout为abstract的,子类必须重载实现自己的位置逻辑。

3.measure操作完成后得到的是对每个View经测量过的measuredWidth和measuredHeight,layout操作完成之后得到的是对每个View进行位置分配后的mLeft、mTop、mRight、mBottom,这些值都是相对于父View来说的。

4.凡是layout_XXX的布局属性基本都针对的是包含子View的ViewGroup的,当对一个没有父容器的View设置相关layout_XXX属性是没有任何意义的。

5.使用View的getWidth()和getHeight()方法来获取View测量的宽高,必须保证这两个方法在onLayout流程之后被调用才能返回有效值。

draw的过程

View的绘制过程遵循以下几步:

1)绘制背景background.draw(canvas)
2)绘制自己(onDraw)
3)绘制 children(dispatchDraw)
4)绘制装饰(onDrawScrollBars)

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); 
 
            // 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; 
        } 
 
        /* 
         * 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); 
 
        // 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); 
}

View的绘制过程的传递是通过dispatchDraw实现的,dispatchdraw会遍历调用所有子元素的draw方法。如此draw事件就一层一层的传递下去。

draw总结

1.如果该View是一个ViewGroup,则需要递归绘制其所包含的所有子View。

2.View默认不会绘制任何内容,真正的绘制都需要自己在子类中实现。

3.View的绘制是借助onDraw方法传入的Canvas类来进行的。

4.在获取画布剪切区(每个View的draw中传入的Canvas)时会自动处理掉padding,子View获取Canvas不用关注这些逻辑,只用关心如何绘制即可。

5.默认情况下子View的ViewGroup.drawChild绘制顺序和子View被添加的顺序一致,但是你也可以重载ViewGroup.getChildDrawingOrder()方法提供不同顺序。

参考资料
《Android开发艺术探索》

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