View 的 measure 流程-再总结

看图说话

流程图

借用该链接：凶残的程序员-View 的工作流程 的两张图，来表示大致的工作流程。

时序图

来一张时序图：

ViewRootImpl DecorView View FrameLayout ViewGroup 0，performTraversals 1，performMeasure 2，measure 没有重写，直调父 类 3，super.measure 调整宽高为正值 4，onMeasure 规格强置为 EXACTLY 5，super.onMeasure(...) 6，measureChildWithMargins(child, ... ) 7，child.measure(width, height) loop [ 测量每个子 view ] 8，setMeasuredDimension(resolveSizeAndState(...),resolveSizeAndState(...)) 测完子view，测自 己 ViewRootImpl DecorView View FrameLayout ViewGroup

表格对比

再来一张表格，展示 measure 函数 和 onMeasure 函数在不同类实现上的一些区别：

	measure	onMeasure		measure	onMeasure
ViewGroup	null， 调用View#measure	null	View	判断是否要测量， 调用 onMeasure	默认的设置尺寸逻辑
ViewGroup子类	null	根据自身布局的特点来测量子 View，其中会调用到 ViewGroup 的 measureChildWithMargins 函数 和 getChildMeasureSpec 函数；测出子 View 的规格后。for循环：调用 child.measure 触发子 View 的测量流程，并传递规格给子 View	View子类	null	完全重写，根据自身特点来测量， 调用View#setMeasuredDimension来保存测量规格

简单源码梳理

第0步 performTraversals

• 0，ViewRootImpl#performTraversals 源码

// 代码段0：
// ViewRootImpl 类

    public final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();

    // These are accessed by multiple threads.
    final Rect mWinFrame; // frame given by window manager.
    
    // 构造函数中
    //mWinFrame = new Rect();
    
    private void performTraversals() {
        // cache mView since it is used so much below...
        final View host = mView;

        WindowManager.LayoutParams lp = mWindowAttributes;

        Rect frame = mWinFrame;

        if (mWidth != frame.width() || mHeight != frame.height()) {
                mWidth = frame.width();
                mHeight = frame.height();                
        }

        // 2832 line
        int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); // 0-1
        int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

        // 2844 line
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); // 0-2
        
        ...

注释0-1处，调用 getRootMeasureSpec 函数，测量 DecorView 的宽高 MeasureSpec ;

注释0-2处，调用 performMeasure 函数 ，传入0-1处的测量结果，继续流程。

注意，入参的 lp 的值来自 mWindowAttributes，而 mWindowAttributes 在 ViewRootImpl#setView 中设置了值：

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
      synchronized (this) {
    if (mView == null) {
        mView = view;

        mWindowAttributes.copyFrom(attrs);
        // 略。。。
        // Schedule the first layout -before- adding to the window
        // manager, to make sure we do the relayout before receiving
        // any other events from the system.
        requestLayout();

接着 goin 注释0-1 getRootMeasureSpec(mWidth, lp.width)

// ViewRootImpl 类

    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;
    }

逻辑如表格所示：

	MATCH_PARENT	WRAP_CONTENT	default
mode	EXACTLY	AT_MOST	EXACTLY
size	windowSize	windowSize	rootDimension

执行该函数得到的宽高 MeasureSpec，传入 0-2 处的 performMeasure 函数，流程继续。

第1步 performMeasure

• 1，ViewRootImpl#performMeasure 源码

// 代码段1
// ViewRootImpl 类

    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        if (mView == null) {
            return;
        }
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            // 1-1
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); 
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

在注释 1-1 处，调用 DecorView 的 measure 函数，流程继续。

第2步 DecorView#measure

• 2，DecorView#measure

DecorView 类没有重写 measure 函数，直接调用 View#measure；

DecorView 是 View 的子类，View 的 measure 函数是 final 修饰的，任何子类都必须直接使用 View 的 measure 函数，不能重写。

第3步 View#measure

• 3，View#measure 源码

// 代码段2
// 代码来源于：/sdk/sources/android-28/android/view/View.java
// View 类：

    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
                int oHeight = insets.top  + insets.bottom;
            // 2-1，可能需要调整一下，使其不为负数
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;

	   // 。。。
        final boolean needsLayout = specChanged
                && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);

        if (forceLayout || needsLayout) { // 2-2 判断是否需要测量

            int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                     onMeasure(widthMeasureSpec, heightMeasureSpec);  // 2-3
            } 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);
            }

		// 。。。
        }

         mOldWidthMeasureSpec = widthMeasureSpec; // 记录旧规格
        mOldHeightMeasureSpec = heightMeasureSpec;
    }

在 2-2 处，判断是否需要测量；
在 2-3 处，调用 onMeasure 函数，其参数 widthMeasureSpec 和 heightMeasureSpec，就是入参的两个函数 ，有可能调整过，有可能原封不动。
流程继续。

第4步 DecorView#onMeasure

• 4，DecorView#onMeasure 源码

// 代码段3：
// DecorView 类

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

        final int widthMode = getMode(widthMeasureSpec);
        final int heightMode = getMode(heightMeasureSpec);

        boolean fixedWidth = false;
        mApplyFloatingHorizontalInsets = false;
        if (widthMode == AT_MOST) {
            final TypedValue tvw = isPortrait ? mWindow.mFixedWidthMinor : mWindow.mFixedWidthMajor;
            if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
                final int w;
                if (tvw.type == TypedValue.TYPE_DIMENSION) {
                    w = (int) tvw.getDimension(metrics);
                } else if (tvw.type == TypedValue.TYPE_FRACTION) {
                    w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels);
                } else {
                    w = 0;
                }
                if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed width: " + w);
                final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
                if (w > 0) {
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            Math.min(w, widthSize), EXACTLY);
                    fixedWidth = true;
                } else {
                    widthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            widthSize - mFloatingInsets.left - mFloatingInsets.right,
                            AT_MOST);
                    mApplyFloatingHorizontalInsets = true;
                }
            }
        }

        // heightMode == AT_MOST， 跟 widthMode 一样的处理逻辑
        // ...

        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        // ...
    }

在 DecorView 的 onMeasure 函数中，若 widthMode == AT_MOST，强制将其规格模式设置为 EXACTLY，然后调用父类 FrameLayout 的 onMeasure 函数。

第5步 FrameLayout#onMeasure

• 5，FrameLayout#onMeasure 源码

// 代码段3.1
// FrameLayout 类

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();

        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                // 3.1-1 测量 child 的 MeasureSpec，
                // 并调用 child.measure，开启 child 的测量流程
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                // 。。。
            }
        }

        // 。。。

        // 3.1-2，调用基类 View 类的 setMeasuredDimension 函数，设置大小
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));

    }

3.1-1 处，循环的调用 measureChildWithMargins 函数，是在 ViewGroup 中定义的，接着到第7步，看源码：

第6步 measureChildWithMargins

• 6，ViewGroup#measureChildWithMargins

// 代码段3.1.1
// ViewGroup 类

    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);
    }

其内部又调用了 getChildMeasureSpec 函数，源码：

// 代码段3.1.1.1
// ViewGroup 类

    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;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

MeasureSpec类帮助我们来测量View，它是一个32位的int值，高两位为specMode （测量的模式），低30位为specSize （测量的大小），测量模式分为三种：

• UNSPECIFIED：未指定模式，View想多大就多大，父容器不做限制，一般用于系统内部的测量。
• AT_MOST：最大模式，对应于wrap_comtent属性，只要尺寸不超过父控件允许的最大尺寸就行。
• EXACTLY：精确模式，对应于match_parent属性和具体的数值，父容器测量出View所需要的大小，也就是specSize的值。

测量逻辑可用表格展示，引用：该链接：

该步骤执行完后，会得到 子view 宽高的规格；

再回到第7步，代码段3.1.1，在 measureChildWithMargins 函数的末尾，调用了子类的 measure 函数，开始子 view 的测量工作，此时来到第八步：

第7步 child.measure

• 7，child.measure(childWidthMeasureSpec, childHeightMeasureSpec)

measure 函数依然是调用 View#measure 函数，跟第4步一样…

接着回到第6步，代码段3.1，看注释 3.1-2 处，调用 View 类的 setMeasuredDimension 函数，来到第9步：

第8步 setMeasuredDimension

• 8，setMeasuredDimension(resolveSizeAndState(…),resolveSizeAndState(…)) 源码

// 代码段3.1.2
// View 类

    protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;

            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        setMeasuredDimensionRaw(measuredWidth, measuredHeight);
    }


    private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

最终调用 setMeasuredDimensionRaw 函数，把宽高的测量规格赋值给 mMeasuredWidth、mMeasuredHeight 变量；

后续流程，可以通过下述函数获取：

    public final int getMeasuredWidth() {
        return mMeasuredWidth & MEASURED_SIZE_MASK;
    }
    
    public final int getMeasuredWidthAndState() {
        return mMeasuredWidth;
    }
    
    public final int getMeasuredWidthAndState() {
        return mMeasuredWidth;
    }
    
    public final int getMeasuredHeight() {
        return mMeasuredHeight & MEASURED_SIZE_MASK;
    }

注意 final 修饰的 setMeasuredDimension 函数，所以 View 的子类不能重写，只能调用…

总结

从 measure 这个流程看下来，发现它是一个先测完子view，再测自己的递归的流程；

从第七步开始的 child.measure(width, height)，child 也就是 DecorView 的第一个 子View，即系统布局 R.layout.screen_title 或 R.layout.screen_simple 或 其他类似的，统一放在源码工程的 frameworks/base/core/res/res/layout/ 路径下；

R.layout.screen_title 源码 和 R.layout.screen_simple 源码

因为 R.layout.screen_title 这类的布局文件是 ViewGroup 类型的，所以第七步 child 就是它们的 根view：LinearLayout；

其实，这整个流程看下来，只是对 measure 流程有一个大概的了解，具体的View 子类和 ViewGroup 子类的 onMeasure 函数都有自己的实现，需要去具体分析…

链接汇总

R.layout.screen_title 源码

R.layout.screen_simple 源码

ViewRootImpl 源码

DecorView 源码

FrameLayout 源码

View 源码

ViewGroup 源码

ViewStub extends View

LinearLayout 源码

TextView 源码

查源码的链接