自定义控件View之onMeasure调用时机源码分析

先上测试代码：

MainActivity.java

import android.app.Activity;import android.os.Bundle;import android.util.Log;public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        Log.e("hty", "before setContextView");        setContentView(R.layout.activity_main);        Log.e("hty", "after setContextView");    }    @Override    protected void onResume() {        super.onResume();        Log.e("hty", "onResume");    }    @Override    protected void onDestroy() {        super.onDestroy();        Log.e("hty", "onDestroy");    }}

MyView.java

import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.util.AttributeSet;import android.util.Log;import android.view.View;public class MyView extends View {    Paint paint;    public MyView(Context context) {        this(context, null);    }    public MyView(Context context, AttributeSet attrs) {        super(context, attrs);        Log.e("hty","view constructor");        paint = new Paint();        paint.setColor(Color.RED);        paint.setTextSize(20);    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        super.onMeasure(widthMeasureSpec, heightMeasureSpec);        Log.e("hty","view onMeasure");    }    @Override    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {        super.onLayout(changed, left, top, right, bottom);        Log.e("hty","view onLayout");    }    String str = "这里是测试";    @Override    protected void onDraw(Canvas canvas) {        super.onDraw(canvas);        Log.e("hty","view onDraw");        canvas.drawText(str, getWidth()/2-paint.measureText(str)/2,getHeight()/2, paint);    }}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?><RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:tools="http://schemas.android.com/tools"    android:id="@+id/activity_main"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:paddingBottom="@dimen/activity_vertical_margin"    android:paddingLeft="@dimen/activity_horizontal_margin"    android:paddingRight="@dimen/activity_horizontal_margin"    android:paddingTop="@dimen/activity_vertical_margin"    tools:context="com.zqc.mytest.MainActivity">    <com.zqc.mytest.MyView        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="Hello World!" /></RelativeLayout>

正常运行后，查看对应的Log：

从Log输出可以看出在一个View的绘制过程中，onMeasure是被多次调用了的。下面通过源码来一步步分析 onMeasure(int widthMeasureSpec, int heightMeasureSpec)函数，尤其是传过来的两个参数到底是从哪里来的。

首先看下MainActivity里面的setContentView，进入该函数后，其对应的代码如下：

Activity.java

    public void setContentView(@LayoutRes int layoutResID) {        getWindow().setContentView(layoutResID);        initWindowDecorActionBar();    }

即调用了getWindow()的setContentView方法，查看getWindow方法，其返还的是类Window的一个实例mWindow，该类是一个抽象类，其具体实现类是PhoneWindow，即调用的是PhoneWindow的setContentView方法，查看相应的代码如下：

PhoneWindow.java

    @Override    public void setContentView(int layoutResID) {        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window        // decor, when theme attributes and the like are crystalized. Do not check the feature        // before this happens.        if (mContentParent == null) {            installDecor();        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {            mContentParent.removeAllViews();        }        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,                    getContext());            transitionTo(newScene);        } else {            mLayoutInflater.inflate(layoutResID, mContentParent);        }        mContentParent.requestApplyInsets();        final Callback cb = getCallback();        if (cb != null && !isDestroyed()) {            cb.onContentChanged();        }    }

该方法首先判断mContentParent是否为空，不为空则调用installDecor()方法来初始化mContentParent，查看具体的代码：

PhoneWindow.java

    private void installDecor() {        if (mDecor == null) {            mDecor = generateDecor();//这里生成了mDecor,它是所有应用窗口的根View 。            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);            mDecor.setIsRootNamespace(true);            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);            }        }        if (mContentParent == null) {            mContentParent = generateLayout(mDecor);//这里就生成了mContentParent，这个generateLayout会根据设定的style来布局显示的界面            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.            mDecor.makeOptionalFitsSystemWindows();            ....        }    }

其中generateDecor方法就直接返回一个DecorView，代码如下：

PhoneWindow.java

    protected DecorView generateDecor() {        return new DecorView(getContext(), -1);    }

而generateLayout(mDecor)方法会根据程序Activity设定的style来布局显示的界面，其代码如下：

PhoneWindow.java

    protected ViewGroup generateLayout(DecorView decor) {        // Apply data from current theme.        TypedArray a = getWindowStyle();//获取窗口的style                。。。。        if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {            requestFeature(FEATURE_NO_TITLE);//看到没，你在xml里面设置的FEATURE_NO_TITLE，在这里生效了        } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {            // Don't allow an action bar if there is no title.            requestFeature(FEATURE_ACTION_BAR);        }        。。。。        final Context context = getContext();                。。。。        WindowManager.LayoutParams params = getAttributes();        if (!hasSoftInputMode()) {            params.softInputMode = a.getInt(                    R.styleable.Window_windowSoftInputMode,                    params.softInputMode);        }        if (a.getBoolean(R.styleable.Window_backgroundDimEnabled,                mIsFloating)) {            /* All dialogs should have the window dimmed */            if ((getForcedWindowFlags()&WindowManager.LayoutParams.FLAG_DIM_BEHIND) == 0) {                params.flags |= WindowManager.LayoutParams.FLAG_DIM_BEHIND;            }            if (!haveDimAmount()) {                params.dimAmount = a.getFloat(                        android.R.styleable.Window_backgroundDimAmount, 0.5f);            }        }        。。。。        int layoutResource;        int features = getLocalFeatures();                。。。。        View in = mLayoutInflater.inflate(layoutResource, null);//这里把给定的布局加载出来，然后加到decor中        decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));        mContentRoot = (ViewGroup) in;        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);//看到没这个ID_ANDROID_CONTENT，也就是一个窗口的根布局        if (contentParent == null) {            throw new RuntimeException("Window couldn't find content container view");        }        。。。。        mDecor.finishChanging();        return contentParent;    }

通过一张图来分析下一个窗口的布局具体是怎样的。

图上标的很详细，在最外层是一个FramLayout，其实也就是DecorView，是所有窗口的根布局，在该根布局下有一个(0)LinearLayout和一个(1)View，这个(1)View就是状态栏，(0)LinearLayout里面有个FrameLayout，在里面的多个View有固定的id，在图中已经标明，所有在一个Activity通过findViewById获取的ID_ANDROID_CONTENT就是
(0)FrameLayout->(0)LinearLayout->(0)FrameLayout->(1)FrameLayout对应的View。
要知道onMeasure两个参数到底是从哪里来的，还得再找下View是如何绘制的，上一篇文章有分析。View的绘制从ViewRootImpl的performTraversals()函数开始，下面进入该方法中具体分析下。

ViewRootImpl.java

    private void performTraversals() {        // cache mView since it is used so much below...        final View host = mView;        。。。。        mIsInTraversal = true;        mWillDrawSoon = true;        boolean windowSizeMayChange = false;        boolean newSurface = false;        boolean surfaceChanged = false;        WindowManager.LayoutParams lp = mWindowAttributes;        int desiredWindowWidth;        int desiredWindowHeight;        final int viewVisibility = getHostVisibility();        boolean viewVisibilityChanged = mViewVisibility != viewVisibility                || mNewSurfaceNeeded;        WindowManager.LayoutParams params = null;        if (mWindowAttributesChanged) {            mWindowAttributesChanged = false;            surfaceChanged = true;            params = lp;        }                。。。。        Rect frame = mWinFrame;        if (mFirst) {            mFullRedrawNeeded = true;            mLayoutRequested = true;            if (lp.type == WindowManager.LayoutParams.TYPE_STATUS_BAR_PANEL                    || lp.type == WindowManager.LayoutParams.TYPE_INPUT_METHOD) {                // NOTE -- system code, won't try to do compat mode.                Point size = new Point();                mDisplay.getRealSize(size);                desiredWindowWidth = size.x;                desiredWindowHeight = size.y;            } else {                DisplayMetrics packageMetrics =                    mView.getContext().getResources().getDisplayMetrics();                desiredWindowWidth = packageMetrics.widthPixels;                desiredWindowHeight = packageMetrics.heightPixels;            }           。。。。        } else {            desiredWindowWidth = frame.width();            desiredWindowHeight = frame.height();            if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {                if (DEBUG_ORIENTATION) Log.v(TAG,                        "View " + host + " resized to: " + frame);                mFullRedrawNeeded = true;                mLayoutRequested = true;                windowSizeMayChange = true;            }        }        if (viewVisibilityChanged) {            mAttachInfo.mWindowVisibility = viewVisibility;            host.dispatchWindowVisibilityChanged(viewVisibility);            if (viewVisibility != View.VISIBLE || mNewSurfaceNeeded) {                destroyHardwareResources();            }            if (viewVisibility == View.GONE) {                // After making a window gone, we will count it as being                // shown for the first time the next time it gets focus.                mHasHadWindowFocus = false;            }        }        。。。。        boolean layoutRequested = mLayoutRequested && (!mStopped || mReportNextDraw);        if (layoutRequested) {            final Resources res = mView.getContext().getResources();            if (mFirst) {                // make sure touch mode code executes by setting cached value                // to opposite of the added touch mode.                mAttachInfo.mInTouchMode = !mAddedTouchMode;                ensureTouchModeLocally(mAddedTouchMode);            } else {                。。。。                if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT                        || lp.height == ViewGroup.LayoutParams.WRAP_CONTENT) {                    windowSizeMayChange = true;                    if (lp.type == WindowManager.LayoutParams.TYPE_STATUS_BAR_PANEL                            || lp.type == WindowManager.LayoutParams.TYPE_INPUT_METHOD) {                        // NOTE -- system code, won't try to do compat mode.                        Point size = new Point();                        mDisplay.getRealSize(size);                        desiredWindowWidth = size.x;                        desiredWindowHeight = size.y;                    } else {                        DisplayMetrics packageMetrics = res.getDisplayMetrics();                        desiredWindowWidth = packageMetrics.widthPixels;                        desiredWindowHeight = packageMetrics.heightPixels;                    }                }            }            // Ask host how big it wants to be            windowSizeMayChange |= measureHierarchy(host, lp, res,                    desiredWindowWidth, desiredWindowHeight);        }        。。。。            if (!mStopped || mReportNextDraw) {                boolean focusChangedDueToTouchMode = ensureTouchModeLocally(                        (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);                if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()                        || mHeight != host.getMeasuredHeight() || contentInsetsChanged) {                    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//获取                    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);                    if (DEBUG_LAYOUT) Log.v(TAG, "Ooops, something changed!  mWidth="                            + mWidth + " measuredWidth=" + host.getMeasuredWidth()                            + " mHeight=" + mHeight                            + " measuredHeight=" + host.getMeasuredHeight()                            + " coveredInsetsChanged=" + contentInsetsChanged);                     // Ask host how big it wants to be                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//看这里，看这里                    // Implementation of weights from WindowManager.LayoutParams                    // We just grow the dimensions as needed and re-measure if                    // needs be                    int width = host.getMeasuredWidth();                    int height = host.getMeasuredHeight();                    boolean measureAgain = false;                    if (lp.horizontalWeight > 0.0f) {                        width += (int) ((mWidth - width) * lp.horizontalWeight);                        childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,                                MeasureSpec.EXACTLY);                        measureAgain = true;                    }                    if (lp.verticalWeight > 0.0f) {                        height += (int) ((mHeight - height) * lp.verticalWeight);                        childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,                                MeasureSpec.EXACTLY);                        measureAgain = true;                    }                    if (measureAgain) {                        if (DEBUG_LAYOUT) Log.v(TAG,                                "And hey let's measure once more: width=" + width                                + " height=" + height);                        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//看这里，看这里                    }                    layoutRequested = true;                }            }        }         。。。。        mIsInTraversal = false;    }    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");        try {            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);        } finally {            Trace.traceEnd(Trace.TRACE_TAG_VIEW);        }    }

看performTraversals方法中调用的performMeasure的地方，performMeasure即调用了View的measure方法，而measure方法会去调用onMeasure方法。

看下如下两行代码

int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//获取int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

在这两行代码中获取了child的宽高，使用的方法是getRootMeasureSpec，其中参数lp.width是传入的MATCH_PARENT或者WRAP_CONTENT，mWidth是窗口期望的大小，getRootMeasureSpec代码如下：

ViewRootImpl.java

    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;    }

这里应该很好理解了，其中调用了MeasureSpec类中的方法，关于MeasureSpec类网上资料很多，该类中用一个int值的两部分分别表示Mode和具体的尺寸。其中最高两位表示

Mode，而最低的30位表示具体的尺寸值，这里计算完之后就进入了View的measure函数中，代码如下：

View.java

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;            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);        }        // Suppress sign extension for the low bytes        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||                widthMeasureSpec != mOldWidthMeasureSpec ||                heightMeasureSpec != mOldHeightMeasureSpec) {            // first clears the measured dimension flag            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;            resolveRtlPropertiesIfNeeded();            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;            }            // flag not set, setMeasuredDimension() was not invoked, we raise            // an exception to warn the developer            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {                throw new IllegalStateException("View with id " + getId() + ": "                        + getClass().getName() + "#onMeasure() did not set the"                        + " measured dimension by calling"                        + " setMeasuredDimension()");            }            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;        }        mOldWidthMeasureSpec = widthMeasureSpec;        mOldHeightMeasureSpec = heightMeasureSpec;        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension    }

该方法是final的，因而不能被继承，但是里面提供了onMeasure回调，这样子类就可以直接继承onMeasure函数来实现相应的操作。这个View类型的，但是还有一种是ViewGroup类型，也就是容器类型的控件，在具体容器类型的控件里面可以通过重写onMeasure来实现，比如FrameLayout中的onMeasure函数如下：

FrameLayout.java

   protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        int count = getChildCount();        final boolean measureMatchParentChildren =                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;        mMatchParentChildren.clear();        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) {                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());                if (measureMatchParentChildren) {                    if (lp.width == LayoutParams.MATCH_PARENT ||                            lp.height == LayoutParams.MATCH_PARENT) {                        mMatchParentChildren.add(child);                    }                }            }        }        // Account for padding too        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();        // Check against our minimum height and width        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());        // Check against our foreground's minimum height and width        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));        count = mMatchParentChildren.size();        if (count > 1) {            for (int i = 0; i < count; i++) {                final View child = mMatchParentChildren.get(i);                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();                final int childWidthMeasureSpec;                if (lp.width == LayoutParams.MATCH_PARENT) {                    final int width = Math.max(0, getMeasuredWidth()                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()                            - lp.leftMargin - lp.rightMargin);                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(                            width, MeasureSpec.EXACTLY);                } else {                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +                            lp.leftMargin + lp.rightMargin,                            lp.width);                }                final int childHeightMeasureSpec;                if (lp.height == LayoutParams.MATCH_PARENT) {                    final int height = Math.max(0, getMeasuredHeight()                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()                            - lp.topMargin - lp.bottomMargin);                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(                            height, MeasureSpec.EXACTLY);                } else {                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +                            lp.topMargin + lp.bottomMargin,                            lp.height);                }                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);            }        }    }

 大概也就是回调本容器里面的子View的measure函数实现尺寸计算。这里通过方法ViewGroup类中的getChildMeasureSpec()来获取子类期望自己获取的宽高大小。其代码是

ViewGroup.java

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);    }

  在重写onMeasure方法时一定要调用setMeasuredDimension，该方法会将mPrivateFlags经过或使得View知道已经经过了measure这个步骤了。代码如下：

View.java

    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;    }

至此分析结束，所以说一个View的大小是由自己和父类两者共同决定的。