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  • 前几天在给一个同事讲&运算取变量的状态写法，原因是我看到他代码中这样写会产生问题，所以想纠正他一下，于是就跟他说你可以参考下android系统中View在measure时使用的状态是怎么通过&运算获取的。

  • 他也说这几个状态是与View在布局文件中设置width或height时，有个对应关系。就这个机会，深入的了解了下View的宽高的measure是怎么与android.view.View.MeasureSpec.EXACTLY、android.view.View.MeasureSpec.AT_MOST、android.view.View.MeasureSpec.UNSPECIFIED这三种状态一一对应的。于是就有了下文。

在布局文件中设置View的layout_width或layout_height时，可以设置三种值match_parent、wrap_content和具体的长度值，下面看下android系统源码中是怎么处理这几种值的。

做过android开发都知道，如果在Activity的onCreate()方法中通过调用View的getWidth()方法是获取不到View的宽高的，此时返回的是0，因为View还没有初始化完成。那么查看View的getWidth()方法发现，返回的值是通过mRight - mLeft返回的值，也就是说如果调用getWidth()返回值，那么一定是mRight和mLeft这两个变量被赋值了。View的getHeight()方法返回高度同理。在View中mLeft个成员被初始化值在两个方法中，一个是setLeft(int)方法中，一个是setFrame(int,int,int,int)中，实际上这两个方法都是由系统来调用的，尤其是setFrame()方法被标记为隐藏。通过查看源码发现，setLeft(int)这个方法很少被使用，倒是跟踪setFrame(int,int,int,int)方法时，发现一些眉目，最后发现这个这个setFrame()方法是在View的layout()方法中一直调用过来的。OK，那么这个layout()方法的调用轨迹是怎样的呢？可以自定义一个View，然后重写这个View的layout()方法，代码如下所示：

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@Override
public void layout(int l, int t, int r, int b) {
    super.layout(l, t, r, b);
    Throwable th = new Throwable();
    th.printStackTrace(); // 打印被调用的栈信息
}

输出的LOG如下所示：

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W/System.err: java.lang.Throwable
W/System.err:     at com.jacpy.busline.widget.BusLineView.layout(BusLineView.java:243)
W/System.err:     at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055)
W/System.err:     at android.view.View.layout(View.java:14860)
W/System.err:     at android.view.ViewGroup.layout(ViewGroup.java:4643)
W/System.err:     at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
W/System.err:     at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
W/System.err:     at android.view.View.layout(View.java:14860)
W/System.err:     at android.view.ViewGroup.layout(ViewGroup.java:4643)
W/System.err:     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
W/System.err:     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
W/System.err:     at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
W/System.err:     at android.view.View.layout(View.java:14860)
W/System.err:     at android.view.ViewGroup.layout(ViewGroup.java:4643)
W/System.err:     at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
W/System.err:     at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
W/System.err:     at android.view.View.layout(View.java:14860)
W/System.err:     at android.view.ViewGroup.layout(ViewGroup.java:4643)
W/System.err:     at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2013)
W/System.err:     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1770)
W/System.err:     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1019)
W/System.err:     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5725)
W/System.err:     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
W/System.err:     at android.view.Choreographer.doCallbacks(Choreographer.java:574)
W/System.err:     at android.view.Choreographer.doFrame(Choreographer.java:544)
W/System.err:     at android.view.Choreographer$FrameDisplayEventReceiver.run(
W/System.err:     at android.os.Handler.handleCallback(Handler.java:733)
W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:95)
W/System.err:     at android.os.Looper.loop(Looper.java:136)
W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5086)
W/System.err:     at java.lang.reflect.Method.invokeNative(Native Method)
W/System.err:     at java.lang.reflect.Method.invoke(Method.java:515)
W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(
W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
W/System.err:     at dalvik.system.NativeStart.main(Native Method)

这段LOG从下往上看，首先是ZygoteInit启动，也就是手机系统的启动，这部分忽略，只看与应用相关的，从ActivityThread.main()开始。在ActivityThread中启动的Looper主线程用来执行了一个Runnable实例，这个实例是android.view.Choreographer$FrameDisplayEventReceiver类的实例，在这个类的run()方法中执行了Choreographer的doFrame()方法，在这个方法中看到熟悉的日志输出：

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Log.i(TAG, "Skipped " + skippedFrames + " frames!  "
                            + "The application may be doing too much work on its main thread.");

当View初始化时在主线程中做的操作过多导致卡帧时，这个LOG就会输出。

OK，跑题了，继续看日志。在doCallbacks()方法中从mCallbackQueues这个回调队列中根据回调的类型取出要执行的CallbackRecord对象，并调用其run()方法。而ViewRootImpl的TraversalRunnable对象是通过在ViewRootImpl的scheduleTraversals()方法中设置的。代码如下所示：

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void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null); // 将mTranversalRunnable加入到mChoreographer的mCallbackQueues的队列中
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
    }
}

最后在performLayout()方法中看到具体调用View的layout()方法的代码：

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host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

这里的host是ViewRootImpl的mView对象，这个对象是一个PhoneWindow.DecorView对象，后面会有文章分析。

OK，这里可以看到mLeft赋值是0，mRight就是getMeasureWidth()方法的返回值。getMeasureWidth()这个方法代码如下所示：

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public static final int MEASURED_SIZE_MASK = 0x00ffffff;

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

上面的代码可以看出，mMeasureWidth取了后三个字节，也就是本来是int类型的mMeasureWidth实际目前只使用了低位三个字节，对于目前的显示的分辨率来说足够。

OK，重点来了，这个mMeasureWidth的值是怎么来的？继续跟代码发现这个变量在View的setMeasuredDimensionRaw()方法中被赋值，使用方法中的参数赋值。接着发现这个方法在setMeasuredDimension()和measure()方法中被调用。而在measure()方法中cacheIndex这个变量可能是-1，因为mMeasureCache变量中的键值对只在measure()方法最后才放进去的，而最后是调用了onMeasure()方法。代码如下所示：

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public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    // ...
   // Suppress sign extension for the low bytes
    long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL; // 用一个64位的long类型保存两个32位的值，高32位是宽，低32位是高
    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;
		 // ...
        int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                mMeasureCache.indexOfKey(key); // 如果没有找到这个Key也是返回-1
        if (cacheIndex < 0 || sIgnoreMeasureCache) { // 极有可能是走这个判断
            // 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;
        }

		 // ...
    }
    
	 // ...
	 // 保存键值对
    mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
            (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}

而setMeasuredDimension()方法是在onMeasure()方法中有调用。因此，上面的代码不管是走if流程还是else流程，最终都会调用setMeasuredDimensionRaw()这个方法。不管流程怎样，这个值是从measure()这个方法中的参数传进来的。以同样的方法重写View的onMeasure()方法，如下代码所示：

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@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    Throwable t = new Throwable();
    t.printStackTrace();
}

输出的LOG如下所示：

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java.lang.Throwable
    at com.jacpy.busline.widget.BusLineView.onMeasure(BusLineView.java:442)
    at android.view.View.measure(View.java:16540)
    at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:719)
    at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:455)
    at android.view.View.measure(View.java:16540)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
    at android.view.View.measure(View.java:16540)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
    at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
    at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
    at android.view.View.measure(View.java:16540)
    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5137)
    at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
    at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
    at android.view.View.measure(View.java:16540)
    at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1942)
    at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1132)
    at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1321)
    at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1019)
    at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5725)
    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
    at android.view.Choreographer.doCallbacks(Choreographer.java:574)
    at android.view.Choreographer.doFrame(Choreographer.java:544)
    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
    at android.os.Handler.handleCallback(Handler.java:733)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:136)
    at android.app.ActivityThread.main(ActivityThread.java:5086)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
    at dalvik.system.NativeStart.main(Native Method)

从上面的LOG可以看出来，最终是在measure()方法中调到onMeasure()方法，也就是上面分析的流程。

通过上面两段LOG发现，最终都指向了同一个地方，那就是ViewRootImpl的performTraversals()。也就是说，在这个方法中先measure，然后再layout。

从measureHierarchy()方法中可以看出来在调用performMeasure(int,int)方法时，传了两个参数childWidthMeasureSpec和childHeightMeasureSpec，而这两个变量是通过getRootMeasureSpec()方法返回的，如下代码所示：

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private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
        final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
    int childWidthMeasureSpec;
    int childHeightMeasureSpec;
    boolean windowSizeMayChange = false;

    boolean goodMeasure = false;
    if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) {
        // On large screens, we don't want to allow dialogs to just
        // stretch to fill the entire width of the screen to display
        // one line of text.  First try doing the layout at a smaller
        // size to see if it will fit.
        final DisplayMetrics packageMetrics = res.getDisplayMetrics();
        res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
        int baseSize = 0;
        if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
            baseSize = (int)mTmpValue.getDimension(packageMetrics);
        }
        if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize);
        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;
            } else {
                // Didn't fit in that size... try expanding a bit.
                baseSize = (baseSize+desiredWindowWidth)/2;
                if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="
                        + baseSize);
                childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                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) {
                    if (DEBUG_DIALOG) Log.v(TAG, "Good!");
                    goodMeasure = true;
                }
            }
        }
    }

    if (!goodMeasure) {
        childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
        childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
        if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
            windowSizeMayChange = true;
        }
    }

    return windowSizeMayChange;
}

本文的重点来了，getRootMeasureSpec()方法的代码如下所示：

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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;
}

从方法中的switch可以看到，MATCH_PARENT对应MeasureSpec.EXACTLY、WRAP_CONTENT对应MeasureSpec.AT_MOST，默认的可以认为是设置了具体的值对应的是MeasureSpec.EXACTLY。

为了确定一下，可以看下MeasureSpc的makeMeasureSpec()方法的代码，如下所示：

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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);
    }
}

前面说尺寸值是由低三个字节表示，这里可以看到高位第一个字节的前两个位用来表示mode。也就是说int类型的第31、32位表示的是mode值，低30位表示是具体的长度值。

OK，最后是在ViewGroup的measureChildWithMargins()方法中调用每个child的measure()方法去具体测量每个子View的长度，代码如下所示：

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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);
}

而其中被调用的getChildMeasureSpec()方法中子View的大小是根据父View的大小及模式来决定的，代码如下所示：

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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);
}

OK，以上就是整个分析过程。

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