Android 中View的作图机制源码分析 一

尊重原创: http://blog.csdn.net/yuanzeyao/article/details/46765113

差不多半年没有写博客了，一是因为工作比较忙，二是觉得没有什么内容值得写，三是因为自己越来越懒了吧，不过最近我对Android中View的绘制机制有了一些新的认识，所以想记录下来并分享给大家。在之后的几篇博客中，我会给大家分享如下的内容：

1、View中measure(),layout(),draw()函数执行过程分析，带领大家详细分析View的尺寸测量过程，位置计算，并最终绘制到UI上的过程

2、以LinearLayout为例讲解ViewGroup尺寸计算，位置计算，以及绘制过程

3、更深层次的理解LayoutParams的意义

4、LayoutInflater创建View的过程分析，详细分析inflate(int resource, ViewGroup root, boolean attachToRoot)方法中各个参数的意义

掌握上面几个知识点对于自定义View有非常重要的意义的，而且据我所知自定义View在面试过程中是必问知识点。

以上内容都是Android中View系统比较重要的一些内容，View系统的功能主要包括用户输入消息到消息处理的整个过程，以及UI的绘制，用户输入消息以及消息处理的部分我之前也有写过几篇文章，如果读者用兴趣可以去了解下：

Android 系统Touch事件传递机制 上：http://blog.csdn.net/yuanzeyao/article/details/37961997

Android 系统Touch事件传递机制 下：http://blog.csdn.net/yuanzeyao/article/details/38025165

Android 系统Key事件传递机制 上：http://blog.csdn.net/yuanzeyao/article/details/13630909

Android 系统Key事件传递机制 下：http://blog.csdn.net/yuanzeyao/article/details/13631139

由于涉及的内容比较多，所以我打算使用 多篇文章来讲解上述内容，敬请期待。

那么现在就开始学习View的measure过程吧，measure过程主要作用就是计算一个View的大小，这个其实很好理解，因为任何一个View在绘制到UI上时，必须事先知道这个View的大小，不然是无法绘制的。

平时我们在指定一个view的大小时，通常就是在xml文件中设置layout_width和layout_hegiht属性，这里我要提出一个问题：为什么View的宽度和高度对应的属性名前面有layout而不是直接叫width和height？先记住这个问题吧，等你看完本文的内容相信你就明白了。其实measuer过程就将layout_width和layout_height这些属性变为具体的数字大小。

当我们想要将一个xml文件显示到UI上时，通常就是将该xml文件的id传入到Activity的setContentView中去，其实最终就会调用到ViewRoot的performTraversals方法，此方法承担了Android的View的绘制工作，这个方法代码非常多，但是逻辑非常简单，主要包含了三个阶段：

第一个阶段就是我们今天要学习的measure，第二个阶段就是layout，第三个阶段就是draw，measure阶段就是得到每个View的大小，layout阶段就是计算每个View在UI上的坐标，draw阶段就是根据前面两个阶段的数据进行UI绘制。

首先我们看看ViewRoot的performTraversals方法的部分代码(使用的2.3代码，选择2.3代码的原因是因为2.3的版本逻辑比4.x版本简单，而且主要逻辑还是一样的)

    private void performTraversals() {        // Section one mView就是DecorView，        final View host = mView;		//Section two        int desiredWindowWidth;        int desiredWindowHeight;        int childWidthMeasureSpec;        int childHeightMeasureSpec;        ...        Rect frame = mWinFrame;        if (mFirst) {            fullRedrawNeeded = true;            mLayoutRequested = true;            DisplayMetrics packageMetrics =                mView.getContext().getResources().getDisplayMetrics();			//Section three            desiredWindowWidth = packageMetrics.widthPixels;            desiredWindowHeight = packageMetrics.heightPixels;            // For the very first time, tell the view hierarchy that it            // is attached to the window.  Note that at this point the surface            // object is not initialized to its backing store, but soon it            // will be (assuming the window is visible).           ...        } else {			//Section four            desiredWindowWidth = frame.width();            desiredWindowHeight = frame.height();            if (desiredWindowWidth != mWidth || desiredWindowHeight != mHeight) {                if (DEBUG_ORIENTATION) Log.v("ViewRoot",                        "View " + host + " resized to: " + frame);                fullRedrawNeeded = true;                mLayoutRequested = true;                windowResizesToFitContent = true;            }        }           boolean insetsChanged = false;        if (mLayoutRequested) {            // Execute enqueued actions on every layout in case a view that was detached            // enqueued an action after being detached            getRunQueue().executeActions(attachInfo.mHandler);			...			//Section five            childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);            childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);            // Ask host how big it wants to be            if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v("ViewRoot",                    "Measuring " + host + " in display " + desiredWindowWidth                    + "x" + desiredWindowHeight + "...");			//Section six            host.measure(childWidthMeasureSpec, childHeightMeasureSpec);            if (DBG) {                System.out.println("======================================");                System.out.println("performTraversals -- after measure");                host.debug();            }        }        ....    }

上面的代码就是第一阶段的主要代码，请看代码中的Section one部分，这里定义了一个View 类型的变量host，它被赋值mView，这里我想说的仅仅是mView就是一个界面的DecorView，如果你还不熟悉DecorView可以看看我的另外一篇文章：

《窗口的创建过程》，Section two分别定义了4个int 类型的变量，前面两个变量在Section three部分或者Section four部分赋值，通常第一次进来是在Section three里面进行赋值，也就是说desiredWindowWidth和disireWindowHeight分别是手机屏幕的宽和高(当然并不总是这样的，这里我们只用考虑简单的一种情况)，在Section five部分分别对childWidthMeasureSpec和childHeightMeasureSpec进行赋值，这里调用了一个getRootMeasureSpec的方法，我们后面再分析它。在Setion six部分调用host.measure来计算View的大小，到这里performTraversals中mersure的调用过程就算结束了，但是getRootMeasureSpec和host的measure方法我们还不清楚它们到底做了什么，下面就来分析这两个方法吧:

先看看getRootMeasureSpec方法吧。

    private 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;    }

看了实现之后，你是不是觉得这个方法实现超简单，以getRootMeasureSpec(desiredWindowWidth，lp.width)为例，我们知道第一个参数就是屏幕的宽度，第二个参数是一个View的LayoutParams中的width属性，其实这个参数是在Activity的

 void makeVisible() {        if (!mWindowAdded) {            ViewManager wm = getWindowManager();            wm.addView(mDecor, getWindow().getAttributes());            mWindowAdded = true;        }        mDecor.setVisibility(View.VISIBLE);    }

  // The current window attributes.    private final WindowManager.LayoutParams mWindowAttributes =        new WindowManager.LayoutParams();

           public LayoutParams() {            super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);            type = TYPE_APPLICATION;            format = PixelFormat.OPAQUE;        }

这里出现了一个MeasureSpec的陌生类，先看看MeasureSpec是何方圣神。MeasureSpec是定义在View中的一个内部类，这个类里面有几个比较重要的常量：

       private static final int MODE_SHIFT = 30;        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;        /**         * Measure specification mode: The parent has not imposed any constraint         * on the child. It can be whatever size it wants.         */        public static final int UNSPECIFIED = 0 << MODE_SHIFT;        /**         * Measure specification mode: The parent has determined an exact size         * for the child. The child is going to be given those bounds regardless         * of how big it wants to be.         */        public static final int EXACTLY     = 1 << MODE_SHIFT;        /**         * Measure specification mode: The child can be as large as it wants up         * to the specified size.         */        public static final int AT_MOST     = 2 << MODE_SHIFT;

我们知道java中的int类型占用32位，随意这几个变量在内存中的表现形式如下：

MODE_MASK:  11000000 00000000 00000000 00000000

UNSPECIFIED: 000000000 00000000 00000000 00000000

EXACTLY:         01000000 00000000 00000000 00000000

AT_MOST:       10000000 00000000 00000000 00000000

也就是说每个高2位表示的model，第30位才真正表示尺寸的大小

有了上面的基础之后，相信理解下面三个方法就不难了

/**         * Creates a measure specification based on the supplied size and mode.         *         * The mode must always be one of the following:         * <ul>         *  <li>[email protected] android.view.View.MeasureSpec#UNSPECIFIED}</li>         *  <li>[email protected] android.view.View.MeasureSpec#EXACTLY}</li>         *  <li>[email protected] android.view.View.MeasureSpec#AT_MOST}</li>         * </ul>         *         * @param size the size of the measure specification         * @param mode the mode of the measure specification         * @return the measure specification based on size and mode         */        public static int makeMeasureSpec(int size, int mode) {            return size + mode;        }        /**         * Extracts the mode from the supplied measure specification.         *         * @param measureSpec the measure specification to extract the mode from         * @return [email protected] android.view.View.MeasureSpec#UNSPECIFIED},         *         [email protected] android.view.View.MeasureSpec#AT_MOST} or         *         [email protected] android.view.View.MeasureSpec#EXACTLY}         */        public static int getMode(int measureSpec) {            return (measureSpec & MODE_MASK);        }        /**         * Extracts the size from the supplied measure specification.         *         * @param measureSpec the measure specification to extract the size from         * @return the size in pixels defined in the supplied measure specification         */        public static int getSize(int measureSpec) {            return (measureSpec & ~MODE_MASK);        }

看明白了这里，我们就回到getRootMeasureSpec吧，我们知道lp.width属性通常有三种：match_parent(fill_parent)，wrap_content，具体一个大小(如100dip)，而这里通过我们上面的分析，知道宽和高均是match_parent。通过代码我们知道这三种情况对应的mode分别是：

EXACTLY，AT_MOST，EXACTLY，也就是说math_parent和具体的大小(100dip)对应的都是EXACTLY。最后根据得到的mode和屏幕的宽度调用makeMeasureSpec方法得到一个int类型的值赋值给childWidthMeasureSpec，同理得到了childHeightMeasureSpec，并将这两个值传入measure中。下面我们就看看measure做了什么

由于这里调用的是host的measure，而host其实是一个FrameLayout，所以我不打算继续使用这个例子将View的测量过程了，但是ViewGroup是没有改写measure的，所以其实调用的还是View的measure方法，measure方法的源码如下：

    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {        if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||                widthMeasureSpec != mOldWidthMeasureSpec ||                heightMeasureSpec != mOldHeightMeasureSpec) {            // first clears the measured dimension flag            mPrivateFlags &= ~MEASURED_DIMENSION_SET;            if (ViewDebug.TRACE_HIERARCHY) {                ViewDebug.trace(this, ViewDebug.HierarchyTraceType.ON_MEASURE);            }            // measure ourselves, this should set the measured dimension flag back            onMeasure(widthMeasureSpec, heightMeasureSpec);            // flag not set, setMeasuredDimension() was not invoked, we raise            // an exception to warn the developer            if ((mPrivateFlags & MEASURED_DIMENSION_SET) != MEASURED_DIMENSION_SET) {                throw new IllegalStateException("onMeasure() did not set the"                        + " measured dimension by calling"                        + " setMeasuredDimension()");            }            mPrivateFlags |= LAYOUT_REQUIRED;        }        mOldWidthMeasureSpec = widthMeasureSpec;        mOldHeightMeasureSpec = heightMeasureSpec;    }

我们看到measure方法其实是final的，所以ViewGroup是无法改写此方法的。通常一个具体的ViewGroup都是改写onMeasure方法，这点你可以去看看LinearLayout和FrameLayout，他们在onMeasure方法里面都间接调用了ViewGroup的measureChildWithMargins方法，今天我们就以measureChildWithMargins这个方法为入口分析View的测量过程。measureChildWithMargins方法的源码如下：

    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);    }

下面我们分三步来分析measureChildWithMargins方法：

1、获取孩子的LayoutParams

2、调用getChildMeasureSpec方法得到孩子的measureSpec（包括widthSpec和heightSpec）

我们看看getChildMeasureSpec做了什么，先看看它的几个参数，以获取孩子的widthSpec为例 ，第一个参数是ViewGroup的widthSpec,第二个参数是ViewGroup已经被使用的width，第三个是lp.width，接下来看看源码：

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

回到measureChildWithMargins 看第三步：调用了child.measure。并且参数就是第二步中得到的，另外注意这个child就是一个普通的View(因为我们已经假设ViewGroup里面没有ViewGroup，只有View)

由于是View调用measure，所以measure中调用onMeasure也是View中的，我们看看View的onMeasuere方法吧

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

这里出现了一个重要的方法getDefaultSize，其代码如下：

    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;    }

代码如下：

  protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {        mMeasuredWidth = measuredWidth;        mMeasuredHeight = measuredHeight;        mPrivateFlags |= MEASURED_DIMENSION_SET;    }