图+源码，读懂View的Measure方法-六虎

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读懂 View 三大制作办法的文章

图+源码，读懂View的MeasureSpec – ()

图+源码，读懂View的Measure办法 – ()

图+源码，读懂View的Layout办法 – ()

图+源码，读懂View的Draw办法 – ()

前置知识

有Android开发基础
了解 View 系统
了解 View 的 MeasureSpec 办法

前言

本篇是 读懂View 系列的第二篇文章，本文将给咱们正式开端讲解View制作的三大办法，本篇将讲述第一个办法—— Measure 办法。

Measure办法有何效果

讲到Measure办法的效果，咱们需求回顾一下在View系统(下)一文中学到的页面制作流程一图，为方便你检查，我把这个制作流程图搬来这里。

经过此图，咱们能够看到，在履行 performTraversals() 办法的时分，其办法内部会依次履行 performMeasure()、performLayout() 和 performDraw() 办法。下面 performTraversals() 的源码是经过的裁剪的，咱们能够很清楚的看到三者的履行顺序。

private void performTraversals() {
    ...
        if (!mStopped || mReportNextDraw) {
            ...
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
            ...
        }
    if (didLayout) {
        performLayout(lp, mWidth, mHeight);
        ...
            if (!performDraw() && mSyncBufferCallback != null) {
                mSyncBufferCallback.onBufferReady(null);
            }
        ...
    }
}

在上一篇文章中咱们说到，在 performMeasure 办法内部，它是会履行 measure() 办法的。所以说，measure() 办法是三大制作办法中首个履行的办法，其效果是丈量 View 的宽和高。它的效果流程又两个，一个是 View 的效果流程，一个是 ViewGroup 的效果流程。两个流程有所不同，下面咱们细细道来。

View 的 measure 流程

源码剖析

首要咱们打开View的源码，找到 onMeasure() 办法，下面代码中，因为注释占据的篇幅较大，我删去了一些。注释中首要说的是，该段代码是用于丈量View的宽度和高度，该办法会被 measure() 办法调用，如果承继View运用该办法的话，建议重写以供给愈加精确的功用。并且写了一些重写的要求和哪种情况有必要重写。

在 onMeasure() 办法中，咱们能够看到传入的参数正是上一篇文章中咱们讲的 MeasureSpec ，它的参数由 measure() 办法调用的时分传入，而 measure() 办法则是供给给 performMeasure 办法调用来丈量的。

/**
 * ...省掉一大段注释，有爱好的同学可查阅源码的注释
 * @see #getMeasuredWidth()
 * @see #getMeasuredHeight()
 * @see #setMeasuredDimension(int, int)
 * @see #getSuggestedMinimumHeight()
 * @see #getSuggestedMinimumWidth()
 * @see android.view.View.MeasureSpec#getMode(int)
 * @see android.view.View.MeasureSpec#getSize(int)
 */
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                         getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ...
    if (forceLayout || needsLayout) {
        ...
        if (cacheIndex < 0 || sIgnoreMeasureCache) {
            // measure ourselves, this should set the measured dimension flag back
            onMeasure(widthMeasureSpec, heightMeasureSpec);//调用onMeasure()
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        } ...
    }
    ...
}

咱们发现，onMeasure() 里面只有一个 setMeasuredDimension() 办法。咱们接着看一下其代码，它需求传入两个参数，分别是丈量的宽度和高度，看一下代码的履行过程，咱们能够发现，这段代码是用来设置 View 的宽以及高的。

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;
}

咱们看到这里，会发现，咱们仍未看到 View 的宽以及高在何处进行丈量的。

继续点开 getDefaultSize() 的代码，咱们在此处能够看到代码中传入了 View 的大小(size)，View 的 measureSpec 数据。然后代码履行了以下的步骤。

在注释1和2处，经过 MeasureSpec 类，获得了 specMode 和 specSize 两个数据
然后在注释3处，根据不同的形式，放回不同的size大小值

但是在 AT_MOST 和 EXACTLY (就是wrap_content和match_parent)两种形式下，其回来值是一样的，这明显是不对的。所以说，当咱们自定义 View 需求 wrap_content 特点时，需求重写 onMeasure() 办法，对该特点进行处理。

/**
 * Utility to return a default size. Uses the supplied size if the
 * MeasureSpec imposed no constraints. Will get larger if allowed
 * by the MeasureSpec.
 *
 * @param size Default size for this view
 * @param measureSpec Constraints imposed by the parent
 * @return The size this view should be.
 */
public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);//1
    int specSize = MeasureSpec.getSize(measureSpec);//2
    switch (specMode) {//3
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
    }
    return result;
}

上面 getDefaultSize() 的代码中，在 UNSPECIFIED 形式下，是直接回来传入的 size ，而这个 size 则是由 getSuggestedMinimumWidth() 或者是 getSuggestedMinimumHeight() 办法传递得出，两个办法的处理逻辑是一样的，咱们剖析其间一个就可。

查阅 getSuggestedMinimumWidth() 的代码，咱们会发现，它的逻辑是：当无布景时，直接回来 mMinWidth ；而当有布景的时分，回来的是 mMinWidth 和布景(Drawable)最小宽度两者之间的最大值。

/**
 * Returns the suggested minimum width that the view should use. This
 * returns the maximum of the view's minimum width
 * and the background's minimum width
 *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
 * <p>
 * When being used in {@link #onMeasure(int, int)}, the caller should still
 * ensure the returned width is within the requirements of the parent.
 *
 * @return The suggested minimum width of the view.
 */
protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
//getSuggestedMinimumHeight()同理

上述代码中的 mMinWidth ，是能够经过 Android:minWidth 这个特点设置，或者是经过 View 的 setMinimumWidth()这个办法来设置值，若不设置，则为默许值0 。下面给出其get和set代码供咱们检查。

/**
 * Returns the minimum height of the view.
 *
 * @return the minimum height the view will try to be, in pixels
 *
 * @see #setMinimumHeight(int)
 *
 * @attr ref android.R.styleable#View_minHeight
 */
@InspectableProperty(name = "minHeight")
public int getMinimumHeight() {
    return mMinHeight;
}
/**
 * Sets the minimum height of the view. It is not guaranteed the view will
 * be able to achieve this minimum height (for example, if its parent layout
 * constrains it with less available height).
 *
 * @param minHeight The minimum height the view will try to be, in pixels
 *
 * @see #getMinimumHeight()
 *
 * @attr ref android.R.styleable#View_minHeight
 */
@RemotableViewMethod
public void setMinimumHeight(int minHeight) {
    mMinHeight = minHeight;
    requestLayout();
}
//对应的Width办法同理

接着，咱们看一下 mBackground.getMinimumWidth() 这个布景宽度的获取代码，因为这个布景类是 Drawable 类型的，所以这个办法也是在 Drawable 类下面的。咱们看到办法中对 intrinsicWidth 进行判断，而当他未被设置固有宽度的时分 intrinsicWidth 则为-1，那么回来的值将为0 。反之，则回来固有的宽度。

/**
 * Returns the minimum width suggested by this Drawable. If a View uses this
 * Drawable as a background, it is suggested that the View use at least this
 * value for its width. (There will be some scenarios where this will not be
 * possible.) This value should INCLUDE any padding.
 *
 * @return The minimum width suggested by this Drawable. If this Drawable
 *         doesn't have a suggested minimum width, 0 is returned.
 */
public int getMinimumWidth() {
    final int intrinsicWidth = getIntrinsicWidth();
    return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
/**
 * Returns the drawable's intrinsic width.
 * <p>
 * Intrinsic width is the width at which the drawable would like to be laid
 * out, including any inherent padding. If the drawable has no intrinsic
 * width, such as a solid color, this method returns -1.
 *
 * @return the intrinsic width, or -1 if no intrinsic width
 */
public int getIntrinsicWidth() {
    return -1;
}

View的Measure流程图

根据上面的源码剖析，得出该过程的图为

ViewGroup 的 measure 流程

源码剖析

对于 ViewGroup 的 measure 流程，与 View 不同的当地就是：它不只要丈量本身，还要遍历的调用子元素的measure办法。

咱们知道，ViewGroup 是承继自 View 的，所以，它能够运用 View(实际上让子类重写完成) 的 measure() 和 onMeasure() 办法。咱们直接检查它完成遍历子类的办法即可。

其遍历子类的办法是 measureChildren() 。阅览其代码可发现，它遍历每一个子元素，调用的是 measureChild() 办法。而 measureChild() 办法内部，是获取到子元素(本身)的 LayoutParams (注释1)和父布局的 parentWidthMeasureSpec() (注释2)一起传入到 getChildMeasureSpec() 中，从而得出子布局的 MeasureSpec 信息。这和上一篇文章中，根布局(DecorView)获取 MeasureSpec 的条件是不同的。由此咱们可知，除根布局外，其他View的 MeasureSpec 都与本身的 LayoutParams 和父布局的 MeasureSpec 有关。

/**
 * Ask all of the children of this view to measure themselves, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * We skip children that are in the GONE state The heavy lifting is done in
 * getChildMeasureSpec.
 *
 * @param widthMeasureSpec The width requirements for this view
 * @param heightMeasureSpec The height requirements for this view
 */
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);
        }
    }
}
/**
 * Ask one of the children of this view to measure itself, taking into
 * account both the MeasureSpec requirements for this view and its padding.
 * The heavy lifting is done in getChildMeasureSpec.
 *
 * @param child The child to measure
 * @param parentWidthMeasureSpec The width requirements for this view
 * @param parentHeightMeasureSpec The height requirements for this view
 */
protected void measureChild(View child, int parentWidthMeasureSpec,
                            int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();//1
    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                                                          mPaddingLeft + mPaddingRight, lp.width);//2
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                                                           mPaddingTop + mPaddingBottom, lp.height);
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

接着咱们来看一下，这里丈量一般View的 getChildMeasureSpec() 办法，是怎么履行的。

咱们能够看到，其流程和获得 DecorView 的 getRootMeasureSpec() 办法是差不多的。有一个不同且需求留意的当地是下面注释1处，咱们发现当父布局的形式为AT_MOST时，子元素无论是 MATCH_PARENT 还是 WRAP_CONTENT ，他们的回来值都是如出一辙的。所以，当咱们要在运用特点为 WRAP_CONTENT 时，指定默许的宽和高。

/**
 * Does the hard part of measureChildren: figuring out the MeasureSpec to
 * pass to a particular child. This method figures out the right MeasureSpec
 * for one dimension (height or width) of one child view.
 *
 * The goal is to combine information from our MeasureSpec with the
 * LayoutParams of the child to get the best possible results. For example,
 * if the this view knows its size (because its MeasureSpec has a mode of
 * EXACTLY), and the child has indicated in its LayoutParams that it wants
 * to be the same size as the parent, the parent should ask the child to
 * layout given an exact size.
 *
 * @param spec The requirements for this view
 * @param padding The padding of this view for the current dimension and
 *        margins, if applicable
 * @param childDimension How big the child wants to be in the current
 *        dimension
 * @return a MeasureSpec integer for the child
 */
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;//1
            }
            break;
            // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let them 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);
}

到此就讲完 ViewGroup 源码的measure了，对其子类完成 onMeasure() 办法的感爱好的同学，能够检查一下源码

ViewGroup的measure流程图

给出 ViewGroup 的流程图，希望能更好的帮助理解

参阅

View.java – Android Code Search

Drawable.java – Android Code Search

LinearLayout.java – Android Code Search

图+源码，读懂View的Measure方法

前置知识

前言

Measure办法有何效果

View 的 measure 流程

源码剖析

View的Measure流程图

ViewGroup 的 measure 流程

源码剖析

ViewGroup的measure流程图

参阅

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