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Android进阶基础系列:View的工作原理 全面理解!

1、ViewRoot 和 DecorView

  1. ViewRoot对应ViewRootImpl类,是连接WindowManager和DecorView的纽带。View的三大流程是通过ViewRoot完成的。 在ActivityThread中,当Activity对象被创建完毕时,会将DecorView添加到Window中,同时会创建ViewRootImpl,且ViewRootImpl和DecorView会建立关联。如下代码,WindowManagerGlobal的addView()方法:
public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow) {
			...
            root = new ViewRootImpl(view.getContext(), display);
            root.setView(view, wparams, panelParentView);
            ...
            }
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  1. View绘制流程从 performTraversals开始,经过Measure、layout、draw。流程图如下

在这里插入图片描述
3. DecorView是顶级View,是一个FrameLayout,上面是标题栏、下面是内容栏。内容栏就是setContengView的内容view,id是content。事件 经过DecorView 然后传给我们自己的View。

2、 MeasureSpec

MeasureSpec封装了从父级传递到子级的布局要求。系统把view的LayoutParams 根据 父容器施加的规则(父容器的SpecMode) 转换成 view的MeasureSpec,然后使用这个MeasureSpec确定view的测量宽高(不一定是最终宽高)。

2.1MeasureSpec

1.MeasureSpec---view的测量规格:高2位的SpecMode,低30位的SpecSize。 2.SpecMode的分类: UNPECIFIED,父容器对view不限制,要多大给多大,一般系统内部使用。 EXACTLY,父容器检测出view所需大小,view最终大小就是SpecSize的值。对应 LayoutParams中的matchParent、具体数值 两种模式。 AT_MOST,父容器制定了可用大小即SpecSize,view的大小不能大于这个值,具体要看view的具体实现。对应LayoutParams中的wrap_content。

2.2MeasureSpec和LayoutParams的对应关系

前面说了View的MeasureSpec是由LayoutParams和父容器的MeasureSpec共同决定。顶级view,即DecorView,是由窗口尺寸和自身LayoutParams决定

1、DecorView,ViewRootImpl中measureHierarchy()方法(performTraversals中执行),代码如下,desiredWindowWidth、desiredWindowHeight是屏幕的尺寸。

private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
            final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
            ...
            	childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
            	childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
            	performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
            ...
            }
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performMeasure()内部是调用mView.measure(childWidthMeasureSpec, childHeightMeasureSpec),mView就是DecorVIew。继续看getRootMeasureSpec()方法如下:

/**
     * Figures out the measure spec for the root view in a window based on it's
     * layout params.
     *
     * @param windowSize
     *            The available width or height of the window
     *
     * @param rootDimension
     *            The layout params for one dimension (width or height) of the
     *            window.
     *
     * @return The measure spec to use to measure the root view.
     */
    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;
    }
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DecorView的MeasureSpec就明确了,根据其LayoutParams:

  • MATCH_PARENT:精确模式,就是窗口大小;
  • WRAP_CONTENT:最大值模式,最大值不能超过窗口大小;
  • 固定值(如100dp):精确模式,就是LayoutParams的指定值。

2、普通View,测量过程从ViewGroup传递下来,看ViewGroup的measureChildWithMargins()方法:

/**
     * Ask one of the children of this view to measure itself, taking into
     * account both the MeasureSpec requirements for this view and its padding
     * and margins. The child must have MarginLayoutParams The heavy lifting is
     * done in getChildMeasureSpec.
     *
     * @param child The child to measure
     * @param parentWidthMeasureSpec The width requirements for this view
     * @param widthUsed Extra space that has been used up by the parent
     *        horizontally (possibly by other children of the parent)
     * @param parentHeightMeasureSpec The height requirements for this view
     * @param heightUsed Extra space that has been used up by the parent
     *        vertically (possibly by other children of the parent)
     */
    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);
    }
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即先获取child的MeasureSpec,再调child.measure()。可以看到,child的MeasureSpec是由父容器的MeasureSpec、父容器的padding、child的LayoutParams、child的marging 共同决定。继续看getChildMeasureSpec()方法:

/**
     * 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);
        
        //padding,就是已被占用的空间,就是 父容器的padding+child的marging
		//size,是ViewGroup本身size减去已使用的空间,是ViewGroup能提供给child的最大值。
        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);
    }
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可见,view的MeasureSpec由 viewParent的MeasureSpec和自身layoutParams确定。另外,child的可利用的尺寸是parent尺寸减去padding,上面代码已有注释,这很好理解。 梳理如下:

parentSpecMode
/childLayoutParams
EXACTLY AT_MOST UNSPECIFIED
dp/px EXACTLY
childSize
EXACTLY
childsize
EXACTLY
childsize
match_parent EXACTLY
parentSize
AT_MOST
parentSize
UNSPECIFIED
0
wrap_content AT_MOST
parentSize
AT_MOST
parentSize
UNSPECIFIED
0

注意,parentSize是父容器可使用的大小。

更新,看到鸿洋公众号的文章关于UNSPECIFIED说明:

MeasureSpec.UNSPECIFIED是不是真的不常见?

在日常定制View时,确实很少会专门针对这个模式去做特殊处理,大多数情况下,都会把它当成MeasureSpec.AT_MOST一样看待,就比如最最常用的TextView,它在测量时也是不会区分UNSPECIFIED和AT_MOST的。

不过,虽说这个模式比较少直接接触到,但很多场景下,我们已经在不知不觉中用上了,比如RecyclerView的Item,如果Item的宽/高是wrap_content且列表可滚动的话,那么Item的宽/高的测量模式就会是UNSPECIFIED。 还有就是NestedScrollViewScrollView,因为它们都是扩展自FrameLayout,所以它们的子View会测量两次,第一次测量时,子View的heightMeasureSpec的模式是写死为UNSPECIFIED的。 我们在自定义ViewGroup过程中,如果允许子View的尺寸比ViewGroup大的话,在测量子View时就可以把Mode指定为UNSPECIFIED。

看到ScrollView重写了measureChild方法,指定高度的mode是UNSPECIFIED

ScrollView重写了measureChild方法,指定高度的mode是UNSPECIFIED

3、View的工作流程

View的三大流程,measure、layout、draw。measure确定view的测量宽高,layout确定view的最终宽高和四个顶点位置,draw绘制到屏幕。

3.1 Measure过程

view的测量过程,由measure()方法完成。viewGroup测量自身后,还需调用child.measure()遍历测量子view。

3.1.1 view的测量过程

/**
     * <p>
     * This is called to find out how big a view should be. The parent
     * supplies constraint information in the width and height parameters.
     * </p>
     *
     * <p>
     * The actual measurement work of a view is performed in
     * {@link #onMeasure(int, int)}, called by this method. Therefore, only
     * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
     * </p>
     *
     *
     * @param widthMeasureSpec Horizontal space requirements as imposed by the
     *        parent
     * @param heightMeasureSpec Vertical space requirements as imposed by the
     *        parent
     *
     * @see #onMeasure(int, int)
     */
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
	...
	// measure ourselves, this should set the measured dimension flag back
     onMeasure(widthMeasureSpec, heightMeasureSpec);
    ...
}
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可见view的measure()方法是final,不可被子类重写。里面调用onMeasure(),实际真正的测量过程在onMeasure()中。所以只有onMeasure()可以且必须被子类重写。另外,参数widthMeasureSpec、heightMeasureSpec就是上一节最后的表格中的值。继续看onMeasure():

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }
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从名字就可以看出,setMeasuredDimension()就是设置测量的尺寸,且在onMeasure()中必须被调用,否则在测量时会发送异常。getDefaultSize()获取默认的宽/高。所以View类中的onMeasure() 是设置默认的宽高。 继续看getDefaultSize()具体实现:

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;
    }
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UNSPECIFIED,一般是系统使用,不需要关心。这里view大小直接取size,就是getSuggestedMinimumWidth()/getSuggestedMinimumHeight(),意思是 建议的 最小宽高。看下实现:

protected int getSuggestedMinimumWidth() {
        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
    }
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没有背景,就取mMinWidth,就是xml中设置的minWidth属性值;有背景,取 mMinWidth 、背景的MinimumWidth 的较大值。drawable的getMinimumWidth()如下,有固有宽度就取固有宽度(如BitmapDrawable),没有就是0(如ShadeDrawable)。

    public int getMinimumWidth() {
        final int intrinsicWidth = getIntrinsicWidth();
        return intrinsicWidth > 0 ? intrinsicWidth : 0;
    }
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AT_MOST、EXACTLY,直接取specSize,就是上一节最后的表格中的值,作为测量宽高。那这样取specSize是否合适呢? 再来看一遍specSize的来源。

parentSpecMode
/childLayoutParams
EXACTLY AT_MOST UNSPECIFIED
dp/px 1EXACTLY
childSize
2EXACTLY
childsize
EXACTLY
childsize
match_parent 3EXACTLY
parentSize
4AT_MOST
parentSize
UNSPECIFIED
0
wrap_content 5AT_MOST
parentSize
6AT_MOST
parentSize
UNSPECIFIED
0

1、2的情况,具体dp值,取SpecSize没问题,因为是EXACTLY,就是给定的的尺寸。 3的情况,match_parent,取SpecSize,即parentSize,也没问题,因为是EXACTLY,也是确定的尺寸。 4的情况,match_parent,但父容器又是wrap_content,系统就给了AT_MOST+parentSize,限制最大尺寸为parentSize。而这里直接取specSize即parentSize,似乎也没问题。这个看一个例子一,如下,view是match_parent,可见view取得确实是parentSize。

在这里插入图片描述
5、6的情况,wrapContent即AT_MOST+parentSize,取specSize也就是parentSize,所以和3、4一样都是parentSize,即 View类 中 默认wrapContent等同于match_parent。

再看一个情况例子二,如下,View换成TextView(继承View),尺寸就不是parentSize了,而是内容尺寸,说明TextView在onMeasure中做了处理。

在这里插入图片描述
继续看,例子三如下,同时有TextView、View,此时textView又是取parentSize(可用空间):
在这里插入图片描述
所以得出结论: 通常直接继承View的自定义View,在onMeasure()需要处理 : a、wrap_content的情况,否则wrap_content就等同于match_parent; b、match_parent+父容器wrap_content的情况,否则就像例子一,父容器wrap_content是无效的,处理方式就是例子二中的textView。 总结就是,直接继承View的自定义View,需要处理AT_MOST时的宽高

处理方式如下:

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        
        if (widthMode == MeasureSpec.AT_MOST && heightMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(mWidth, mHeight);
        } else if (widthMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(mWidth, heightSize);
        } else if (heightMode == MeasureSpec.AT_MOST) {
            setMeasuredDimension(widthSize, mHeight);
        }
    }
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实际就是在 AT_MOST时 设置一个指定的尺寸mWidth、mHeight,其他情况沿用系统。至于mWidth、mHeight是多少,则要具体看你的view的逻辑了。例如TextView,可以参考其源码的实现。

3.1.2 ViewGroup的测量过程

ViewGroup需要完成自身的测量,还要遍历子view调用measure()方法进行测量。

ViewGroup是抽象类,没有重写onMeasure,因为无法做到统一,是让具体继承ViewGroup的子类重写自己的逻辑。但是提供一些方便的方法给子类调用。如measureChildren()、measureChild()、measureChildWithMargins(),上面第二节分析过measureChildWithMargins(),这里我们看下measureChildren():

/**
     * 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);
            }
        }
    }
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就是遍历子view,调用measureChild(),继续看:

protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }
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通过getChildMeasureSpec()获取child的MeasureSpec,然后调用child.measure(),测量就传到child内部了,很好理解。measureChild()相比measureChildWithMargins() 没有考虑child的margin值。

上面说了,ViewGroup没有重写onMeasure,因为无法做到统一,让具体继承ViewGroup的子类重写自己的逻辑。具体看下LinearLayout的测量过程

@Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        if (mOrientation == VERTICAL) {
            measureVertical(widthMeasureSpec, heightMeasureSpec);
        } else {
            measureHorizontal(widthMeasureSpec, heightMeasureSpec);
        }
    }
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继续看measureVertical():

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
	...
	//下面这句官方注释:看每个人多高,也记住最大宽度。想想这不就是计算竖向LinearLayout宽高的思路嘛!
	// See how tall everyone is. Also remember max width.
        for (int i = 0; i < count; ++i) {
        	...
            final View child = getVirtualChildAt(i);
            ...
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            ...
            	// Determine how big this child would like to be. If this or
                // previous children have given a weight, then we allow it to
                // use all available space (and we will shrink things later
                // if needed).
                final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
                //这里测量child(里面就是measureChildWithMargins())
                measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
                        heightMeasureSpec, usedHeight);

                final int childHeight = child.getMeasuredHeight();
                ...
                final int totalLength = mTotalLength;
                //这里mTotalLength加上child的高度、margin,就是child高度累积。
                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                       lp.bottomMargin + getNextLocationOffset(child));
                ...
                //这里记录最大宽度(包含margin)
                final int margin = lp.leftMargin + lp.rightMargin;
            	final int measuredWidth = child.getMeasuredWidth() + margin;
            	maxWidth = Math.max(maxWidth, measuredWidth);
            ...
        }
        //遍历完了:高度加上自身的上下padding
		// Add in our padding
        mTotalLength += mPaddingTop + mPaddingBottom;
        int heightSize = mTotalLength;
        // Check against our minimum height
        heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
        
        //这里很重要:调用resolveSizeAndState--决定 计算的高度(高度累加)和 LinearLayout的父容器约束的高度,取哪一个。
        // Reconcile our calculated size with the heightMeasureSpec
        int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
        heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
	...
		//最大宽度加上左右margin
		maxWidth += mPaddingLeft + mPaddingRight;

        // Check against our minimum width
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

		//设置最终的测量尺寸(宽也也同样调用resolveSizeAndState决定取哪个)
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                heightSizeAndState);
}
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所以,简单概括就是: 1.先测量所有child; 2.根据child的情况获取自身宽高(累加高度、最大宽度)。

那么,是否就取 累加高度、最大宽度?再看下resolveSizeAndState():

/**
     * Utility to reconcile a desired size and state, with constraints imposed
     * by a MeasureSpec. Will take the desired size, unless a different size
     * is imposed by the constraints. The returned value is a compound integer,
     * with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and
     * optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the
     * resulting size is smaller than the size the view wants to be.
     *
     * @param size How big the view wants to be. --想要的尺寸
     * @param measureSpec Constraints imposed by the parent. --父布局给的measureSpec
     * @param childMeasuredState Size information bit mask for the view's
     *                           children.
     * @return Size information bit mask as defined by
     *         {@link #MEASURED_SIZE_MASK} and
     *         {@link #MEASURED_STATE_TOO_SMALL}.
     */
    public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
        final int specMode = MeasureSpec.getMode(measureSpec);
        final int specSize = MeasureSpec.getSize(measureSpec);
        final int result;
        switch (specMode) {
            case MeasureSpec.AT_MOST:
            	//AT_MOST时,想要的尺寸大于约束的尺寸,就只能取 约束的尺寸。
                if (specSize < size) {
                    result = specSize | MEASURED_STATE_TOO_SMALL;
                } else {
                    result = size;
                }
                break;
            case MeasureSpec.EXACTLY:
            	//dp值、match_parent且父EXACTLY,就是SpecSize
                result = specSize;
                break;
            case MeasureSpec.UNSPECIFIED:
            default:
                result = size;
        }
        return result | (childMeasuredState & MEASURED_STATE_MASK);
    }
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这个过程就是 限制 AT_MOST时,即wrap_content(或match_parent且父wrap_content)时高度不能大于parent的剩余空间。

3.1.3 获取View宽高的时机

Measure过程完成,就可通过getMeasuredWidth()、getMeasuredHeight()获取测量宽高。但某些极端情况 需要多次Measure才能确定最终宽高。所以在onLayout方法中获取测量宽高是真正ok的。 我们知道,activity的onCreate中无法获取到view的宽高。实际onCreate、onStart、onResume都不能保证view已完成测量,所以可能获取的都是0。因为view的measure和activity生命周期不是同步的。

以下是保证可以获取view测量宽高的方法

1、Activity/View # onWindowFocusChanged

onWindowFocusChanged:View已初始化完毕,宽高已准备ok。 但会多次调用,获取焦点、失去焦点都回调用。(这个回调是ViewRootIml中分发到DecorView,接着到Activity、到各级View。)

@Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);
        if (hasFocus) {
            int measuredWidth = scoreView.getMeasuredWidth();
            int measuredHeight = scoreView.getMeasuredHeight();
        }
    }
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2、view.post(runnable)

view.post可以把runnable放入消息队列,等待looper到此runnable是view已经初始化完成。v详细原理参考【Android源码解析】View.post()到底干了啥

@Override
    protected void onStart() {
        super.onStart();
        scoreView.post(new Runnable() {
            @Override
            public void run() {
                int measuredWidth = scoreView.getMeasuredWidth();
                int measuredHeight = scoreView.getMeasuredHeight();
            }
        });
    }
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3、ViewTreeObserver

ViewTreeObserver有很多回调,其中有个OnGlobalLayoutListener,当View树的状态发生改变或者View树内部view的可见性发生改变时 方法 onGlobalLayout()都会被调用。所以是会回调多次。 此时也可以获取view的宽高:

ViewTreeObserver observer = view.getViewTreeObserver();
        observer.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
            @Override
            public void onGlobalLayout() {
                mDefaultControlLayout.getViewTreeObserver().removeGlobalOnLayoutListener(this);
                if (mIsGroupListAnimating) {
                    mIsGroupListAnimationPending = true;
                } else {
                    updateLayoutHeightInternal(animate);
                }
            }
        });
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3.2Layout过程

layout()的作用是View用来确定view本身位置,内部调用onLayout()来确定子view的位置。 layout过程比measure过程简单很多。看View的layout方法:

    public void layout(int l, int t, int r, int b) {
        if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
            onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
            mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
        }

        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        //使用setFrame方法设置4个顶点,就确定位置了~
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
        	//这里调用onLayout,是个空实现。ViewGroup中重写了,还是空实现,但加了abstract,即ViewGroup的子类必须重写onLayout确定子View的位置。
            onLayout(changed, l, t, r, b);

            if (shouldDrawRoundScrollbar()) {
                if(mRoundScrollbarRenderer == null) {
                    mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
                }
            } else {
                mRoundScrollbarRenderer = null;
            }

            mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;

            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnLayoutChangeListeners != null) {
                ArrayList<OnLayoutChangeListener> listenersCopy =
                        (ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
                int numListeners = listenersCopy.size();
                for (int i = 0; i < numListeners; ++i) {
                    listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
                }
            }
        }

        ...
    }
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先是用setFrame方法设置4个顶点,就确定位置了,即mLeft、mTop、mBottom、mRight确定了。 然后调用onLayout,是个空实现。ViewGroup中重写了onLayout,还是空实现,但加了abstract,即ViewGroup的子类必须重写onLayout确定子View的位置。 那就看看LinearLayout的onLayout

@Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        if (mOrientation == VERTICAL) {
            layoutVertical(l, t, r, b);
        } else {
            layoutHorizontal(l, t, r, b);
        }
    }
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继续看layoutVertical():

void layoutVertical(int left, int top, int right, int bottom) {
        final int paddingLeft = mPaddingLeft;

        int childTop;
        int childLeft;

        // Where right end of child should go
        final int width = right - left;
        int childRight = width - mPaddingRight;

        // Space available for child
        int childSpace = width - paddingLeft - mPaddingRight;

        final int count = getVirtualChildCount();

        final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
        final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

        switch (majorGravity) {
           case Gravity.BOTTOM:
               // mTotalLength contains the padding already
               childTop = mPaddingTop + bottom - top - mTotalLength;
               break;

               // mTotalLength contains the padding already
           case Gravity.CENTER_VERTICAL:
               childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
               break;

           case Gravity.TOP:
           default:
               childTop = mPaddingTop;
               break;
        }
		//遍历子view
        for (int i = 0; i < count; i++) {
            final View child = getVirtualChildAt(i);
            if (child == null) {
                childTop += measureNullChild(i);
            } else if (child.getVisibility() != GONE) {
            	//获取child的测量宽高
                final int childWidth = child.getMeasuredWidth();
                final int childHeight = child.getMeasuredHeight();

                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();

                int gravity = lp.gravity;
                if (gravity < 0) {
                    gravity = minorGravity;
                }
                final int layoutDirection = getLayoutDirection();
                final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
                switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                    case Gravity.CENTER_HORIZONTAL:
                        childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                                + lp.leftMargin - lp.rightMargin;
                        break;

                    case Gravity.RIGHT:
                        childLeft = childRight - childWidth - lp.rightMargin;
                        break;

                    case Gravity.LEFT:
                    default:
                        childLeft = paddingLeft + lp.leftMargin;
                        break;
                }

                if (hasDividerBeforeChildAt(i)) {
                    childTop += mDividerHeight;
                }

                childTop += lp.topMargin;
                //以上就是获取子view的左、上的位置,即宽高,然后调用setChildFrame
                setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                        childWidth, childHeight);
                 //top位置加上高度和margin,就是下一个view的top
                childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

                i += getChildrenSkipCount(child, i);
            }
        }
    }
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就是遍历子view,确认childLeft、childTop,调用setChildFrame确认子view的位置:

private void setChildFrame(View child, int left, int top, int width, int height) {
		//这里width、height就是 上面获取的 测量宽高
        child.layout(left, top, left + width, top + height);
    }
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也就是调用child的layout方法,这样就走child的layout过程了。

一个问题:getMeasuredWidth() 与 getWidth()有何区别? 答曰:一般情况,getMeasuredWidth() 与 getWidth()两者无区别。 先看,getWidth():

public final int getWidth() {
        return mRight - mLeft;
    }
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在上面分析LinearLayout时,child.layout的参数中 mRight就是mLeft + measuredWidth,所以getWidth()就是measuredWidth。只不过是measuredWidth在测量过程产生,getWidth()在layout过程产生。 只要不重写view的layout()方法(也不需要重写)改变顶点位置就不会出现不同的情况,例如下面这个最终宽高比测量宽高大100。

    public void layout(int l, int t, int r, int b) {
    	super.layout(l,t,r+100,b+100);
    }
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3.3Draw过程

draw过程: 1、画背景 2、画自己-- onDraw,自己实现 3、画子view-- dispatchDraw 4、画装饰

public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            drawAutofilledHighlight(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // Step 7, draw the default focus highlight
            drawDefaultFocusHighlight(canvas);

            if (debugDraw()) {
                debugDrawFocus(canvas);
            }

            // we're done...
            return;
        }
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ViewGroup一般不用onDraw画自己,只需要画子View就可以了。但明确需要画自己的话,需要调用setViewNotDraw(false);

以上View的三大流程就分析完了。

4、自定义View

自定义view涉及view层次结构、事件分发、工作原理,有一定复杂度,但也是有章可循的。

4.1自定义view的分类

  1. 继承View:重写onDraw,要处理wrap_content、padding。
  2. 继承ViewGroup:重写onMeasure测量自己、子View,重写onLayout布局子View。
  3. 继承特定View(如TextView):扩展自己的功能。
  4. 继承特定ViewGroup(如LinearLayout):扩展自己的功能。

4.2 自定义view 注意点

  1. 支持wrap_content:直接继承View或ViewGroup的,要在onMeasure中处理wrap_content的情况。
  2. 支持padding:直接继承View在onDraw中处理;直接继承ViewGroup,在onMeasure、onLayout中处理padding和子view的margin。
  3. 不要在View中使用handler,因为本身提供了post方法。
  4. 在View#onDetachedFromWindow中停止动画或线程。
  5. 处理好嵌套滑动。

4.3 例子

自定义ViewGroup实例:横向滑动HorizontalView

4.4 自定义view的思想

先掌握基本功,弹性滑动、滑动冲突、绘制原理等,然后选择自定义的类别,按照注意事项多做就可以了。

.

好了,今天就到这里,欢迎留言讨论~

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