View绘制流程二:测量布局绘制

Posted <天各一方>

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文章目录


上篇博客我们详细分析了布局是如何添加到我们的界面上的,谈到ViewRootImpl的三个方法也就停止了,今天我们详细的分析measure、layout、draw三个过程。

Measure

理解MeasureSpec

什么是MeasureSpec

当我们要对我们的View进行测量时,要有一定的规则,我们不仅要考虑开发者在xml中给我们指定的layout,而且还要结合父View给我们指定的测量规则,这两个属性共同决定了我们子View的MeasureSpec,也就是子View的测量规则,按照这个MeasureSpec,我们就可以很轻松的解决测量时可能会遇到的问题。

MeasureSpec是一个32位的int值,高2位是SpecMode,它指的是测量模式,低30位是、SpecSize,它指的是在某种测量模式下的规格大小。MeasureSpec是将这两个打包成一个int值来避免过多对象内存分配,在MeasureSpec类内部提供了对这两个属性的操作。

MeasureSpec分类

  • UNSPECIFIED

    父View不会对View有任何的限制,要多大给多大,这种情况一般用于系统内部。

  • EXACTLY

    父View已经测出View所需的精确的大小,这个时候View的最终大小就是SpecSize所指定的值。它对应LayoutParams中的match_parent和具体数值这两种模式。

  • AT_MOST

    父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值,具体是什么要看不同View的具体实现。它对应wrap_content属性。

MeasureSpec的创建

通过getChildMeasure就可以创建出子View的MeasureSpec:

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;
    
    //noinspection ResourceType
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);

上述代码一图以蔽之:

View的measure

有了MeasureSpec,我们也可以放心的去测量我们的View了。View的measure过程是由其measure方法来完成,measure方法是一个final方法,子类不可以重写此方法,在View的measure方法中会调用View的onMeasure方法,因此我们直接看onMeasure方法:

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

这个方法里,只用了一个方法就完成了View的measure过程,而通过方法名我们也知道这个方法时设置测量尺寸,那么参数就是测量过的尺寸了,所以我们看getDefaultSize这个方法:

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;

这个方法也是很简单,UNSPECIFIED这个分支我们不去管他,而AT_MOST和EXACTLY这两个返回的结果是一样的,都是specSize,我们通过上面的表格也可以看出specSize是parentSize,是父View提供的可用空间。这就给我们自定义View提供了一个思路:继承View需要重写onMeasure并对wrap_content属性设置默认宽高,否则效果同match_parent一样,从源码我们也可以看出原因来。

ViewGroup的measure

对于ViewGroup来说,除了完成自己的measure过程以外,还要遍历的去调用所有子元素的measure方法,各个子元素再递归去执行这个过程。和View不同的是,ViewGroup是一个抽象类,因此它没有重写View的onMeasure方法,但是它提供了一个叫measureChildren的方法:

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);
        
    

这个方法里,会对每一个子元素去执行ViewGroup的measureChild方法:

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);

这个方法里,获取了子View的MeasureSpec,然后对子View进行测量。在继承ViewGroup实现自定义ViewGroup时,测量完子View的宽高后,就可以确定ViewGroup自身的宽高了。

Layout

Layout的作用是ViewGroup用来确定子元素的位置的,当ViewGroup的位置被确定后,它会在onLayout方法中遍历所有的子元素并调用其layout方法,在layout方法中,onLayout方法又会被调用。layout方法是确定其本身的位置,而onLayout方法是去确定所有子元素的位置。

简单看一看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;

    boolean changed = isLayoutModeOptical(mParent) ?
            setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

    if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) 
        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);
            
        
    

    final boolean wasLayoutValid = isLayoutValid();

    mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
    mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;

    if (!wasLayoutValid && isFocused()) 
        mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
        if (canTakeFocus()) 
            // We have a robust focus, so parents should no longer be wanting focus.
            clearParentsWantFocus();
         else if (getViewRootImpl() == null || !getViewRootImpl().isInLayout()) 
            // This is a weird case. Most-likely the user, rather than ViewRootImpl, called
            // layout. In this case, there's no guarantee that parent layouts will be evaluated
            // and thus the safest action is to clear focus here.
            clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
            clearParentsWantFocus();
         else if (!hasParentWantsFocus()) 
            // original requestFocus was likely on this view directly, so just clear focus
            clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
        
        // otherwise, we let parents handle re-assigning focus during their layout passes.
     else if ((mPrivateFlags & PFLAG_WANTS_FOCUS) != 0) 
        mPrivateFlags &= ~PFLAG_WANTS_FOCUS;
        View focused = findFocus();
        if (focused != null) 
            // Try to restore focus as close as possible to our starting focus.
            if (!restoreDefaultFocus() && !hasParentWantsFocus()) 
                // Give up and clear focus once we've reached the top-most parent which wants
                // focus.
                focused.clearFocusInternal(null, /* propagate */ true, /* refocus */ false);
            
        
    

    if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) 
        mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
        notifyEnterOrExitForAutoFillIfNeeded(true);
    

    notifyAppearedOrDisappearedForContentCaptureIfNeeded(true);

通过setFrame来设置View四个顶点的位置,即初始化mLeft、mRight、mTop、mBottom。四个顶点的位置确定了,View在父容器的位置也就确定了。

Draw

Draw的过程就简单了,它的作用就是将View绘制到屏幕上面。View的绘制遵循着如下几步:

  • 绘制背景 background.draw(canvas)
  • 绘制自己 onDraw
  • 绘制children dispatchDraw
  • 绘制装饰 onDrawScrollBars

这四个步骤在源码中也明显可以看出。View的onDraw是空实现,不同的控件对内容的绘制都不同,所以留给控件自己实现,我们自定义View时需要重写onDraw方法来绘制我们自己的控件。在ViewGroup中,也有对dispatchDraw的实现,dispatchDraw中对子View进行遍历,并调用了drawChild方法,其中又调用了View的draw方法,完成对子View的绘制。

自定义View时注意以下问题

  1. 让View支持wrap_content。
  2. 如果有必要,支持padding。
  3. 尽量不要在View中使用Handler。
  4. View中如果有线程或者动画,及时停止。
  5. View带有滑动嵌套情景时,处理好滑动冲突。

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