一. View的工做流程

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相关文章

• 一. View的工做流程
• 二. View的事件分发机制
• 三. View的经常使用API
• 四. View的动画效果

本文概要:

1. 概述
   • Activity的UI层次及工做流程入口
2. 理解MeasureSpe
3. View的工做流程
4. 自定义View的方式及注意事项

一. 概述

如无特别说明,本次源码版本均基于8.0.

1.1 Activity的UI层次

在平常开发咱们在Activity的OnCreate中经过setContentView设置自定义布局时,实际上调用了PhoneWindow的setContentView方法,在此方法里面首先会初始化一个DecorView.最后将咱们自定义布局加载到DecorView布局中id为content的FrameLayout.

Activity的UI结构层次图-来自《进阶之光》

为了证明上述结论的正确性,咱们能够参考源码.

对应源码

//Activity.java
    public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }

//PhoneWindow.java
 @Override
    public void setContentView(int layoutResID) {
       
        if (mContentParent == null) {
            /**
             *  1.初始化DecorView
             */
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            /*
             *2.解析layoutResID布局文件,将它嵌入到*mContentParent的内部
             */
            mLayoutInflater.inflate(layoutResID, mContentParent);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }
    
//PhoneWindow#installDecor方法
private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            //1.生成DecorView
            mDecor = generateDecor(-1);
            mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
            mDecor.setIsRootNamespace(true);
            if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
                mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
            }
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            //2.找到ID为content的ViewGroup
            mContentParent = generateLayout(mDecor);

            // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
            mDecor.makeOptionalFitsSystemWindows();

            final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                    R.id.decor_content_parent);
    //...省略部分代码
   }         
}

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1.2 View的工做流程入口

咱们知道ActivityThread做为启动入口,会经过handleLaunchActivity,建立一个Activity对象.其中在performLaunchActivity中调用OnCreate方法,也就是上一节所讲内容.接着在handleResumeActivity方法里,开始将View加载到Window最后.这里咱们只看与View绘制相关流程.

1. 首先经过WindowManager将DecorView添加到Window中,与此同时将ViewRootImpl对象经过setView将DecorView关联起来.

对应源码

//WindowManagerImpl.java
@Override
    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        //调用WindowManagerGlobal的addView
        mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
    }

//WindowManagerGlobal.java
    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
     
        //...省略部分代码
        
        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {

          //...省略部分代码

            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);

            // do this last because it fires off messages to start doing things
            try {
                //将DecorView关联起来
                root.setView(view, wparams, panelParentView);
            } catch (RuntimeException e) {
                // BadTokenException or InvalidDisplayException, clean up.
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
                throw e;
            }
        }
    }
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2. 在加载到Window以前,其ViewRootImpl内部会经过requestLayout方法来布局整个DecorView树,最终调用performTravels,依次执行DecorView的measure、layout、draw三大过程.

对应源码

//ViewRootImpl.java
 @Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true;
            //执行开始布局
            scheduleTraversals();
        }
    }

private void performTraversals() {
//...省略部分代码
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
//...省略部分代码
    performLayout(lp, mWidth, mHeight);
//...省略部分代码
    performDraw();
//...省略部分代码
}
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了解了相关启动加载与绘制流程后,下面就开始进入正题-View相关的重要知识点.

二. 理解MeasureSpec

MeasureSpec能够理解为规格限制,其由specSize和specMode构成.

提供相应的打包与拆包方法: makeMeasureSpec/getMode,getSize

Mode 三种模式:

1. EXACTLY: 精确模式
   • 父View对子view有精确的大小,大小就是specSize.
   • 对应于MACTCH_PARENT和具体数值.
2. AT_MOST:最大模式
   • 父View对子view最大值有要求,大小就是specSize.
   • 对应于WRAP_CONTENT
3. UNSPECIFIED

MeasureSpec 的计算方式:

• 对于DecorView,它的MeasureSpec由窗口与自身的layoutParams共同决定---在viewRootImpl完成.

• 对于普通View,它的MeasureSpec由父容器的MeasureSpec与自身的layoutParams共同决定---在父容器中完成.

对应源码:

//ViewRootImpl.java
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
           
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:

            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }


//ViewGroup.java
protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        
        //对于在父元素的已占用的空间=父元素自身padding+子元素的magrgin
        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);
    }

   /** @param 父元素的MeasureSpec
     * @param 对于在父元素的已占用的空间=父元素自身padding+子元素的magrgin
     * @param 自身的lp参数
     * @return 子元素的MeasureSpec
     */
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);
        
        //父元素可用空间 = 父元素总的大小-已占用的空间
        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) {
               
                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) {
                
                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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对于上述源码咱们重点注意getChildMeasureSpec方法,其中第二个参数表明的是父元素已占用的空间.该方法体现了生成普通View的MeasureSpec的规格.

遵循如下规则,UNSPECIFIED除外.前者表示子元素的layoutParams参数.

• dp/px
  • EXACTLY与childSize.
• match_parent
  • 父元素的specMode为EXACTLY,那么规格就是EXACTLY与parentSize.
  • 父元素的specMode为AT_MOST,那么规格就是AT_MOST与parentSize.
• wrap_content
  • AT_MOST与parentSize.

从上面规则能够发现,不管是match_parent,仍是wrap_content,最终返回的大小都是parentSize,也就是父元素的可用空间.而View在默认测量时getDefaultSize会直接返回该大小,因此在测量过程当中,须要对wrap_content进行特殊处理.具体如何处理下一篇在讲解.

三. View的工做流程

measure、layout、draw 是调度方法,由父容器调用,进行预处理.

在测量以前父元素都须要生成对应子view的测量规格MeasureSpec.具体见上一节.

3.1 measure过程

做用: 肯定测量宽高

measure-onMeasure-setMeasureDimension

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1. 当子View是纯View时,View的测量过程

onMeasure 实际测量方法

1. 在onMeasure里计算出本身的尺寸,经过setMeasureDimension保存测量尺寸,完成自我测量.

2. 当子View是ViewGroup时,ViewGroup的测量过程

1. 在onMeasure里,首先须要遍历调用全部子view的measure方法,让子View完成自我测量,依次递归.
   • 调用子view的measure方法以前需先获取到子view的MeasureSpec,具体见上一节.
2. 而后根据子View的尺寸,得出子View的位置,并保存尺寸和位置.
3. 最后根据这些子View的尺寸位置,计算出本身尺寸,经过setMeasureDimension保存测量尺寸,完成自我测量.

对应源码

//View.java
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //获取默认值并保存
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

//获取默认值
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;//直接返回父元素可用空间,因此须要对wrap_content作特殊处理.
            break;
        }
        return result;
    }


//ViewGroup.java
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
        final int size = mChildrenCount;
        final View[] children = mChildren;
        //遍历全部的子View
        for (int i = 0; i < size; ++i) {
            final View child = children[i];
            if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
            }
        }
    }

protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        final LayoutParams lp = child.getLayoutParams();
        //得到子View的MeasureSpec
        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);
        //调用ziview的measure方法
        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

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3.2 layout过程

做用: 肯定最终宽高和位置

layout-onLayout
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• layout 保存传入的位置,肯定了View自己在其父View中的位置.

• 若是子View是纯View,那么在onLayout里,什么都不作.

• 若是是ViewGroup,那么在onLayout里,首先须要遍历调用全部子view的layout方法,让子View完成自我布局,依次递归.

对应源码

@SuppressWarnings({"unchecked"})
    public void layout(int l, int t, int r, int b) {
        
        //...省略部分代码
        int oldL = mLeft;
        int oldT = mTop;
        int oldB = mBottom;
        int oldR = mRight;
        
        //setFrame设置View四个顶点位置,肯定在父容器的位置
        boolean changed = isLayoutModeOptical(mParent) ?
                setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);

        if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
            //开始布局子view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);
                }
            }
        }

        //...省略部分代码
    }
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从测量与布局两个阶段能够看出两个特色.

1. 测量阶段由父View统一保存全部子View的尺寸位置信息(后面简称信息)
2. 布局阶段由本身保存信息,该信息从父View传进来.

3.3 draw过程

做用: 绘制到屏幕上

draw方法里流程

• drawBackground 绘制背景
• onDraw 绘制本身
• dispatchDraw 绘制子view
• onDrawScrollBars 绘制装饰

四. 自定义View的方式及注意事项

• 基本方式:

1. 继承原生View
   • 继承已存在的成熟控件(如:TextView)
2. 继承原生ViewGroup
   • 继承已存在的成熟控件(如:LinearLayout)

• 注意事项:

1. 继承原生的,不管是View仍是ViewGroup,都须要在onMeasure处理warp_content时状况的宽高.
2. • 对于view绘制时须要考虑padding.不然这些属性都不生效.
   • 对于viewGroup测量和布局时,须要考虑自身的padding和子view的margin.

因为本人技术有限,若有错误的地方,麻烦你们给我提出来,本人不胜感激,你们一块儿学习进步.

参考连接:

• blog.csdn.net/lfdfhl/arti…
• hencoder.com/ui-2-1/
• item.jd.com/12125491.ht…
• blog.csdn.net/singwhatiwa…