Android View底层到底是怎么绘制的详解手机开发

Android绘制链图：

网上很多讲Android  view的绘制流程往往只讲到了Measure Layout Draw。

但是，这只是一个大体的流程，而我们需要探讨的是Android在我们调用setcontentView（）之后，系统给我们干了什么事情，这个完整的逻辑是什么样的，却很少有人讲，还是先看下系统代码吧。

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

而最终调用了initWindowDecorActionBar这个方法，我们看下这个方法里面都实现了什么

 span span private void initWindowDecorActionBar() { Window window = getWindow(); // Initializing the window decor can change window feature flags. // Make sure that we have the correct set before performing the test below. window.getDecorView(); if (isChild() || !window.hasFeature(Window.FEATURE_ACTION_BAR) || mActionBar != null) { return; } mActionBar = new WindowDecorActionBar(this); mActionBar.setDefaultDisplayHomeAsUpEnabled(mEnableDefaultActionBarUp); mWindow.setDefaultIcon(mActivityInfo.getIconResource()); mWindow.setDefaultLogo(mActivityInfo.getLogoResource()); }

根据人家给我们的注释，这段代码是创建一个actionbar,初始化这个view和actionbar。这里面有一段很重要的代码：

window.getDecorView();

正式这段代码告知系统可以从view的根节点开始绘制了，
通过DecorView方法，decorview调用了performTraversals方法，我们来看下performTraversals源码：

 span private void performTraversals() { 

 final View host = mView; 

 ... 

 host.measure(childWidthMeasureSpec, childHeightMeasureSpec); 

 ... 

 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); 

 ... 

 draw(fullRedrawNeeded); 

} 

调用然后系统再调用Measure Layout Draw实现了View的绘制。

我们看一下完整的绘制流程，直接上一张图，或许更能说明这个意思：

到这里，系统会调用我们之前的比较熟悉的几个方法:Measure Layout Draw

Measure

Measure过程是计算视图大小，View中视图measure过程相关的方法主要有三个

public final void measure(int widthMeasureSpec, int heightMeasureSpec) 

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) 

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) 

measure调用onMeasure，onMeasure测量完成后setMeasureDimension，setMeasureDimension是final类型，view的子类不需要重写。

measure 源码：

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; 

} 

我们看一下OnMearsure方法：

 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 

 setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), 

 getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); 

 }

这个方法主要是实现setMeasuredDimension，这个方法是测量view的大小：

 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); 

 }

而对于这个measuredWidth和measuredHeight参数，系统却调了一个getDefaultSize();

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; 

 }

widthMeasureSpec和heightMeasureSpec决定了Mode和Size的值，
widthMeasureSpec和heightMeasureSpec来自父视图，这两个值都是由父视图经过计算后传递给子视图的，说明父视图会在一定程度上决定子视图的大小。但是最外层的根视图，它的widthMeasureSpec和heightMeasureSpec又是从哪里得到的呢？这就需要去分析ViewRoot中的源码了.

关于视图的measure过程可以阅读以下LinearLayout源码。

Layout measure过程确定视图的大小，而layout过程确定视图的位置。

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); 

 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); 

 mPrivateFlags = ~PFLAG_FORCE_LAYOUT; 

 mPrivateFlags3 |= PFLAG3_IS_LAID_OUT; 

 }

 函数中参数l、t、r、b是指view的左、上、右、底的位置，通过这几个参数来确定view在Windows的位置。

在layout函数中，重载了一个空函数

protected void onLayout(boolean changed, int left, int top, int right, int bottom) { 

 } 

这个需要子类去实现的。

比如Linearlayout:

 protected void onLayout(boolean changed, int l, int t, int r, int b) { 

 if (mOrientation == VERTICAL) { 

 layoutVertical(); 

 } else { 

 layoutHorizontal(); 

 } 

具体实现请自行看源码。

而在最后无论是layoutVertical还是layoutHorizontal都会掉一个setChildFrame方法来控制显示位置。

private void setChildFrame(View child, int left, int top, int width, int height) { 

 child.layout(left, top, left + width, top + height); 

} 

从上面看出，layout也是一个自上而下的过程，先设置父视图位置，在循环子视图，父视图位置一定程度上决定了子视图位置。

draw过程调用顺序在measure()和layout()之后，同样的，performTraversals()发起的draw过程最终会调用到mView的draw()函数，对于activity来说就是调用的PhoneWindow.DecorView。

* 1. Draw the background 

* 2. If necessary, save the canvas layers to prepare for fading 

* 3. Draw views content 

* 4. Draw children 

* 5. If necessary, draw the fading edges and restore layers 

* 6. Draw decorations (scrollbars for instance)

根据view源码的注释，

1，绘制背景

2，保存画布图层

3，调用了onDraw方法，子类中实现onDraw方法

4，使用的dispatchDraw方法

View或ViewGroup的子类不用再重载ViewGroup中该方法，因为它已经有了默认而且标准的view系统流程。dispatchDraw()内部for循环调用drawChild()分别绘制每一个子视图，而drawChild()内部又会调用draw（）函数完成子视图的内部绘制工作。

有兴趣的可以看看onDraw的源码。

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