Android View底层到底是怎么绘制的

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 style="font-size: 16px;"> </span><span style="font-size:14px;"> 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());
}</span>

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 style="font-size:14px;">private void performTraversals() {
final View host = mView;
...
host.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
draw(fullRedrawNeeded);
} </span>

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

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 view's 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。