本文主要是介绍安卓Android ViewGroup onDraw为什么没调用?，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
原帖 : link
前言
通过本篇文章，你将了解到：
1、ViewGroup onDraw不执行的原因
2、怎么让ViewGroup onDraw执行
3、setWillNotDraw(boolean)作用
如果对原理不感兴趣，请拉到最后的总结查看解决办法～
小例子
我们知道自定义view的时候会重写onDraw()方法，如下：
public class MyView extends View {<span class="token keyword">private</span> <span class="token class-name">Paint</span> paint<span class="token punctuation">;</span> <span class="token keyword">private</span> <span class="token class-name">Rect</span> rect<span class="token punctuation">;</span> <span class="token keyword">private</span> <span class="token class-name">Bitmap</span> bitmap<span class="token punctuation">;</span> <span class="token keyword">private</span> <span class="token class-name">Matrix</span> matrix<span class="token punctuation">;</span><span class="token keyword">public</span> <span class="token class-name">MyView</span><span class="token punctuation">(</span><span class="token class-name">Context</span> context<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">(</span>context<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">public</span> <span class="token class-name">MyView</span><span class="token punctuation">(</span><span class="token class-name">Context</span> context<span class="token punctuation">,</span> <span class="token annotation punctuation">@Nullable</span> <span class="token class-name">AttributeSet</span> attrs<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span> attrs<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">public</span> <span class="token class-name">MyView</span><span class="token punctuation">(</span><span class="token class-name">Context</span> context<span class="token punctuation">,</span> <span class="token annotation punctuation">@Nullable</span> <span class="token class-name">AttributeSet</span> attrs<span class="token punctuation">,</span> <span class="token keyword">int</span> defStyleAttr<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span> attrs<span class="token punctuation">,</span> defStyleAttr<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>paint <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Paint</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>paint<span class="token punctuation">.</span><span class="token function">setAntiAlias</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>paint<span class="token punctuation">.</span><span class="token function">setColor</span><span class="token punctuation">(</span><span class="token class-name">Color</span><span class="token punctuation">.</span>BLUE<span class="token punctuation">)</span><span class="token punctuation">;</span>rect <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Rect</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token number">100</span><span class="token punctuation">,</span> <span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token annotation punctuation">@Override</span> <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">draw</span><span class="token punctuation">(</span><span class="token class-name">Canvas</span> canvas<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">draw</span><span class="token punctuation">(</span>canvas<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token annotation punctuation">@Override</span> <span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">onDraw</span><span class="token punctuation">(</span><span class="token class-name">Canvas</span> canvas<span class="token punctuation">)</span> <span class="token punctuation">{</span>canvas<span class="token punctuation">.</span><span class="token function">drawRect</span><span class="token punctuation">(</span>rect<span class="token punctuation">,</span> paint<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
}
效果图：
image.png
蓝色的方块即是我们自定义MyView绘制的。
现在自定义一个ViewGroup：MyFrameLayout，并把MyView添加到MyFrameLayout里。
public class MyFrameLayout extends FrameLayout {<span class="token keyword">private</span> <span class="token class-name">Paint</span> paint<span class="token punctuation">;</span> <span class="token keyword">private</span> <span class="token class-name">RectF</span> rectF<span class="token punctuation">;</span><span class="token keyword">public</span> <span class="token class-name">MyFrameLayout</span><span class="token punctuation">(</span><span class="token annotation punctuation">@NonNull</span> <span class="token class-name">Context</span> context<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">(</span>context<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">public</span> <span class="token class-name">MyFrameLayout</span><span class="token punctuation">(</span><span class="token annotation punctuation">@NonNull</span> <span class="token class-name">Context</span> context<span class="token punctuation">,</span> <span class="token annotation punctuation">@Nullable</span> <span class="token class-name">AttributeSet</span> attrs<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span> attrs<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">public</span> <span class="token class-name">MyFrameLayout</span><span class="token punctuation">(</span><span class="token annotation punctuation">@NonNull</span> <span class="token class-name">Context</span> context<span class="token punctuation">,</span> <span class="token annotation punctuation">@Nullable</span> <span class="token class-name">AttributeSet</span> attrs<span class="token punctuation">,</span> <span class="token keyword">int</span> defStyleAttr<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span> attrs<span class="token punctuation">,</span> defStyleAttr<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">init</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token function">addView</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">MyView</span><span class="token punctuation">(</span><span class="token function">getContext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>paint <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Paint</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>paint<span class="token punctuation">.</span><span class="token function">setAntiAlias</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>paint<span class="token punctuation">.</span><span class="token function">setColor</span><span class="token punctuation">(</span><span class="token class-name">Color</span><span class="token punctuation">.</span>RED<span class="token punctuation">)</span><span class="token punctuation">;</span>rectF <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">RectF</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">200</span><span class="token punctuation">,</span> <span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token annotation punctuation">@Override</span> <span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">dispatchDraw</span><span class="token punctuation">(</span><span class="token class-name">Canvas</span> canvas<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">super</span><span class="token punctuation">.</span><span class="token function">dispatchDraw</span><span class="token punctuation">(</span>canvas<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token annotation punctuation">@Override</span> <span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">onDraw</span><span class="token punctuation">(</span><span class="token class-name">Canvas</span> canvas<span class="token punctuation">)</span> <span class="token punctuation">{</span>canvas<span class="token punctuation">.</span><span class="token function">drawRect</span><span class="token punctuation">(</span>rectF<span class="token punctuation">,</span> paint<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span>
}
MyFrameLayout 里重写了onDraw()方法，试图绘制一个红色的矩形，并且将MyView作为子view添加到MyFrameLayout里。最后将MyFrameLayout添加到xml作为activity布局文件：
<com.fish.myapplication.MyFrameLayoutandroid:id="@+id/myFrameLayout"android:layout_width="400dp"android:layout_height="400dp"></com.fish.myapplication.MyFrameLayout>
来看看效果：
image.png
想象中红色的矩形并没有显示出来，猜测一下原因：
1、MyView遮挡了MyFrameLayout的绘制？这个可以排除了，因为MyView蓝色区域大小：100px * 100px，而MyFrameLayout红色区域大小：200px * 200px。
2、MyFrameLayout的onDraw()没有调用。
寻根溯源
这里涉及到view的绘制流程，不进行深入阐述，大致挑重点说一下：
1、从根view(viewGroup)开始，先绘制自身，再绘制子view
2、子view继续按照第一步递归
3、如果当前view不是viewGroup，那么不会绘制子view（因为它没有孩子啊）
view绘制源码，对应上述步骤。
public void draw(Canvas canvas) {<span class="token comment">//省略</span><span class="token keyword">boolean</span> horizontalEdges <span class="token operator">=</span> <span class="token punctuation">(</span>viewFlags <span class="token operator">&</span> FADING_EDGE_HORIZONTAL<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">;</span><span class="token keyword">boolean</span> verticalEdges <span class="token operator">=</span> <span class="token punctuation">(</span>viewFlags <span class="token operator">&</span> FADING_EDGE_VERTICAL<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">;</span><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>verticalEdges <span class="token operator">&&</span> <span class="token operator">!</span>horizontalEdges<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token comment">// 绘制自身内容</span><span class="token function">onDraw</span><span class="token punctuation">(</span>canvas<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// 绘制子view</span><span class="token function">dispatchDraw</span><span class="token punctuation">(</span>canvas<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//省略</span><span class="token comment">// we're done...</span><span class="token keyword">return</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token comment">//省略</span> <span class="token punctuation">}</span>
回到我们之前的试验，MyFrameLayout onDraw()没有执行，但是MyView onDraw()执行了，说明MyFrameLayout dispatchDraw()方法执行了，我们有理由相信MyFrameLayout draw()方法没有执行，只执行了MyFrameLayout dispatchDraw()，继续探究。
硬件加速
现在Android默认开启硬件加速，什么是硬件加速呢？为了加快Android绘制速度，适当解放cpu资源，Android将一部分绘制放到gpu执行。而对应的Android里面的canvas，也分为是否支持硬件加速，因此绘制流程也有所差异，流程图简示如下：
image.png
[]表示该调用该类里的对应方法。
()表示方法里的参数
从上图可以看出，不管是否开启硬件加速，都会经历“跳过绘制”的逻辑判断，而该判断的分支就决定了viewGroup的ondraw()方法是否执行。如果“跳过绘制”成立，那么调用dispatchDraw()方法，继而调用子view进行绘制（如果有子view）。如果“跳过绘制”不成立，那么调用draw(x1)，该方法上面分析过了：会调用dispatchDraw()和ondraw()方法。
决定是否绘制的因素
从代码上分析结果来看，初步符合我们的猜想：MyFrameLayout因为某种原因，跳过了绘制，只调用了dispatchDraw()方法，从而onDraw()方法没有得到执行，最终导致没有绘制自身的内容。接下来看看“跳过绘制”的判断依据。
软件绘制：
view.java draw(x1,x2,x3)方法...// Fast path for layouts with no backgroundsif ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {mPrivateFlags &= ~PFLAG_DIRTY_MASK;dispatchDraw(canvas);} else {draw(canvas);}...
硬件加速：
view.java updateDisplayListIfDirty方法//省略// Fast path for layouts with no backgroundsif ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {dispatchDraw(canvas);drawAutofilledHighlight(canvas);if (mOverlay != null && !mOverlay.isEmpty()) {mOverlay.getOverlayView().draw(canvas);}if (debugDraw()) {debugDrawFocus(canvas);}} else {draw(canvas);}//省略
可以看出不管是否支持硬件加速，其判断依据是通过PFLAG_SKIP_DRAW标记来确定的，现在就需要找到这个标记什么时候赋值与清空的。MyFrameLayout onDraw()没执行，而MyView onDraw()执行了，那么猜测MyFrameLayout设置了 PFLAG_SKIP_DRAW标记，MyView没有设置。MyFrameLayout简单继承了FrameLayout，MyView简单继承了View，我们并没有对两者进行单独设置标记，进而猜测是viewGroup和View初始化时对于PFLAG_SKIP_DRAW标记做了不同的处理。
来看看ViewGroup初始化：
image.png
viewGroup初始化的时候，默认设置了WILL_NOT_DRAW，从字面意思来看是“不会绘制”标记，这个标记是否和PFLAG_SKIP_DRAW有联系呢？继续查看setFlags方法：
vew.java setFlags方法//省略if ((changed & DRAW_MASK) != 0) {if ((mViewFlags & WILL_NOT_DRAW) != 0) {if (mBackground != null|| mDefaultFocusHighlight != null|| (mForegroundInfo != null && mForegroundInfo.mDrawable != null)) {mPrivateFlags &= ~PFLAG_SKIP_DRAW;} else {mPrivateFlags |= PFLAG_SKIP_DRAW;}} else {mPrivateFlags &= ~PFLAG_SKIP_DRAW;}requestLayout();invalidate(true);}//省略
到此处就比较明朗，将两个标记值联系起来了：
1、如果设置了WILL_NOT_DRAW标记，那么继续检查background、foreground(mDrawable字段)、focusHighLight是否有值，如果三者任意一个设置了，那么将PFLAG_SKIP_DRAW标记清除，否则将该标记加上。
2、如果没有设置WILL_NOT_DRAW标记，那么将PFLAG_SKIP_DRAW标记清除。
至此，我们知道了MyFrameLayout onDraw()方法没有执行的原因：viewGroup默认设置了WILL_NOT_DRAW标记，进而设置了PFLAG_SKIP_DRAW标记，而在绘制的时候通过判断PFLAG_SKIP_DRAW标记来决定是否调用MyFrameLayout draw(x)方法，最终调用onDraw()方法。而view默认没有设置WILL_NOT_DRAW标记，也就没有后面的事了。
如何让viewGroup onDraw()执行
既然知道了MyFrameLayout没有绘制的原因，那么就有方法让它执行绘制流程。
先来看看WILL_NOT_DRAW
view.java/*** If this view doesn't do any drawing on its own, set this flag to* allow further optimizations. By default, this flag is not set on* View, but could be set on some View subclasses such as ViewGroup.** Typically, if you override {@link #onDraw(android.graphics.Canvas)}* you should clear this flag.** @param willNotDraw whether or not this View draw on its own*/public void setWillNotDraw(boolean willNotDraw) {setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);}<span class="token comment">/*** Returns whether or not this View draws on its own.** @return true if this view has nothing to draw, false otherwise*/</span> <span class="token annotation punctuation">@ViewDebug</span><span class="token punctuation">.</span><span class="token class-name">ExportedProperty</span><span class="token punctuation">(</span>category <span class="token operator">=</span> <span class="token string">"drawing"</span><span class="token punctuation">)</span> <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">willNotDraw</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">return</span> <span class="token punctuation">(</span>mViewFlags <span class="token operator">&</span> DRAW_MASK<span class="token punctuation">)</span> <span class="token operator">==</span> WILL_NOT_DRAW<span class="token punctuation">;</span> <span class="token punctuation">}</span>
View类里暴露了设置WILL_NOT_DRAW标记的接口：
setWillNotDraw(boolean willNotDraw)，可以在MyFrameLayout里使用setWillNotDraw(false)。
不想设置该标记也是可行的，前面说过即使设置了WILL_NOT_DRAW，后面还是有判断background、foreground、focusHighLight是否有值。
background:view背景
foreground(mDrawable字段):view前景
focusHighLight:view获得焦点时高亮
我们只要设置了其中一个值，PFLAG_SKIP_DRAW标记将会被清空。
来看看这三个值如何影响PFLAG_SKIP_DRAW标记
view.java
public void setBackgroundDrawable(Drawable background) {
if (background != null) {if ((mPrivateFlags & PFLAG_SKIP_DRAW) != 0) {mPrivateFlags &= ~PFLAG_SKIP_DRAW;requestLayout = true;}}
}
public void setForeground(Drawable foreground) {
if (foreground != null) {
if ((mPrivateFlags & PFLAG_SKIP_DRAW) != 0) {
mPrivateFlags &= ~PFLAG_SKIP_DRAW;
}
}
}
private void setDefaultFocusHighlight(Drawable highlight) {
mDefaultFocusHighlight = highlight;
mDefaultFocusHighlightSizeChanged = true;
if (highlight != null) {
if ((mPrivateFlags & PFLAG_SKIP_DRAW) != 0) {
mPrivateFlags &= ~PFLAG_SKIP_DRAW;
}
}
}
探究了原理，我们来看看实际效果，现在给MyFrameLayout加上背景，再来看看效果：
image.png
蓝色是MyView绘制出来的
红色是MyFrameLayout绘制出来的
绿色是MyFrameLayout设置的背景
看得出来，MyFrameLayout内容已经绘制出来（红色区域）
总结
若要ViewGroup onDraw()执行，只需要setWillNotDraw(false)、设置背景、设置前景、设置焦点高亮，4个选项其中一项满足即可。
当然如果不想在MyFrameLayout onDraw里绘制，也可以重写MyFrameLayout dispatchDraw()方法，在该方法里绘制MyFrameLayout内容。
@Overrideprotected void dispatchDraw(Canvas canvas) {canvas.drawRect(rectF, paint);super.dispatchDraw(canvas);}<span class="token annotation punctuation">@Override</span> <span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">onDraw</span><span class="token punctuation">(</span><span class="token class-name">Canvas</span> canvas<span class="token punctuation">)</span> <span class="token punctuation">{</span>
// canvas.drawRect(rectF, paint);
}
需要注意的是，super.dispatchDraw(canvas)要放到后边执行，不然子view内容会被MyFrameLayout覆盖。
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