源码分析Android中View的绘制流程

在开发中自定义控件的使用是比较频繁的，而自定义控件的基础之一就是View的测量以及绘制。这篇文章从源码的角度简要分析一下View的测量绘制。

在了解View绘制流程之前，必须先要了解一个类，MeasureSpec，它是View的内部类，专门来进行对测量的数据和类型进行打包和解包，看一下源码就会清楚不少：

public static class MeasureSpec {        private static final int MODE_SHIFT = 30;        private static final int MODE_MASK  = 0x3 << MODE_SHIFT;        public static final int UNSPECIFIED = 0 << MODE_SHIFT;        public static final int EXACTLY     = 1 << MODE_SHIFT;        public static final int AT_MOST     = 2 << MODE_SHIFT;        public static int makeMeasureSpec(int size, int mode) {            return size + mode;        }        public static int getMode(int measureSpec) {            return (measureSpec & MODE_MASK);        }        public static int getSize(int measureSpec) {            return (measureSpec & ~MODE_MASK);        }        public static String toString(int measureSpec) {            int mode = getMode(measureSpec);            int size = getSize(measureSpec);            StringBuilder sb = new StringBuilder("MeasureSpec: ");            if (mode == UNSPECIFIED)                sb.append("UNSPECIFIED ");            else if (mode == EXACTLY)                sb.append("EXACTLY ");            else if (mode == AT_MOST)                sb.append("AT_MOST ");            else                sb.append(mode).append(" ");            sb.append(size);            return sb.toString();        }    }

类还是比较简单，使用getMode和getSize方法对一个32位的int值进行解包，解释出前2位和后30位分别作为测量模式和大小的标注。使用makeMeasureSpec来进行打包。这里暂时只需要知道可以用一个int值来存储这两种类型的数据就可以了，后面还会接触到。

ViewGroup中存放了许多的View，那么对View的测量一定会经过ViewGroup，在ViewGroup中的measureChildren方法完成了对子View的测量，来看一下：

 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);            }        }    }

在方法中只是遍历了一下然后调用了measureChild方法，进入看一下：

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);    }

可以看到方法中生成了子View的MeasureSpec，然后简单的调用了子View的measure方法。来看一下getChildMeasureSpec方法：

 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 = 0;                resultMode = MeasureSpec.UNSPECIFIED;            } else if (childDimension == LayoutParams.WRAP_CONTENT) {                // Child wants to determine its own size.... find out how                // big it should be                resultSize = 0;                resultMode = MeasureSpec.UNSPECIFIED;            }            break;        }        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);    }

先解释一下参数，spec是当前ViewGroup的MeasureSpec，padding是当前ViewGroup的padding，childDimension是子View的大小（可以为MATCH_PARENT和WRAP_CONTENT）。

当ViewGroup的MeasureSpec是EXACTLY时：

①子View的大小是大于等于0的，那么给子View设置的大小就是childDimension，MeasureSpec为EXACTLY。

②子View的大小是MATCH_PARENT，给子View设置的大小是当前ViewGroup大小-padding大小，MeasureSpec为EXACTLY。

③子View的大小是WRAP_CONTENT，给子View设置的大小是当前ViewGroup的大小-padding大小，MeasureSpec为AT_MOST。

当ViewGroup的MeasureSpec是AT_MOST时：

①子View的大小是大于等于0的，那么给子View设置的大小就是childDimension，MeasureSpec为EXACTLY。

②子View的大小是MATCH_PARENT，给子View设置的大小是当前ViewGroup大小-padding大小，MeasureSpec为AT_MOST。

③子View的大小是WRAP_CONTENT，给子View设置的大小是当前ViewGroup的大小-padding大小，MeasureSpec为AT_MOST。

剩下的一种情况是ViewGroup的MeasureSpec为UNSPECIFIED，这种情况基本不会出现，在这里就不深入考虑了。

了解了这些后ViewGroup对子View的测量就完成了，可以看到，子View宽高的测量值是由父ViewGroup和其自身的LayoutParams共同决定的。看完了ViewGroup对子View的分发测量再来看一下View的measure方法：

 public final void measure(int widthMeasureSpec, int heightMeasureSpec) {        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||                widthMeasureSpec != mOldWidthMeasureSpec ||                heightMeasureSpec != mOldHeightMeasureSpec) {            // first clears the measured dimension flag            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;            resolveRtlPropertiesIfNeeded();            // 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 & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {                throw new IllegalStateException("onMeasure() did not set the"                        + " measured dimension by calling"                        + " setMeasuredDimension()");            }            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;        }        mOldWidthMeasureSpec = widthMeasureSpec;        mOldHeightMeasureSpec = heightMeasureSpec;    }

在这里参数就是ViewGroup中创建的，因为这个方法是final的，我们并不能修改，所以我们只看一下能修改的onMeasure方法：

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

在方法中直接调用getDefaultSize生成了测量结果，然后进行赋值，跟进一下这个方法：

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;    }

由前面的分析我们知道，基本上不管子View中的LayoutParams设置的大小为什么，它的MeasureSpec都会是AT_MOST和EXACTLY中的一个，那么它最后的测量大小必定等于specSize。并且还由刚才的分析可以知道，当我们把子View指定为WRAP_CONTENT时它的默认大小就是覆盖了整个ViewGroup。大家可以简单的做个实验，在这里就不贴实验的代码了。

如果想要让WRAP_CONTENT起作用，我们需要在onMeasure中加入逻辑的判断以确定其大小。

在onMeasure方法结束后，调用了setMeasuredDimension方法为mMeasuredWidth和mMeasuredHeight赋值：

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {        mMeasuredWidth = measuredWidth;        mMeasuredHeight = measuredHeight;        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;    }

我们可以调用getMeasuredWidth对这个测量结果进行获取（但是必须在onMeasure调用结束后），getMeasuredHeight同理。

既然已经测量过了，现在就来看一下layout方法：

ViewGroup的layout方法也是简单的调用了父类View的layout方法，所以这里只看一下View的方法。

public void layout(int l, int t, int r, int b) {        int oldL = mLeft;        int oldT = mTop;        int oldB = mBottom;        int oldR = mRight;        boolean changed = 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;    }

在layout中首先判断了一下View的大小位置是否发生了改变，并且在setFram方法时将传入的参数赋给本地变量进行保存，而后调用onLayout方法去处理子View，因为onLayout方法和具体布局相关，所以在View和ViewGroup中都没有对其给出实现。

layout基本看完了，下面就是和绘画相关的draw。

public void draw(Canvas canvas) {        final int privateFlags = mPrivateFlags;        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;        /*         * Draw traversal performs several drawing steps which must be executed         * in the appropriate order:         *         *      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)         */        // Step 1, draw the background, if needed        int saveCount;        if (!dirtyOpaque) {            final Drawable background = mBackground;            if (background != null) {                final int scrollX = mScrollX;                final int scrollY = mScrollY;                if (mBackgroundSizeChanged) {                    background.setBounds(0, 0,  mRight - mLeft, mBottom - mTop);                    mBackgroundSizeChanged = false;                }                if ((scrollX | scrollY) == 0) {                    background.draw(canvas);                } else {                    canvas.translate(scrollX, scrollY);                    background.draw(canvas);                    canvas.translate(-scrollX, -scrollY);                }            }        }        // skip step 2 & 5 if possible (common case)        final int viewFlags = mViewFlags;        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;        if (!verticalEdges && !horizontalEdges) {            // Step 3, draw the content            if (!dirtyOpaque) onDraw(canvas);            // Step 4, draw the children            dispatchDraw(canvas);            // Step 6, draw decorations (scrollbars)            onDrawScrollBars(canvas);            // we're done...            return;        }}

注释写的也比较清楚，绘制背景->绘制自己->绘制子View->绘制ScrollBar，值得一提的是在View中dispatchDraw方法是空的，在ViewGroup中给出了实现。