Android自定义View

前言

Android自定义View的详细步骤是我们每一个Android开发人员都必须掌握的技能，因为在开发中总会遇到自定义View的需求。为了提高自己的技术水平，自己就系统的去研究了一下，在这里写下一点心得，有不足之处希望大家及时指出。

流程

在Android中对于布局的请求绘制是在Android framework层开始处理的。绘制是从根节点开始，对布局树进行measure与draw。在RootViewImpl中的performTraversals展开。它所做的就是对需要的视图进行measure(测量视图大小)、layout(确定视图的位置)与draw(绘制视图)。下面的图能很好的展现视图的绘制流程：  

当用户调用requestLayout时，只会触发measure与layout，但系统开始调用时还会触发draw

下面来详细介绍这几个流程。

measure

measure是View中的final型方法不可以进行重写。它是对视图的大小进行测量计算，但它会回调onMeasure方法，所以我们在自定义View的时候可以重写onMeasure方法来对View进行我们所需要的测量。它有两个参数widthMeasureSpec与heightMeasureSpec。其实这两个参数都包含两部分，分别为size与mode。size为测量的大小而mode为视图布局的模式

我们可以通过以下代码分别获取：

int widthSize = MeasureSpec.getSize(widthMeasureSpec); 
int heightSize = MeasureSpec.getSize(heightMeasureSpec); 
int widthMode = MeasureSpec.getMode(widthMeasureSpec); 
int heightMode = MeasureSpec.getMode(heightMeasureSpec);

获取到的mode种类分为以下三种：

setMeasuredDimension

通过以上逻辑获取视图的宽高，最后要调用setMeasuredDimension方法将测量好的宽高进行传递出去。其实最终是调用setMeasuredDimensionRaw方法对传过来的值进行属性赋值。调用super.onMeasure()的调用逻辑也是一样的。

下面以自定义一个验证码的View为例，它的onMeasure方法如下：

@Override 
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { 
        int widthSize = MeasureSpec.getSize(widthMeasureSpec); 
        int heightSize = MeasureSpec.getSize(heightMeasureSpec); 
        int widthMode = MeasureSpec.getMode(widthMeasureSpec); 
        int heightMode = MeasureSpec.getMode(heightMeasureSpec); 
        if (widthMode == MeasureSpec.EXACTLY) { 
            //直接获取精确的宽度 
            width = widthSize; 
        } else if (widthMode == MeasureSpec.AT_MOST) { 
            //计算出宽度(文本的宽度+padding的大小) 
            width = bounds.width() + getPaddingLeft() + getPaddingRight(); 
        } 
        if (heightMode == MeasureSpec.EXACTLY) { 
            //直接获取精确的高度 
            height = heightSize; 
        } else if (heightMode == MeasureSpec.AT_MOST) { 
            //计算出高度(文本的高度+padding的大小) 
            height = bounds.height() + getPaddingBottom() + getPaddingTop(); 
        } 
        //设置获取的宽高 
        setMeasuredDimension(width, height); 
    }

可以对自定义View的layout_width与layout_height进行设置不同的属性，达到不同的mode类型，就可以看到不同的效果

measureChildren

如果你是对继承ViewGroup的自定义View那么在进行测量自身的大小时还要测量子视图的大小。一般通过measureChildren(int widthMeasureSpec, int heightMeasureSpec)方法来测量子视图的大小。

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); 
            } 
        } 
    }

通过上面的源码会发现，它其实是遍历每一个子视图，如果该子视图不是隐藏的就调用measureChild方法，那么来看下measureChild源码：

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); 
    }

会发现它首先调用了getChildMeasureSpec方法来分别获取宽高，最后再调用的就是View的measure方法，而通过前面的分析我们已经知道它做的就是对视图大小的计算。而对于measure中的参数是通过getChildMeasureSpec获取，再来看下其源码：

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 = 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); 
    }

是不是容易理解了点呢。它做的就是前面所说的根据mode的类型，获取相应的size。根据父视图的mode类型与子视图的LayoutParams类型来决定子视图所属的mode,最后再将获取的size与mode通过MeasureSpec.makeMeasureSpec方法整合返回。最后传递到measure中，这就是前面所说的widthMeasureSpec与heightMeasureSpec中包含的两部分的值。整个过程为measureChildren->measureChild->getChildMeasureSpec->measure->onMeasure->setMeasuredDimension,所以通过measureChildren就可以对子视图进行测量计算。

layout

layout也是一样的内部会回调onLayout方法，该方法是用来确定子视图的绘制位置，但这个方法在ViewGroup中是个抽象方法，所以如果要自定义的View是继承ViewGroup的话就必须实现该方法。但如果是继承View的话就不需要了，View中有一个空实现。而对子视图位置的设置是通过View的layout方法通过传递计算出来的left、top、right与bottom值，而这些值一般都要借助View的宽高来计算,视图的宽高则可以通过getMeasureWidth与getMeasureHeight方法获取，这两个方法获取的值就是上面onMeasure中setMeasuredDimension传递的值，即子视图测量的宽高。

getWidth、getHeight与getMeasureWidth、getMeasureHeight是不同的，前者是在onLayout之后才能获取到的值，分别为left-right与top-bottom;而后者是在onMeasure之后才能获取到的值。只不过这两种获取的值一般都是相同的，所以要注意调用的时机。

下面以定义一个把子视图放置于父视图的四个角的View为例：

@Override 
    protected void onLayout(boolean changed, int l, int t, int r, int b) { 
        int count = getChildCount(); 
        MarginLayoutParams params; 
         
        int cl; 
        int ct; 
        int cr; 
        int cb; 
             
        for (int i = 0; i < count; i++) { 
            View child = getChildAt(i); 
            params = (MarginLayoutParams) child.getLayoutParams(); 
                 
            if (i == 0) { 
                //左上角 
                cl = params.leftMargin; 
                ct = params.topMargin; 
            } else if (i == 1) { 
                //右上角 
                cl = getMeasuredWidth() - params.rightMargin - child.getMeasuredWidth(); 
                ct = params.topMargin; 
            } else if (i == 2) { 
                //左下角 
                cl = params.leftMargin; 
                ct = getMeasuredHeight() - params.bottomMargin - child.getMeasuredHeight() 
                 - params.topMargin; 
            } else { 
                //右下角 
                cl = getMeasuredWidth() - params.rightMargin - child.getMeasuredWidth(); 
                ct = getMeasuredHeight() - params.bottomMargin - child.getMeasuredHeight() 
                 - params.topMargin; 
            } 
            cr = cl + child.getMeasuredWidth(); 
            cb = ct + child.getMeasuredHeight(); 
            //确定子视图在父视图中放置的位置 
            child.layout(cl, ct, cr, cb); 
        } 
    }

至于onMeasure的实现源码我后面会给链接，如果要看效果图的话，我后面也会贴出来，前面的那个验证码的也是一样

draw

draw是由dispatchDraw发动的，dispatchDraw是ViewGroup中的方法，在View是空实现。自定义View时不需要去管理该方法。而draw方法只在View中存在，ViewGoup做的只是在dispatchDraw中调用drawChild方法，而drawChild中调用的就是View的draw方法。那么我们来看下draw的源码：

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) { 
            drawBackground(canvas); 
        } 
          
        // 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); 
              
            // Overlay is part of the content and draws beneath Foreground 
            if (mOverlay != null && !mOverlay.isEmpty()) { 
                            mOverlay.getOverlayView().dispatchDraw(canvas); 
            } 
                          
            // Step 6, draw decorations (foreground, scrollbars) 
            onDrawForeground(canvas); 
                        
            // we're done... 
            return; 
        } 
        //省略2&5的情况 
        .... 
}

源码已经非常清晰了draw总共分为6步;

绘制背景

如果需要的话，保存layers

绘制自身文本

绘制子视图

如果需要的话，绘制fading edges

绘制scrollbars

其中 第2步与第5步不是必须的。在第3步调用了onDraw方法来绘制自身的内容，在View中是空实现，这就是我们为什么在自定义View时必须要重写该方法。而第4步调用了dispatchDraw对子视图进行绘制。还是以验证码为例：

@Override 
    protected void onDraw(Canvas canvas) { 
        //绘制背景 
        mPaint.setColor(getResources().getColor(R.color.autoCodeBg)); 
        canvas.drawRect(0, 0, getMeasuredWidth(), getMeasuredHeight(), mPaint); 
 
        mPaint.getTextBounds(autoText, 0, autoText.length(), bounds); 
        //绘制文本 
        for (int i = 0; i < autoText.length(); i++) { 
             mPaint.setColor(getResources().getColor(colorRes[random.nextInt(6)])); 
            canvas.drawText(autoText, i, i + 1, getWidth() / 2 - bounds.width() / 2 + i * bounds.width() / autoNum 
                    , bounds.height() + random.nextInt(getHeight() - bounds.height()) 
                    , mPaint); 
        } 
  
        //绘制干扰点 
        for (int j = 0; j < 250; j++) { 
             canvas.drawPoint(random.nextInt(getWidth()), random.nextInt(getHeight()), pointPaint); 
        } 
  
        //绘制干扰线 
        for (int k = 0; k < 20; k++) { 
            int startX = random.nextInt(getWidth()); 
            int startY = random.nextInt(getHeight()); 
            int stopX = startX + random.nextInt(getWidth() - startX); 
            int stopY = startY + random.nextInt(getHeight() - startY); 
             linePaint.setColor(getResources().getColor(colorRes[random.nextInt(6)])); 
            canvas.drawLine(startX, startY, stopX, stopY, linePaint); 
        } 
    }

图，与源码链接

示例图