前言
Android自訂View的詳細步驟是我們每個Android開發人員都必須掌握的技能，因為在開發中總是會遇到自訂View的需求。為了提升自己的技術水平，自己就係統的去研究了一下，在這裡寫下一點心得，有不足之處希望大家及時指出。
流程
在Android中對於佈局的請求繪製是在Android framework層開始處理的。繪製是從根節點開始，對佈局樹進行measure與draw。在RootViewImpl中的performTraversals展開。它所做的就是對需要的視圖進行measure(測量視圖大小)、layout(確定視圖的位置)與draw(繪製視圖)。下面的圖能很好的展現視圖的繪製流程：
當使用者呼叫requestLayout時，只會觸發measure與layout，但係統開始呼叫時還會觸發draw
下面來詳細介紹這幾個流程。
measure
measure是View中的final型方法不可以進行重寫。它是對視圖的大小進行測量計算，但它會回調onMeasure方法，所以我們在自訂View的時候可以重寫onMeasure方法來對View進行我們所需要的測量。它有兩個參數widthMeasureSpec與heightMeasureSpec。其實這兩個參數都包含兩個部分，分別為size與mode。 size為測量的大小而mode為視圖佈局的模式
我們可以透過以下程式碼分別取得：
int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); int widthMode = MeasureSpec.getMode(widthMeasureSpec); int heightMode = MeasureSpec.getMode(heightMeasureSpec);
所取得的mode種類分為以下三種：
setMeasuredDimension
透過以上邏輯視圖的邏輯視圖寬高，最後要呼叫setMeasuredDimension方法將測量好的寬高進行傳遞出去。其實最後是呼叫setMeasuredDimensionRaw方法對傳過來的值進行屬性賦值。呼叫super.onMeasure()的呼叫邏輯也是一樣的。
下面以自訂一個驗證碼的View為例，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);
}可以對自訂View的layout_width與layout_height進行設定不同的屬性，達到不同的mode類型，就可以看到不同的效果
measureChildren
如果你是對繼承ViewGroup的自訂View那麼在進行測量自身的大小時還要測量子視圖的大小。一般透過measureChildren(int widthMeasureSpec, int heightMeasureSpec)方法來測量子視圖的大小。
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);
}
}
}透過上面的原始碼會發現，它其實是遍歷每一個子視圖，如果該子視圖不是隱藏的就調用measureChild方法，那麼來看下measureChild源碼：
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);
}會發現它首先調用了getChildMeasureSpec方法來分別取得寬高，最後再呼叫的就是View的measure方法，而透過前面的分析我們已經知道它所做的就是對視圖大小的計算。而對於measure中的參數是透過getChildMeasureSpec獲取，再來看下其原始碼：
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);
}是不是容易理解了點呢。它做的就是前面所說的根據mode的類型，取得對應的size。根據父視圖的mode類型與子視圖的LayoutParams類型來決定子視圖所屬的mode,最後再將取得的size與mode透過MeasureSpec.makeMeasureSpec方法整合回傳。最後傳遞到measure中，這就是前面所說的widthMeasureSpec與heightMeasureSpec中包含的兩部分的值。整個過程為measureChildren->measureChild->getChildMeasureSpec->measure->onMeasure->setMeasuredDimension,所以透過measureChildren就可以對子視圖進行測量計算。
layout
layout也是一樣的內部會回調onLayout方法，該方法是用來確定子視圖的繪製位置，但這個方法在ViewGroup中是個抽象方法，所以如果要自定義的View是繼承ViewGroup的話就必須實作該方法。但如果是繼承View的話就不需要了，View裡有一個空實作。而子視圖位置的設定是透過View的layout方法透過傳遞計算出來的left、top、right與bottom值，而這些值一般都要藉助View的寬高來計算,視圖的寬高則可以透過getMeasureWidth與getMeasureHeight方法獲取，這兩個方法所獲得的值就是上面onMeasure中setMeasuredDimension傳遞的值，即子視圖測量的寬度。
getWidth、getHeight與getMeasureWidth、getMeasureHeight是不同的，前者是在onLayout之後才能得到的值，分別為left-right與top-bottom;而後者是在onMeasure之後才能獲得到的值。只不過這兩種取得的值一般都是相同的，所以要注意呼叫的時機。
下面以定義一個把子視圖放置於父視圖的四個角的View為例：
@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);
}
}至於onMeasure的實現源碼我後面會給鏈接，如果要看效果圖的話，我後面也會貼出來，前面的驗證碼的也是一樣
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
