1.背景

学习自定义控件，了解Android各种坐标系及一些API的坐标含义绝对算一个小而不可忽视的技能；所谓Android自定义View那几大主要onXXX()方法的重写实质大多数都是在处理坐标逻辑运算。所以，我们一起学习下Android坐标系。

2.Android坐标系

2.1Android屏幕区域划分

device-2016-12-09-142518.png

通过上图我们可以很直观的看到Android对于屏幕的划分定义。下面我们就给出这些区域里常用区域的一些坐标或者度量方式。如下：

//获取手机屏幕区域高度public int getWindowArea() {    DisplayMetrics metrics = new DisplayMetrics();    getWindowManager().getDefaultDisplay().getMetrics(metrics);    int widthPixels = metrics.widthPixels;    int heightPixels = metrics.heightPixels;    return heightPixels;}//获取应用区域高度public int getApplicationArea() {    Rect rect = new Rect();    getWindow().getDecorView().            getWindowVisibleDisplayFrame(rect);    int width = rect.width();    int height = rect.height();    return height;}//获取状态栏高度public int getstatusBarArea() {    Rect rect = new Rect();    getWindow().getDecorView().getWindowVisibleDisplayFrame(rect);    int statusBarHeight = rect.top;    return statusBarHeight;}//获取view绘制区域高度public int getDrawArea() {    Rect rect = new Rect();    getWindow().findViewById(Window.ID_ANDROID_CONTENT)            .getDrawingRect(rect);    int width = rect.width();    int height = rect.height();    return height;}//获取标题栏高度public int gettitleBarArea() {    return getApplicationArea()-getDrawArea();}@Overridepublic void onWindowFocusChanged(boolean hasFocus) {    super.onWindowFocusChanged(hasFocus);    android.util.Log.i("TAG","onWindowFocusChanged");    if (hasFocus) {        //获取view绘制区域（即不包括标titlebar的内容区域）距离手机屏幕顶部的距离        android.util.Log.i("TAG","屏幕区域高度:"+getWindowArea());        android.util.Log.i("TAG","应用区域高度:"+getApplicationArea());        android.util.Log.i("TAG","view绘制区域高度:"+getDrawArea());        android.util.Log.i("TAG","状态栏高度:"+getstatusBarArea());        android.util.Log.i("TAG","标题栏高度:"+gettitleBarArea());    }}

...
onWindowFocusChanged()：当Activity的当前Window获得或失去焦点时会被回调此方法。当回调了这个方法时表示Activity是完全对用户可见的(只是可见，还一片黑呼呼的，有待draw..)。当对话框弹起/消失及Activity新创建及回退等都会调用此方法。此时能获取屏幕区域的准确信息。

2.2Android View绝对相对坐标系

2.2.1View的坐标

view的位置坐标和父容器的关系.jpg（图片来源于网络）
自定义View学习笔记（二）-Android坐标系简介_第3张图片

view.getX和view.getTranslationX区别.png（图片来源于网络）

通过上图我们可以很直观的给出View一些坐标相关的方法解释，不过必须要明确的是上面这些方法必须要在layout之后才有效，如下：

2.2.2MotionEvent坐标

MotionEvent坐标.jpg（图片来源于网络）

手指触摸屏幕时MotionEvent提供的一些方法解释，如下：

2.3Android View滑动相关坐标系

2.3.1为什么scrollTo和scrollBy滚动的是View的内容而非View本身？

/** * Set the scrolled position of your view. This will cause a call to * {@link #onScrollChanged(int, int, int, int)} and the view will be * invalidated. * @param x the x position to scroll to * @param y the y position to scroll to */public void scrollTo(int x, int y) {    if (mScrollX != x || mScrollY != y) {        int oldX = mScrollX;        int oldY = mScrollY;        mScrollX = x;        mScrollY = y;        invalidateParentCaches();        onScrollChanged(mScrollX, mScrollY, oldX, oldY);        if (!awakenScrollBars()) {            postInvalidateOnAnimation();        }    }}/** * Move the scrolled position of your view. This will cause a call to * {@link #onScrollChanged(int, int, int, int)} and the view will be * invalidated. * @param x the amount of pixels to scroll by horizontally * @param y the amount of pixels to scroll by vertically */public void scrollBy(int x, int y) {    scrollTo(mScrollX + x, mScrollY + y);}

因为View在ViewGroup中的位置是由LayoutParams的margin等参数决定的，要想滚动View或者说要想改变View的位置只需要改变LayoutParams的相关参数就可以。但是scrollTo和scrollBy改变的只是mScrollX和mScrollY的值，这两个值对于改变View在ViewGroup里面的位置是毫无关系的；这就排除了scrollTo或者scrollBy滚动的是View本身了。
为什么说View滚动的是内容？

/** * Draws the background onto the specified canvas. * @param canvas Canvas on which to draw the background */private void drawBackground(Canvas canvas) {    final Drawable background = mBackground;    if (background == null) {        return;    }    setBackgroundBounds();    // Attempt to use a display list if requested.    if (canvas.isHardwareAccelerated() && mAttachInfo != null            && mAttachInfo.mHardwareRenderer != null) {        mBackgroundRenderNode = getDrawableRenderNode(background, mBackgroundRenderNode);        final RenderNode renderNode = mBackgroundRenderNode;        if (renderNode != null && renderNode.isValid()) {            setBackgroundRenderNodeProperties(renderNode);            ((DisplayListCanvas) canvas).drawRenderNode(renderNode);            return;        }    }    final int scrollX = mScrollX;    final int scrollY = mScrollY;    if ((scrollX | scrollY) == 0) {        background.draw(canvas);    } else {        canvas.translate(scrollX, scrollY);        background.draw(canvas);        canvas.translate(-scrollX, -scrollY);    }}

上面的代码可以看出mScrollX和mScrollY这两个变量正式交给显示View内容的Canvas来操作的！所以我们说scrollTo或者scrollBy滚动的是View的内容，而不是改变View在parentView显示的位置关系！

2.3.2 mScrollX和mScrollY的变换规律

  /** * The offset, in pixels, by which the content of this view is scrolled * horizontally. * {@hide} */@ViewDebug.ExportedProperty(category = "scrolling")protected int mScrollX;/** * The offset, in pixels, by which the content of this view is scrolled * vertically. * {@hide} */@ViewDebug.ExportedProperty(category = "scrolling")protected int mScrollY;  /** * Return the scrolled left position of this view. This is the left edge of * the displayed part of your view. You do not need to draw any pixels * farther left, since those are outside of the frame of your view on * screen. * * @return The left edge of the displayed part of your view, in pixels. */public final int getScrollX() {    return mScrollX;}/** * Return the scrolled top position of this view. This is the top edge of * the displayed part of your view. You do not need to draw any pixels above * it, since those are outside of the frame of your view on screen. * * @return The top edge of the displayed part of your view, in pixels. */public final int getScrollY() {    return mScrollY;}

mScrollX和mScrollY的变换规律.png（图片来源于网络）

自定义View学习笔记（二）-Android坐标系简介