UiDevice提供对设备状态信息的访问。 也可以使用此类来模拟设备上的用户操作，例如按键盘或按Home和Menu按钮。UiDevice类的完整源码 UiDevice.java

废话不多说，我们首先根据用法来来看看Android Uiautomator 访问设备的原理。

device = UiDevice.getInstance(getInstrumentation());
device.pressHome();

// Bring up the default launcher by searching for a UI component  
// that matches the content description for the launcher button.  

UiObject allAppsButton = device
.findObject(new UiSelector().description("Apps"));

// Perform a click on the button to load the launcher.  

allAppsButton.clickAndWaitForNewWindow();

根据这个用法实例，我们先看一下它的构造函数：

/** Private constructor. Clients should use {@link UiDevice#getInstance(Instrumentation)}. */
private UiDevice(Instrumentation instrumentation) {
mInstrumentation = instrumentation;
UiAutomation uiAutomation = instrumentation.getUiAutomation();
mUiAutomationBridge = new InstrumentationUiAutomatorBridge(
instrumentation.getContext(), uiAutomation);
// Enable multi-window support for API level 21 and up
if (UiDevice.API_LEVEL_ACTUAL >= Build.VERSION_CODES.LOLLIPOP) {
// Subscribe to window information
AccessibilityServiceInfo info = uiAutomation.getServiceInfo();
info.flags |= AccessibilityServiceInfo.FLAG_RETRIEVE_INTERACTIVE_WINDOWS;
uiAutomation.setServiceInfo(info);
}
}
/**
* Retrieves a singleton instance of UiDevice
*
* @return UiDevice instance
*/
public static UiDevice getInstance(Instrumentation instrumentation) {
if (sInstance == null) {
sInstance = new UiDevice(instrumentation);
}
return sInstance;
}

这是一个单例模式的类，负责创建自己的对象，以确保UiDevcie对象的唯一性。在构造方法中除了初始化唯一的uiDevice对象,还初始化了UiAutomoatorBridge对象。UiAutomatorBridge这个类非常的重要，获取设备界面信息、注入事件以及截图都需要经过UiAutomatorBridge。稍后会做详细的介绍。

接下来我们来看，是如何获取设备信息以及模拟用户操作的。

/**
* Returns the display size in dp (device-independent pixel)
*
* The returned display size is adjusted per screen rotation. Also this will return the actual
* size of the screen, rather than adjusted per system decorations (like status bar).
*
* @return a Point containing the display size in dp
*/
public Point getDisplaySizeDp() {
Tracer.trace();
Display display = getAutomatorBridge().getDefaultDisplay();
Point p = new Point();
display.getRealSize(p);
DisplayMetrics metrics = new DisplayMetrics();
display.getRealMetrics(metrics);
float dpx = p.x / metrics.density;
float dpy = p.y / metrics.density;
p.x = Math.round(dpx);
p.y = Math.round(dpy);
return p;
}

getDisplaySizeDp()方法返回的是一个以dp为单位的坐标点。同样可以获取设备的宽度或者高度：

/**
* Gets the width of the display, in pixels. The width and height details
* are reported based on the current orientation of the display.
* @return width in pixels or zero on failure
*/
public int getDisplayWidth() {
Display display = getDefaultDisplay();
Point p = new Point();
display.getSize(p);
return p.x;
}

以上都是设备属性，接下来看模拟用户的操作。操作的第一步就是获取屏幕的焦点。获取到UiObject后就可以对屏幕上的这些控件进行模拟用户操作。

/**
* Returns a UiObject which represents a view that matches the specified selector criteria.
*
* @param selector
* @return UiObject object
*/
public UiObject findObject(UiSelector selector) {
return new UiObject(this, selector);
}

获取到屏幕上布局对象以后，操作无外乎就是点击、长按、滑动 以及键盘等操作。

/**
* Simulates a short press on the HOME button.
* @return true if successful, else return false
* @since API Level 16
*/
public boolean pressHome() {
Tracer.trace();
waitForIdle();
return getAutomatorBridge().getInteractionController().sendKeyAndWaitForEvent(
KeyEvent.KEYCODE_HOME, 0, AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED,
KEY_PRESS_EVENT_TIMEOUT);
}

/**
* Perform a click at arbitrary coordinates specified by the user
*
* @param x coordinate
* @param y coordinate
* @return true if the click succeeded else false
* @since API Level 16
*/
public boolean click(int x, int y) {
Tracer.trace(x, y);
if (x >= getDisplayWidth() || y >= getDisplayHeight()) {
return (false);
}
return getAutomatorBridge().getInteractionController().clickNoSync(x, y);
}
/**
* Performs a swipe from one coordinate to another using the number of steps
* to determine smoothness and speed. Each step execution is throttled to 5ms
* per step. So for a 100 steps, the swipe will take about 1/2 second to complete.
*
* @param startX
* @param startY
* @param endX
* @param endY
* @param steps is the number of move steps sent to the system
* @return false if the operation fails or the coordinates are invalid
* @since API Level 16
*/
public boolean swipe(int startX, int startY, int endX, int endY, int steps) {
Tracer.trace(startX, startY, endX, endY, steps);
return getAutomatorBridge().getInteractionController()
.swipe(startX, startY, endX, endY, steps);
}

不难看出，所有的操作都离不开 uiAutomatorBridge。在该类的方法getInteractionController()获取InteractionController 对象。InteractionController类将用户的键盘事件注入到android系统中，与系统进行交互。稍后会做详细的介绍。

除了这些操作，UiDevice还提供了其他的方法，如打开通知栏、点亮屏幕、执行adb命令、检查电源管理器，屏幕是否打开、等待屏幕更新、获取当前窗口的布局层次结构、截图等。