实现添加悬浮窗的核心源码代码WindowManagerService#addWindow方法中有以下几个关键：

1.权限检查

mPolicy.checkAddPermission(attrs, appOp);

2.悬浮窗参数调整

mPolicy.adjustWindowParamsLw(win.mAttrs);

查看不同android版本的源码对比

结论：

android 4.4(api 19)之前使用TYPE_TOAST 可以显示，但是不能接收触摸事件和按键事件

mPolicy实际上是PhoneWindowManager。

对比checkAddPermission()方法变化结论：

android 4.3(api 18) 开始 TYPE_TOAST根本没有做权限检查, 直接break出去了, 最后返回WindowManagerGlobal.ADD_OKAY。这个就是使用TYPE_TOAST不需要权限显示悬浮窗的原因

//android 4.3_r2.1

public int checkAddPermission(WindowManager.LayoutParams attrs, int[] outAppOp) {

int type = attrs.type;

outAppOp[0] = AppOpsManager.OP_NONE;

if (type < WindowManager.LayoutParams.FIRST_SYSTEM_WINDOW

|| type > WindowManager.LayoutParams.LAST_SYSTEM_WINDOW) {

return WindowManagerGlobal.ADD_OKAY;

}

String permission = null;

switch (type) {

case TYPE_TOAST:

// XXX right now the app process has complete control over

// this... should introduce a token to let the system

// monitor/control what they are doing.

break;

case TYPE_DREAM:

case TYPE_INPUT_METHOD:

case TYPE_WALLPAPER:

// The window manager will check these.

break;

case TYPE_PHONE:

case TYPE_PRIORITY_PHONE:

case TYPE_SYSTEM_ALERT:

case TYPE_SYSTEM_ERROR:

case TYPE_SYSTEM_OVERLAY:

permission = android.Manifest.permission.SYSTEM_ALERT_WINDOW;

outAppOp[0] = AppOpsManager.OP_SYSTEM_ALERT_WINDOW;

break;

default:

permission = android.Manifest.permission.INTERNAL_SYSTEM_WINDOW;

}

if (permission != null) {

if (mContext.checkCallingOrSelfPermission(permission)

!= PackageManager.PERMISSION_GRANTED) {

return WindowManagerGlobal.ADD_PERMISSION_DENIED;

}

}

return WindowManagerGlobal.ADD_OKAY;

}

//android 4.4_r1

public int checkAddPermission(WindowManager.LayoutParams attrs, int[] outAppOp) {

int type = attrs.type;

outAppOp[0] = AppOpsManager.OP_NONE;

if (type < WindowManager.LayoutParams.FIRST_SYSTEM_WINDOW

|| type > WindowManager.LayoutParams.LAST_SYSTEM_WINDOW) {

return WindowManagerGlobal.ADD_OKAY;

}

String permission = null;

switch (type) {

case TYPE_TOAST:

// XXX right now the app process has complete control over

// this... should introduce a token to let the system

// monitor/control what they are doing.

break;

case TYPE_DREAM:

case TYPE_INPUT_METHOD:

case TYPE_WALLPAPER:

case TYPE_PRIVATE_PRESENTATION:

// The window manager will check these.

break;

case TYPE_PHONE:

case TYPE_PRIORITY_PHONE:

case TYPE_SYSTEM_ALERT:

case TYPE_SYSTEM_ERROR:

case TYPE_SYSTEM_OVERLAY:

permission = android.Manifest.permission.SYSTEM_ALERT_WINDOW;

outAppOp[0] = AppOpsManager.OP_SYSTEM_ALERT_WINDOW;

break;

default:

permission = android.Manifest.permission.INTERNAL_SYSTEM_WINDOW;

}

if (permission != null) {

if (mContext.checkCallingOrSelfPermission(permission)

!= PackageManager.PERMISSION_GRANTED) {

return WindowManagerGlobal.ADD_PERMISSION_DENIED;

}

}

return WindowManagerGlobal.ADD_OKAY;

}

对比adjustWindowParamsLw()方法变化结论：

android 4.4(api 19) 之前的TYPE_TOAST不能接收触摸事件和按键事件

//android 4.3_r2.1

public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {

switch (attrs.type) {

case TYPE_SYSTEM_OVERLAY:

case TYPE_SECURE_SYSTEM_OVERLAY:

case TYPE_TOAST:

// These types of windows can't receive input events.

attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE

| WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;

attrs.flags &= ~WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH;

break;

}

}

//android 4.4_r1

public void adjustWindowParamsLw(WindowManager.LayoutParams attrs) {

switch (attrs.type) {

case TYPE_SYSTEM_OVERLAY:

case TYPE_SECURE_SYSTEM_OVERLAY:

// These types of windows can't receive input events.

attrs.flags |= WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE

| WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;

attrs.flags &= ~WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH;

break;

}

}