基于Android M及以上版本写一个系统级的Windows

由于Android M在安全机制上有了更进一步的加强，所以写一个系统级别的window就不仅仅只是在Android Manifests里面配置SYSTEM_ALERT_WINDOW权限这么简单，它需要用户手动去打开权限，由此代码会有些许改动。

package com.example.ubuntu.windowmanagerdemo;

import android.content.Context;
import android.content.Intent;
import android.graphics.PixelFormat;
import android.net.Uri;
import android.os.Build;
import android.provider.Settings;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Gravity;
import android.view.WindowManager;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

    WindowManager mWindowManager;
    Button mWindowButton;
    WindowManager.LayoutParams layoutParams;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
        mWindowButton = new Button(this);
        mWindowButton.setText("Window Button");
        layoutParams = new WindowManager.LayoutParams(
                WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.WRAP_CONTENT,
                0, 0, PixelFormat.TRANSPARENT);
        layoutParams.flags = WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL
                | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                | WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED;
        layoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ERROR;//表明是系统级别的window
        layoutParams.gravity = Gravity.LEFT | Gravity.TOP;
        layoutParams.x = 100;
        layoutParams.y = 300;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (!Settings.canDrawOverlays(getApplicationContext())) {
                //启动Activity让用户授权
                Intent intent = new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION);
                intent.setData(Uri.parse("package:" + getPackageName()));
                startActivityForResult(intent, 100);
            }else {
                //用户已经授权
                mWindowManager.addView(mWindowButton, layoutParams);
            }
        } else {
            //用户版本低于Android M
            mWindowManager.addView(mWindowButton, layoutParams);
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == 100) {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                if (Settings.canDrawOverlays(this)) {
                    mWindowManager.addView(mWindowButton, layoutParams);
                }
            }
        }
    }
}