Activity

常见容器视图示例：

类名称	description
LinearLayout	在一行或一列里显示视图。
RelativeLayout	相对某个视图放置其他视图。
FrameLayout	ViewGroup 包含一个子视图。
ScrollView	一种 FrameLayout，旨在让用户能够在视图中滚动查看内容。
ConstraintLayout	这是更新的 viewgroup；可以灵活地放置视图。在这节课的稍后阶段，我们将学习 ConstraintLayout。

Activity 会创建视图来向用户显示信息，并使用户与 Activity 互动。视图是 Android UI 框架中的类。它们占据了屏幕上的方形区域，负责绘制并处理事件。Activity 通过读取 XML 布局文件确定要创建哪些视图（并放在何处）。这些 XML 文件存储在标记为 layouts 的 res 文件夹内。

• onCreate的super方法放在第一句, onPause以后的super的方法放在最后一句.

onCreate
onStart
onResume

onPaush
onStop
onDestroy

package com.example.android.lifecycle;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.TextView;

//udacity官方实例activity生命周期代碼
public class MainActivity extends AppCompatActivity {

    /*
     * This tag will be used for logging. It is best practice to use the class's name using
     * getSimpleName as that will greatly help to identify the location from which your logs are
     * being posted.
     */
    private static final String TAG = MainActivity.class.getSimpleName();

    /* Constant values for the names of each respective lifecycle callback */
    private static final String ON_CREATE = "onCreate";
    private static final String ON_START = "onStart";
    private static final String ON_RESUME = "onResume";
    private static final String ON_PAUSE = "onPause";
    private static final String ON_STOP = "onStop";
    private static final String ON_RESTART = "onRestart";
    private static final String ON_DESTROY = "onDestroy";
    private static final String ON_SAVE_INSTANCE_STATE = "onSaveInstanceState";

    /*
     * This TextView will contain a running log of every lifecycle callback method called from this
     * Activity. This TextView can be reset to its default state by clicking the Button labeled
     * "Reset Log"
     */
    private TextView mLifecycleDisplay;

    /**
     * Called when the activity is first created. This is where you should do all of your normal
     * static set up: create views, bind data to lists, etc.
     *
     * Always followed by onStart().
     *
     * @param savedInstanceState The Activity's previously frozen state, if there was one.
     */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mLifecycleDisplay = (TextView) findViewById(R.id.tv_lifecycle_events_display);

        // COMPLETED (1) Use logAndAppend within onCreate
        logAndAppend(ON_CREATE);
    }

    // COMPLETED (2) Override onStart, call super.onStart, and call logAndAppend with ON_START
    /**
     * Called when the activity is becoming visible to the user.
     *
     * Followed by onResume() if the activity comes to the foreground, or onStop() if it becomes
     * hidden.
     */
    @Override
    protected void onStart() {
        super.onStart();

        logAndAppend(ON_START);
    }

    // COMPLETED (3) Override onResume, call super.onResume, and call logAndAppend with ON_RESUME
    /**
     * Called when the activity will start interacting with the user. At this point your activity
     * is at the top of the activity stack, with user input going to it.
     *
     * Always followed by onPause().
     */
    @Override
    protected void onResume() {
        super.onResume();

        logAndAppend(ON_RESUME);
    }

    // COMPLETED (4) Override onPause, call super.onPause, and call logAndAppend with ON_PAUSE
    /**
     * Called when the system is about to start resuming a previous activity. This is typically
     * used to commit unsaved changes to persistent data, stop animations and other things that may
     * be consuming CPU, etc. Implementations of this method must be very quick because the next
     * activity will not be resumed until this method returns.
     *
     * Followed by either onResume() if the activity returns back to the front, or onStop() if it
     * becomes invisible to the user.
     */
    @Override
    protected void onPause() {
        super.onPause();

        logAndAppend(ON_PAUSE);
    }

    // COMPLETED (5) Override onStop, call super.onStop, and call logAndAppend with ON_STOP
    /**
     * Called when the activity is no longer visible to the user, because another activity has been
     * resumed and is covering this one. This may happen either because a new activity is being
     * started, an existing one is being brought in front of this one, or this one is being
     * destroyed.
     *
     * Followed by either onRestart() if this activity is coming back to interact with the user, or
     * onDestroy() if this activity is going away.
     */
    @Override
    protected void onStop() {
        super.onStop();

        logAndAppend(ON_STOP);
    }

    // COMPLETED (6) Override onRestart, call super.onRestart, and call logAndAppend with ON_RESTART
    /**
     * Called after your activity has been stopped, prior to it being started again.
     *
     * Always followed by onStart()
     */
    @Override
    protected void onRestart() {
        super.onRestart();

        logAndAppend(ON_RESTART);
    }

    // COMPLETED (7) Override onDestroy, call super.onDestroy, and call logAndAppend with ON_DESTROY
    /**
     * The final call you receive before your activity is destroyed. This can happen either because
     * the activity is finishing (someone called finish() on it, or because the system is
     * temporarily destroying this instance of the activity to save space. You can distinguish
     * between these two scenarios with the isFinishing() method.
     */
    @Override
    protected void onDestroy() {
        super.onDestroy();

        logAndAppend(ON_DESTROY);
    }

    /**
     * Logs to the console and appends the lifecycle method name to the TextView so that you can
     * view the series of method callbacks that are called both from the app and from within
     * Android Studio's Logcat.
     *
     * @param lifecycleEvent The name of the event to be logged.
     */
    private void logAndAppend(String lifecycleEvent) {
        Log.d(TAG, "Lifecycle Event: " + lifecycleEvent);

        mLifecycleDisplay.append(lifecycleEvent + "\n");
    }

    /**
     * This method resets the contents of the TextView to its default text of "Lifecycle callbacks"
     *
     * @param view The View that was clicked. In this case, it is the Button from our layout.
     */
    public void resetLifecycleDisplay(View view) {
        mLifecycleDisplay.setText("Lifecycle callbacks:\n");
    }
}

Intent匹配

Data本身又分为两种方式进行匹配：MIMEType和URI。

MIMEType就是指要访问的组件处理的数据类型，例如video/mpeg4、video/mp4、video/avi等。
MIMEType也可以用通配符（）匹配某一类型的数据，例如“audio/”表示所有的音频数据格式.

URI有些类似我们经常使用的Web地址，但要比Web地址范围更广，例如，下面的3行字符串都属于URI。
http: //www.google.com
content: //mobile.android.data/cities
ftp: //192.168.17.168
总结
定位窗口：

• 通过Componentname、Action、Category和Data可以定位一个或多个窗口。
• 传递数据：通过Data和Extra。
• 控制访问组件的行为（窗口、服务和广播）：通过Flags。

注意显示调用过程中action为null, 这可作为判断是否显式调用/隐式调用的条件.

从常理推测＜category＞标签应该是可选的，不过实际上＜category＞也是必须加的，原因就是一个特殊的Category：android.intent.category.DEFAULT。如果调用者不添加Category，按常理会认为过滤条件中不包含Category，系统自然也就不考虑Category了。不过Android系统并不是这样认为的。不管调用者是否添加Category，系统都会认为有一个默认的Category已经被添加。相当于调用者执行如下的代码。intent.addCategory(Intent.CATEGORY_DEFAULT);
既然调用者默认加入了一个Category，那么被调用这自然也需要在过滤器（＜intent-filter＞标签）中加入如下的＜category＞标签了。

常用Intent.FLAG

如果同时设置了launchMode和标志动作,则标志动作优先于相应的launchMode属性值.

• singgletop: FLAG_ ACTIVITY_ SINGLE_ TOP
• singletask FLAG_ACTIVITY_SINGLE_TOP + FLAG_ ACTIVITY_ CLEAR_ TOP

FLAG_ ACTIVITY_ NEW_ TASK 相当于???

在onCreate方法中及时执行了finish方法, 但还是走完oncreate后才ondestroy的.

当收到back键消息时，调用moveTaskToBack，这样的效果和按Home一样，程序并没有退出，而是在后台。

@Override 
public boolean onKeyDown(int keyCode, KeyEvent event) { 
    if (keyCode == KeyEvent.KEYCODE_BACK) { 
        moveTaskToBack(true); 
        return true; 
    } 
    return super.onKeyDown(keyCode, event); 
}

看来flag只是在原有的基础上影响, 不会一票否决

例如是singletop, 代码常规的改法貌似没用, 如果是非常规, 没试过

• singletask 和 singleInstance如果设置为主activity, 在home键后再图标进入后会执行onNewIntent方法, 会清理该activity上所有的activity.
  而singleTop 和 standard却不行, 且我目前发现只是主activity会发生.
  试试???