WeChat program operating mechanism

The mechanism by which the program runs

Front/background status

After the small program starts, the interface is presented to the user, at which point the small program is in the foreline state.

When the user clicks the upper right-hand capsule button to close the small program, or press the device Home key to leave WeChat, the small program does not completely terminate the operation, but into the background state, the small program can also run for a short period of time.

When the user enters WeChat again or opens the small program again, the small program enters the fore desk from the background. However, if the user has not re-entered the small program for a long time, or the system resources are tight, the small program may be destroyed, that is, completely out of operation.

The small program starts

In this way, small program startup can be divided into two situations, one is cold start, the other is hot start.

• Cold Start: If the user opens it for the first time, or if the program is destroyed and then opened by the user again, the small program needs to reload and start, i.e. cold boot.
• Hot start: If the user has opened a small program, and then open the small program again within a certain period of time, at this time the small program has not been destroyed, but from the background state into the foreline state, this process is hot start.

The timing of the destruction of the small program

Typically, a small program is destroyed only if it enters the background for a certain amount of time, or if the system resources are too high. Specifically, there are several scenarios:

• When a small program enters the background, it can be maintained for a short period of time, and if it does not enter the fore desk during that time, the small program is destroyed.
• When the small program occupies the system resources, it may be destroyed by the system or take the initiative to be actively recycled by the WeChat client.On iOS, when the WeChat client continuously receives the system memory alarm in a certain time interval, it will take the initiative to destroy the applet, and prompt the user "to run memory, please reopen the applet."The specific policy will continue to adjust optimization.It is recommended that the applet is necessary when necessary wx.onMemoryWarning Listen to the internal departure warning events, make the necessary memory cleanup.

Basic library 1.1.0 and above, 1.4.0 below: When the user enters the applet from the entrance to sweep, forwarding and other entrances (the scene value is 1007, 1011, 1025), and if there is no top apparent program,The applet will be destroyed.

Start scenario classification

When the user opens a small program, the scene can be divided into two categories: B.

A. Keep the last browsing status. Scene Have the following:

• If you enter a scenario with path, you go to the corresponding path page each time you open a small program
• If you enter a scenario without path: If the small program is hot start, then leave the original state If the small program is cold start, follow the next section of the restart strategy, may be the home page or the last exit page

B. Relaunch to the specified page or home page

Includes scenarios other than Class A

• If you enter a scene with a path, go to the corresponding path page each time you click
• If you enter a scene without path, the home page opens each time you enter

The restart policy for the Class A scenario

Base library 2.8.0 starts to support, and low versions need to be compatible.

After the program is destroyed, the next cold boot, if it belongs to a Class B scenario, will go to a specific page.

The next cold boot, if it belongs to a Class A scenario, will go to the home page of the small program by default. In the json file for the page (or in the window segment of app.json globally), specifying the restartStrategy configuration item can change the default behavior so that the next cold start of a Class A scenario can be returned to that page after exiting from a page.

Code example:

{
  "restartStrategy": "homePage"
}

RestartStrategy optional value:

Note: Even if it is not configured as homePage, if the small program exits too long (the current default time of day, you can use the exit state to adjust), the next cold start will no longer follow the configuration of restartStrategy, but will start directly from the home page cold.

In any case, the state on the page is not preserved, such as the text content in the input box, checkbox check box check state, etc. will not be restored. If you need to restore or partially restore, you need to take advantage of the exit state.

Exit state

The page callback function onSaveExitState is called whenever a small program may be destroyed. If you want to preserve the state on the page, you can "save" some data in this callback function, which you can get the next time you start it through exitState.

Code example:

{
  "restartStrategy": "homePageAndLatestPage"
}

Page({
  onLoad: function() {
    var prevExitState = this.exitState // 尝试获得上一次退出前 onSaveExitState 保存的数据
    if (prevExitState !== undefined) { // 如果是根据 restartStrategy 配置进行的冷启动，就可以获取到
      prevExitState.myDataField === 'myData' 
    }
  },
  onSaveExitState: function() {
    var exitState = { myDataField: 'myData' } // 需要保存的数据
    return {
      data: exitState,
      expireTimeStamp: Date.now() + 24 * 60 * 60 * 1000 // 超时时刻
    }
  }
})

The onSaveExitState return value can contain two items:

Note:

• If you exceed expireTimeStamp, the saved data is discarded and does not follow the configuration of restartStrategy at cold startup, but starts directly from the home page.
• ExpireTimeStamp may be automatically advanced, such as when WeChat clients need to clean up data.
• During the survival of the small program, onSaveExitState may be called multiple times, with the result of the last call as the final result.
• In some special cases, such as when a WeChat client is killed directly by the system, this method will not be called, and the next cold boot will not follow the configuration of restartStrategy, but will start directly from the home page cold.