如何從SWITCH語句中訪問主GUI線程？

Question

我需要從代理函數更新按鈕的BG顏色，但是當我嘗試從switch語句內部訪問主線程時，出現編譯器錯誤。

代碼如下

switch (peripheral.state) { 
     case CBManagerStatePoweredOn: 
      NSLog(@"Powered on"); 
      [peripheral startAdvertising:_beaconPeripheralData]; 
      dispatch_async(dispatch_get_main_queue(), ^{ 
       _adminPageExitBtn.backgroundColor = [UIColor greenColor]; 
      }); 
      break; 
     case CBManagerStatePoweredOff: 
      NSLog(@"Powered Off"); 
      [peripheral stopAdvertising]; 
      break; 
     case CBManagerStateUnsupported: 
      NSLog(@"Device not supported"); 
      break; 
     default: 
      break; 
    } 

該錯誤消息如下

/Users/mdouhan/Documents/Documents - Mattias’s MacBook/NWMobileTill/NWMobileTill/AdminView.m:161:9: Cannot jump from switch statement to this case label 
/Users/mdouhan/Documents/Documents - Mattias’s MacBook/NWMobileTill/NWMobileTill/AdminView.m:150:55: Jump enters lifetime of block which strongly captures a variable 

如何正確更新我的按鈕從委託方法的背景色？

Answer 1

您的代碼看起來正確。我的猜測是你的插座沒有連接。

嘗試添加的NSLog語句裏面調用：

switch (peripheral.state) { 
    case CBManagerStatePoweredOn: 
    NSLog(@"Powered on"); 
    [peripheral startAdvertising:_beaconPeripheralData]; 
    dispatch_async(dispatch_get_main_queue(), ^{ 
     //------------------------------------------------- 
     //Add the line below 
     NSLog("_adminPageExitBtn = %@", _adminPageExitBtn); 
     //------------------------------------------------- 
     _adminPageExitBtn.backgroundColor = [UIColor greenColor]; 
    }); 
    break; 

...

如果顯示爲零，則需要出口連接在Interface Builder。

Answer 2

根據C規範的定義，case語句的範圍很奇怪。

或更具體地說，case語句不定義範圍。範圍由switch（）{...}自身定義，並且這些案例實際上只是標準標籤，其中交換機將goto樣式跳轉到正確的標籤。塊聲明的工作方式會影響交換機的跳轉行爲。

最簡潔的解決方法是簡單地確保所有情況下具有獨特的範圍：

switch (peripheral.state) { 
     case CBManagerStatePoweredOn: 
     { 
      NSLog(@"Powered on"); 
      [peripheral startAdvertising:_beaconPeripheralData]; 
      dispatch_async(dispatch_get_main_queue(), ^{ 
       _adminPageExitBtn.backgroundColor = [UIColor greenColor]; 
      }); 
     } 
      break; 
     case CBManagerStatePoweredOff: 
     { 
      NSLog(@"Powered Off"); 
      [peripheral stopAdvertising]; 
     } 
      break; 
     case CBManagerStateUnsupported: 
     { 
      NSLog(@"Device not supported"); 
     } 
      break; 
     default: 
      break; 
    } 

，或者，只需要調用在每種情況下的方法。 [self peripheralPoweredOn];,[self peripheralPoweredOff];和[self peripheralUnsupported];。

這樣，您不僅可以解決範圍問題，還可以使其他事情更容易地驅動外設的狀態，而不僅僅是開關/外殼處理。